Class 7 Science Chapter 3: Electricity: Circuits and their Components
- 4 topics
- 100 MCQs
- 28 short answers
- 12 long answers
- 68 flashcards
- 20 games
- ~150 min read
- English
Cells and Batteries
A torch runs on electric cells. Every cell has a positive terminal at its metal cap and a negative one at its flat disc. Joining cells positive to negative makes a battery.
What you will be able to do
- Identify the positive and negative terminals on an electric cell
- Explain what makes a combination of cells a battery
- State why a device may use more than one cell
- Describe why cells must be placed in a particular order in a torch
- State the safety rule about which electrical sources may be used in experiments
Key points
Every electric cell has two terminals, one positive and one negative.
The protruding metal cap is the positive terminal and the flat metal disc is the negative one.
A cell carries its own store of electrical energy about with it.
Two or more cells joined together form a battery.
In a battery the plus end of each cell presses on the next cell minus end.
More cells supply energy for a longer time, or supply more energy, or both.
People say battery of one lone cell as well, such as the one in a mobile phone.
Experiments must use only cells and batteries, never a home or school power supply.
Full explanation
The Switch That Does Two Things
Pick up a torch and slide its switch. The lamp glows. Slide it back and the lamp goes dark. Nothing about the torch has changed except the position of one small piece of plastic, and yet the light appears and disappears. This chapter is about what that switch is actually doing, and the first step is to open the torch and look inside.
Before Anything Else, One Rule
You will see danger signs on electric poles and on appliances. They mean what they say. No experiment in this chapter, or any other, uses the power supply at your home or your school. Portable generators are dangerous too. The only electricity you will work with comes from cells, the kind inside torches, wall clocks, radios and remotes.
What Is Inside the Torch
Open the torch and you will find two or more electric cells sitting in a row. These are what light the lamp. A cell carries its energy with it, which is why a torch works in a field at night with no wall socket anywhere near. That is what makes it a portable source of electrical energy.
Two Ends, Two Jobs
Take one cell out and turn it over in your hand. At one end there is a small metal cap that sticks out. At the other there is a flat metal disc. Look closely and you will find a plus sign and a minus sign printed on the cell. These two ends are not interchangeable, and the rest of the chapter depends on telling them apart.
Which End Is Which
The rule is short. The protruding metal cap is the positive terminal. The flat metal disc is the negative terminal. Every cell you will ever pick up follows this, whatever its size, so once you have learned the shape you never need to hunt for the printed symbol.
Why a Cell Needs Both
A cell cannot do anything with one terminal connected. Electricity has to leave by one end and come back by the other, so both terminals must join the rest of the circuit. A single wire from the cap to a lamp will light nothing at all, however good the cell is.
Try Rearranging the Cells
Here is an experiment worth doing. Open a torch that uses two cells, take both out, and put them back in a different order. Slide the switch and see whether the lamp glows. Then turn one cell around and try again. You will find that some arrangements work and others do not.
The Order Is Not Random
Look carefully at the arrangement that works. Each cell has its plus end pressed against the minus end of its neighbour. Nose to tail, in other words. Turn one cell around and you have two positive terminals facing each other, and the lamp stays dark no matter how good both cells are.
That Arrangement Has a Name
Two or more cells joined that way are called a battery. So a torch with two cells in it holds a battery, not just two cells, and the difference is the connection between them. A cell holder does the joining for you with a metal strip, which is why the holder has a spring at one end.
Why Bother With More Than One
A single cell would light a lamp. Devices often use several because connecting more cells gives the circuit energy for a longer time, or more energy, or both at once. A wall clock runs for months on one cell. A bright torch wants two or four, and that is the reason.
Cells Side by Side
Sometimes a device holds its cells side by side rather than end to end, and then the nose-to-tail rule is hard to see. Inside the compartment you should spot a thick wire, or a metal strip, bridging one cell's positive terminal and the next one's negative. The plus and minus symbols are usually printed in there to help you.
A Confusing Habit of Language
One thing will trip you up. People say battery for a single cell as well. A mobile phone is said to have a battery and there is only one cell inside it. Both uses are accepted, so in this chapter take battery to mean two or more cells joined together unless something tells you otherwise.
Reading a Cell Before You Use It
Get into the habit of checking a cell before it goes anywhere. Find the cap and the disc, note which way the compartment wants them, and only then put it in. Two minutes spent doing that saves the puzzle of a torch that will not light with two perfectly good cells inside it.
A Dead Cell Is Not a Broken Torch
When a torch dims after long use, the cells have given up most of their energy. The torch is fine. This is worth knowing because the first thing people do with a dim torch is shake it or hit it. The only thing that will help is a fresh set of cells, put in the right way round.
What Comes Next
You now have the source sorted out. The next topic looks at the thing the cells are feeding, which is the lamp, and you will find there are two very different kinds. One of them does not care which way round you connect it. The other refuses to work unless you get it right.
Concepts and terms
A two-terminal store of electrical energy you can carry about. The small protruding metal cap is the positive terminal and the flat metal disc at the other end is the negative one. Both are marked on the cell with a plus and a minus sign.
One of the two points on a cell where it joins a circuit. Current leaves by one and returns by the other, so a cell can only work when both are connected. Which terminal is which decides the direction of the current.
Two or more cells set in a line, each one plus end resting on the next one minus end. The word is also used loosely for a single cell, which is why a phone is said to have a battery.
Connecting more cells gives the circuit energy for a longer time, or more energy, or both. That is why a bright torch holds two or four cells while a wall clock manages on one.
Danger signs on poles and appliances warn that electricity can harm you. No experiment should ever use the supply at home or school, or a portable generator. Cells from a torch, clock, radio or remote are the only safe source.
Watch out!
Myth or fact?
Real world: the torch that would not light on a dark road
Scenario:
Imran family is driving home when their car stops on an unlit road. He fetches the torch from under the seat and slides the switch, and nothing happens. His sister says the torch is finished and they should use a phone instead. Imran opens the cell compartment first. There are two cells inside and the plus and minus symbols are printed on the inside of the case. He finds that one cell has its flat disc pressing where the symbols show a cap should go. He turns that cell round, closes the lid, and the lamp lights at full brightness. The cells were never the problem.
Analysis:
Imran checked the cheapest cause before replacing anything. That is the habit worth copying. A torch needs its cells joined nose to tail. The positive terminal of one must meet the negative terminal of the next, and the printed symbols inside the case show how. One cell turned round puts two like terminals together. No amount of energy in the cells will then light the lamp. His sister conclusion was reasonable but untested. It would have cost them a working torch and a set of good cells. Notice too that the full brightness afterwards proves the cells still held their energy.
Activity:
Find a torch or a remote at home that takes two or more cells. Before you open it, draw the compartment and predict which way each cell faces. Then open it and compare your drawing with the printed symbols inside. Write one sentence saying what you got wrong, if anything.
Try it at home
Title:
Make a battery and test the order
Safety:
Use only the cells from a torch, clock, radio or remote. Never use a home or school power supply, or a generator.
What You Notice:
Swapping the two cells changes nothing, because each still meets the next nose to tail. Reversing one stops the lamp completely. So it is the direction of each cell that matters, not which cell sits where.
Did you know
The word battery is used even for a single cell, which is why a mobile phone is said to have one.
A cell is called portable because it carries its energy with it, so a torch works far from any wall socket.
Reversing just one cell in a two-cell torch is enough to stop the lamp glowing.
Memory tricks
Cap is plus, disc is minus: the bump sticking out is the positive terminal.
Nose to tail makes a battery: the plus of one cell meets the minus of the next.
Practice MCQs with answers (25)
- A the negative terminal
- B filament
- C positive terminal
- D switch
Show answer
Why:
The cap that sticks out is the positive terminal, and the flat disc is the negative one.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and II are correct. III is not: cells joined together make a BATTERY, and a circuit is the complete path the current then has.
- A a device that measures out the electric current
- B a switch that completes the whole of a circuit
- C a material which current cannot ever pass through
- D a carry-anywhere store of electrical energy
Show answer
Why:
A cell carries its energy with it, which is why a torch works away from any socket.
- A filament
- B battery
- C circuit diagram
- D conductor
Show answer
Why:
That combination is what the word battery names.
- A the positive of one meets the positive terminal of the next
- B the negative of one meets the negative terminal of the next
- C both terminals of one meet both of the next
- D the positive of one meets the negative of the next
Show answer
Why:
Nose to tail is the rule. Like terminals facing each other will not work.
- A does not glow, because two like terminals face each other
- B glows more brightly than it did before
- C glows exactly as it did before the change
- D glows, but it shows a noticeably different colour of light
Show answer
Why:
Reversing one cell breaks the nose-to-tail arrangement, so the lamp stays dark.
- A Because one cell alone can never light a lamp
- B To make the device heavier and more stable
- C To give energy for longer, or more energy, or both
- D Because cells only work when there are two
Show answer
Why:
More cells extend how long the energy lasts and can supply more of it.
- A a phone must really contain several hidden cells
- B the word cell has now gone out of use entirely
- C a phone cell is not a true electric cell
- D people say battery of one lone cell as well
Show answer
Why:
Both uses are accepted, so battery sometimes names a single cell.
- A The power supply in your school laboratory
- B Cells from a torch, clock, radio or remote
- C The wall socket at home, if an adult helps
- D A small portable generator, used carefully
Show answer
Why:
Only cells and batteries are safe. Home and school supplies and generators are not.
- A electricity can be dangerous if not handled carefully
- B the pole has recently been painted
- C only electricians may look at the pole
- D the supply to that pole has been switched off for repairs
Show answer
Why:
They are a warning about the danger of electricity itself.
- A glows, because the positive terminal is enough
- B glows rather dimly, at about half of its usual brightness
- C glows only if the cell is a fresh one
- D does not glow, since both terminals must be connected
Show answer
Why:
Electricity leaves by one terminal and must return by the other, so one connection does nothing.
- A They are not joined to each other at all in this arrangement
- B By the plastic case of the device itself
- C By a thick wire or metal strip inside the compartment
- D By the narrow air gap between the two cells
Show answer
Why:
A strip inside the compartment joins the positive of one to the negative of the next.
- A Show which way round each cell should face
- B Show how much energy each cell still holds
- C Mark where the switch should be fitted
- D Warn that the device is dangerous to open
Show answer
Why:
They guide correct placement so the cells form a proper battery.
- A protruding metal cap, the positive terminal
- B flat metal disc, the negative terminal
- C glass bulb of the lamp
- D plastic covering of the wire
Show answer
Why:
The spring meets the flat disc, which is the negative terminal of the cell.
- A the torch case itself has become slightly loose
- B the switch has been slid across only part of the way
- C the lamp itself has been fitted the wrong way round
- D the cells have given up most of their energy
Show answer
Why:
Cells run down with use. The torch itself is fine and fresh cells will fix it.
- A a switch out of separate cells
- B a circuit diagram of the device
- C a battery out of separate cells
- D an insulator between the cells
Show answer
Why:
Joining cells positive to negative is exactly what makes them a battery.
- A negative
- B positive
- C neutral
- D fused
Show answer
Why:
The protruding cap is the positive terminal.
- A False, because electricity must leave and return
- B False, but only for very old cells
- C True, as long as it is the positive terminal
- D True, provided the cell is a fresh one
Show answer
Why:
Both terminals must be in the circuit, because electricity leaves by one and returns by the other.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, and the reason is exactly why the rule exists. The danger signs on poles and appliances carry the same warning.
- A all four of them have their positive terminals together
- B all four are joined positive to negative in turn
- C only two of the four of them are actually being used
- D the order of the cells makes no difference
Show answer
Why:
A glowing lamp means the nose-to-tail arrangement is unbroken all the way along.
- A A clock cell holds far more energy than a torch cell
- B Clocks do not really use electrical energy at all
- C It needs very little energy compared with a bright torch
- D A single cell always lasts longer than several
Show answer
Why:
How long a cell lasts depends on how much the device draws, and a clock draws very little.
- A Two, one on each cell
- B Three, since one is shared
- C One, at the switch only
- D Four, two on each cell
Show answer
Why:
Each cell has two terminals, so two cells have four, and all of them take part in the connection.
- A A battery can be made from cells you already own
- B A battery must always be bought ready-made as one unit
- C Cells and batteries work on completely different ideas
- D A battery holds no cells inside it at all
Show answer
Why:
Joining two or more of your own cells positive to negative makes a battery.
- A False, but only in a two-cell device
- B False, the signs show which end is which
- C True, freshness matters more than direction
- D True, because a cell works either way round
Show answer
Why:
Direction matters whatever the cell state, because the terminals must meet nose to tail.
- A use absolutely any supply as long as an adult is present
- B use a generator rather than the wall socket
- C use only cells and batteries, never a mains supply
- D avoid electricity experiments altogether
Show answer
Why:
The caution permits cells and batteries and rules out home and school supplies and generators.
Important short questions (7)
The positive terminal is the small protruding metal cap. The negative terminal is the flat metal disc at the other end.
Two or more cells set in a line, each plus end resting against the minus end that follows it.
Connecting more cells supplies energy for a longer time, or supplies more energy, or does both at once.
The arrangement is broken. Two like terminals now face each other instead of a positive meeting a negative, so the cells cannot work together.
Electricity has to leave the cell by one terminal and return to it by the other. With one connection there is no path back, so nothing happens.
Only cells and batteries may be used in experiments, never a home or school supply or a portable generator. This is because those can be dangerous if not handled carefully.
A thick wire or a metal strip inside the compartment connects one cell plus end across to the minus end of the next. They still form a battery.
Long answer questions (3)
I would take a torch that uses two cells and open its cell compartment. First I would note the arrangement that works and check that the lamp glows. Then I would take both cells out and put them back in a different order, sliding the switch each time to see whether the lamp lights. Next I would reverse the direction of just one cell and try again. The lamp glows only when each cell plus end lands on the next cell minus end. When one cell is reversed, two like terminals face each other and the lamp stays dark, even though both cells are perfectly good.
A cell is a single portable source of electrical energy with two terminals, a positive metal cap and a negative flat disc. A battery is two or more cells joined together so that the plus end of one lands on the minus end of the next. So strictly a battery is made of cells. The confusion comes from everyday language, because people say battery for a single cell as well. A mobile phone is said to have a battery and there is one cell inside it. Both uses are accepted, so the safe approach is to take battery to mean two or more joined cells unless the situation clearly means a single one.
I would tell him not to do it under any circumstances. The supply at home is dangerous if it is not handled carefully, and that is exactly what the danger signs on poles and appliances are warning about. A portable generator is no safer. None of the experiments in this chapter need anything more than cells, and cells are what they are designed for, so he would gain nothing by taking the risk. If he wants a brighter lamp the proper way is to use more cells joined positive to negative, because connecting more cells supplies more energy. That gives him what he wants without touching the mains at all.
Flashcards (17)
Play this topic
- Positive or negative?
- Fact or myth?
- Putting cells into a torch
- Part and job
- Cell sprint
Lamps, Filaments and LEDs
An incandescent lamp glows because current heats a thin filament between its two terminals, and it works either way round. An LED passes current one way only, so its longer wire must go to the positive side.
What you will be able to do
- Name the parts of an incandescent lamp and say which two are its terminals
- Explain what makes a filament glow and what it means for a lamp to be fused
- State why an incandescent lamp works whichever way round it is connected
- Explain why an LED glows in only one direction and identify its positive wire
- Decide, for a given connection, whether an LED will glow
Key points
An incandescent lamp holds a thin filament inside a glass bulb.
Thick wires carry current from the lamp terminals to the two ends of the filament.
The metal tip is one terminal of the lamp and the metal case is the other.
Insulator supports hold the filament in place without letting current pass through them.
Current passing through the filament makes it glow, which is how the lamp gives light.
A fused lamp has a broken filament, so current cannot pass and it will not glow.
An incandescent lamp glows whichever of its terminals meets the positive side of the cell.
An LED passes current in one direction only, so it glows only when connected the right way.
The longer wire of an LED is its positive terminal and must go to the positive side.
Full explanation
Look Into the Glass
Take a torch with an old-style lamp and hold the lamp up to the light. Inside the glass bulb you will see a very thin wire fixed across the middle. That thin wire is the whole point of the lamp. Switch the torch on and watch which part lights up. It is not the glass and not the metal base. It is that thread.
The Filament
The thin wire has a name: the filament. When current passes through it the filament becomes hot enough to glow, and that glow is the light you use. Everything else in the lamp exists to get current to the filament and to hold it in place while it does its job.
How Current Reaches It
Follow the path. Two thick wires run up from the base of the lamp, one to each end of the filament. Those thick wires carry the current without glowing themselves, because they are much thicker than the filament. The thinness of the filament is what makes it the part that lights.
The Two Terminals of a Lamp
Where do those thick wires come from? Look at the base of the lamp. There is a metal tip at the very bottom and a metal case around the sides, and the two are kept apart. The metal tip is one terminal of the lamp and the metal case is the other. Current enters by one and leaves by the other.
What the Insulators Do
Inside the bulb you will also find small supports holding the filament steady. These are insulators, which means current cannot pass through them. That matters twice over. They stop the fine filament from sagging, and because they do not conduct, they do not give the current a shortcut around the filament.
Either Way Round
Here is a convenience worth knowing. With an incandescent lamp it makes no difference which of its terminals meets the positive terminal of the cell. Swap the two wires over and the lamp glows exactly as before. The filament heats up whichever way the current runs through it.
What Fused Means
Sometimes a lamp will not glow even though it is correctly connected to good cells. Usually the filament has broken. We say the lamp has fused. A broken filament leaves a gap, the current has no way across, and no amount of fiddling with the cells will help. The lamp has to be replaced.
Spotting a Fused Lamp
You can often see it. Hold the bulb up and look for the thin thread. If it is in two pieces, or one end has come away, that is your answer. This is a useful thing to check before you decide your cells are dead, because a fused lamp and a flat cell look identical from the outside.
A Different Kind of Lamp
Many torches now use something else entirely: an LED, which stands for light emitting diode. It is small, it has two thin wires coming out of it rather than a metal base, and one of those wires is longer than the other. That difference in length is not an accident.
The LED Is Fussy
Connect an LED to a battery and it glows. Now swap the two wires over and try again. It stays dark. This never happens with a filament lamp, and it is the most important difference between the two. An LED passes current in one direction only.
Which Wire Is Which
The longer wire of an LED is its positive terminal and the shorter wire is its negative one. The LED glows when the longer wire is connected towards the positive terminal of the battery and the shorter wire towards the negative. Get that round the wrong way and no current passes at all.
Why That Is Not a Fault
It would be easy to call this a weakness, and it is not. A component that only lets current through one way is extremely useful in electronics, which is why the LED is built that way on purpose. For now what matters is the practical rule: always check the leg lengths before you connect one.
Testing an LED Fairly
If an LED does not glow, there are two possibilities and you should separate them. Either it is connected the wrong way round, or something else in the circuit is wrong. Swapping the two wires takes a second and settles the first possibility completely, so do that before suspecting anything else.
Two Lamps, Two Habits
So you now have two rules to keep apart. With an incandescent lamp, the direction does not matter and a dark lamp is probably fused. With an LED, direction matters a great deal and a dark LED is quite likely just the wrong way round. Which lamp you are holding decides which thing to check first.
What Comes Next
You have a source and you have something to light. The next topic puts them together into a circuit and asks the question that has been waiting since the first page. What exactly is the switch doing when you slide it?
Concepts and terms
The thin wire fixed in the middle of the glass bulb of an incandescent lamp. Current passing through it makes it glow, and that glow is the light the lamp gives. Thick wires carry the current to its two ends.
The two points by which an incandescent lamp joins a circuit: the metal tip at the bottom and the metal case around it. Current enters by one and leaves by the other, passing through the filament on the way.
A lamp whose filament has broken. The break leaves no path for the current, so the lamp will not glow even when it is connected correctly to good cells. Nothing else about the lamp needs to be wrong.
A part inside the bulb that holds the filament steady without carrying current itself. Without it the fine filament would sag or touch, and with a conducting support the current would bypass the filament altogether.
A light emitting diode, which passes current in one direction only. Its longer wire is the positive terminal. Connected the other way round no current passes and it stays dark, unlike an incandescent lamp.
Watch out!
Myth or fact?
Real world: the science fair stall that kept going dark
Scenario:
Priya and Arun build a model lighthouse for the science fair, with a small LED at the top run from two cells in a holder. It works in the classroom. At the fair they take it apart to carry it and rebuild it on the stall, and now it will not light. Arun is sure a cell has run down on the journey and goes to buy more. Priya instead looks at the LED and notices that the wire going to the positive side of the holder is the shorter of the two. She swaps the two wires and the lighthouse lights immediately. Later another team asks them for help with a torch bulb model that has gone dark, and there the swap changes nothing at all.
Analysis:
Two dark lamps with two different causes, and the kind of lamp is what decides where to look. An LED passes current in one direction only. Its longer wire is the positive terminal, so a rebuilt circuit with the short wire on the positive side will not light however good the cells are. That is why Priya check cost nothing and Arun shopping trip would have cost money and still left it dark. The other team has an incandescent lamp, which glows either way round, so swapping its wires cannot help. For that one the thing to inspect is the filament, because a dark filament lamp with good cells has usually fused.
Activity:
Find a torch at home and work out which kind of lamp it uses. If you can see a thin thread inside a small glass bulb it is incandescent. Write down which kind it is, and then write the one thing you would check first if that torch ever went dark.
Try it at home
Title:
Make an LED glow, then stop it
Safety:
Use only cells and a holder. Never connect an LED to a home or school power supply.
What You Notice:
The LED glows one way round and stays completely dark the other way. Nothing about the cells changed between the two tries, so the direction alone decides whether current passes.
Did you know
The term fused comes from the filament breaking, which is why a dead lamp is said to have fused.
An incandescent lamp works either way round, so there is no right and wrong way to fit it.
In the LED symbol the triangle points the way the current can flow and two arrows show that it gives out light.
Memory tricks
Longer wire, positive side: an LED only glows when its long leg meets the plus.
Fused means broken thread: a dead incandescent lamp usually has a snapped filament.
Practice MCQs with answers (25)
- A terminal
- B insulator
- C filament
- D conductor
Show answer
Why:
The filament is the thin wire, and it is the part that glows when current passes.
- A current passing through the filament makes it glow
- B the glass bulb reflects back the light of the cell
- C the metal case becomes hot and shines
- D the insulator supports give out a glow
Show answer
Why:
The filament heats up as current passes through it, and that glow is the light.
- A the glass bulb and the filament itself
- B the two insulator supports
- C the filament and the metal casing
- D the metal tip and the metal case
Show answer
Why:
The metal tip at the bottom and the metal case around it are the two terminals.
- A They are made out of plastic rather than metal
- B They are much thicker than the filament is
- C No current ever passes through them at all
- D They are kept quite cool by the glass bulb
Show answer
Why:
The same current passes through both, and thinness is what makes the filament glow.
- A the filament can slide freely as it heats
- B the glass bulb does not crack when hot
- C the whole lamp can then be connected either way round
- D current cannot take a shortcut around the filament
Show answer
Why:
A conducting support would let current bypass the filament, and then the lamp would not glow.
- A glow exactly as it did before
- B stay dark until they are swapped back
- C glow much more dimly than before
- D glow with a different colour of light
Show answer
Why:
Direction makes no difference to a filament lamp, so it glows just as before.
- A the glass bulb has gone cloudy
- B its metal case has come loose
- C its filament has broken
- D its cells ran down
Show answer
Why:
Fused means a broken filament, which leaves the current no path.
- A the cells are joined the wrong way round
- B the circuit has too many wires in it
- C the lamp needs a much longer wire
- D the lamp has fused, so its filament is broken
Show answer
Why:
With good cells and a correct connection, a broken filament is the remaining explanation.
- A The glass bulb has turned completely black
- B The thin thread inside is in two pieces
- C The metal case has become slightly warm
- D The lamp feels heavier than a working one
Show answer
Why:
A broken filament is usually visible if you hold the bulb up and look at the thread.
- A light emitting diode
- B low energy device
- C long electric discharge
- D lamp with external drive
Show answer
Why:
An LED is a light emitting diode.
- A needs no cell to make it glow
- B gives out no light at all of any kind
- C has three thin wires coming out instead of two
- D passes current in one direction only
Show answer
Why:
Direction matters for an LED and not for a filament lamp, and that is the key difference.
- A negative terminal
- B filament
- C positive terminal
- D insulator
Show answer
Why:
The longer wire is positive and the shorter wire is negative.
- A its longer wire goes towards the positive terminal
- B its shorter wire goes towards the positive terminal
- C both of its two wires go to the very same terminal
- D it is connected either way round, just like a lamp
Show answer
Why:
The long wire must face the positive side and the short wire the negative side.
- A replace both of the cells at once
- B swap its two wires and try once more
- C add a third cell to the battery
- D rub the LED wires with a cloth
Show answer
Why:
Reversal is the commonest cause and it takes a second to rule out.
- A glow a good deal more brightly
- B glow in a different colour
- C glow exactly as it did before
- D stop glowing altogether
Show answer
Why:
An LED passes current one way only, so reversing it stops the current.
- A Yes, and better LEDs do not have it
- B Yes, and it is caused entirely by poor manufacture
- C No, it is deliberate and useful in electronics
- D No, but only because LEDs are cheap
Show answer
Why:
LEDs are built to pass current one way on purpose, and that property is useful.
- A glowed
- B fused
- C conducted
- D reversed
Show answer
Why:
A broken filament is what fused means.
- A False, it works either way round
- B False, but only for small torch lamps
- C True, the metal tip must be positive
- D True, the metal case must be positive
Show answer
Why:
Only an LED cares about direction. A filament lamp glows either way.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and II are correct. III is not: a fused lamp has a BROKEN filament, and no cell can mend that.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
Both are true, but the reason does not explain the assertion. Being efficient has nothing to do with direction; the LED needs its longer wire towards the positive terminal.
- A shake the torch and listen for a rattle
- B weigh the lamp against a brand new one
- C look at the filament inside the bulb
- D count how many cells are fitted
Show answer
Why:
Only the filament tells you which of the two causes you have.
- A So that the glass itself can carry some of the current
- B So the bulb stays completely cold
- C So the filament can be removed easily
- D So the supports can hold it clear while it glows
Show answer
Why:
The insulator supports hold it clear, which keeps it from sagging or touching.
- A the longer one marks its positive terminal
- B the longer one of them carries much more current
- C it is easier to cut to length later
- D the shorter one is a spare wire
Show answer
Why:
The length difference is a marking, telling you which terminal is positive.
- A False, unless the cells happen to be much larger
- B False, the broken filament has to be replaced
- C True, since fresh cells push current across the gap
- D True, provided that the lamp is left to rest first
Show answer
Why:
No cell can push current across a broken filament. The lamp itself must be replaced.
- A Both kinds of lamp depend on the direction
- B Neither kind of lamp depends on the direction
- C A filament lamp ignores direction; an LED depends on it
- D An LED ignores direction; a filament lamp depends on it
Show answer
Why:
Direction is irrelevant to a filament lamp and decisive for an LED.
Important short questions (7)
The filament is the thin wire fixed inside the glass bulb. Current passing through it makes it hot enough to glow, and that glow is the light the lamp gives.
The metal tip at the bottom of the lamp is one terminal. The metal case around it is the other.
It means the filament inside has broken. The break leaves a gap so current cannot pass through, and the lamp will not glow however good the cells are.
The same current passes through both, but the thick wires are far thicker. Being very thin is what makes the filament heat enough to glow.
An incandescent lamp glows whichever way round it is connected. An LED passes current in one direction only and stays dark if reversed.
The longer wire is the positive terminal. Connected the wrong way round no current passes through the LED, so it does not glow at all.
An insulator cannot carry current. The supports hold the filament steady without offering the current a shortcut that would bypass the filament.
Long answer questions (3)
An incandescent lamp has a glass bulb with a very thin wire fixed across the middle. That wire is the filament. Small insulator supports hold it steady. Two thick wires run from the filament down to the base of the lamp. There the lamp has two terminals: a metal tip at the very bottom and a metal case around the sides, kept apart. Current enters by one terminal and travels up a thick wire. It passes along the whole length of the filament. Then it returns down the other thick wire and leaves by the other terminal. Passing through the thin filament heats it until it glows, and that is the light we use.
With the LED I would suspect the direction first, because an LED passes current in one direction only. I would swap its two wires over and try again, checking that the longer wire is the one going towards the positive terminal of the battery. That takes a second and settles the commonest cause. With the incandescent lamp the direction cannot matter, because it glows either way round, so I would look at the filament instead. I would hold the bulb up and check whether the thin thread is in one piece. If it is broken the lamp has fused and must be replaced. Only after both of those would I start suspecting the cells or the wires.
An LED passes current in one direction only, so it matters which of its two wires goes towards the positive terminal of the battery. The maker therefore leaves one wire longer to mark it. The longer wire is the positive terminal and the shorter wire is the negative one, so you can tell them apart at a glance without any label or printing. That is a help rather than a nuisance. Without the difference in length there would be no way to tell which wire was which, and connecting an LED would be guesswork. The one-way behaviour itself is deliberate too, because a component that only passes current one way is very useful in electronics.
Flashcards (17)
Play this topic
- Which lamp does this describe?
- Fact or myth?
- Finding out why an LED will not glow
- Part and purpose
- Lamp sprint
Circuits, Switches and Circuit Diagrams
A circuit is a complete path for current, taken to flow from the positive terminal to the negative. A switch completes or breaks that path, and standard symbols let any circuit be drawn quickly.
What you will be able to do
- Explain what makes an arrangement of cell, wires and lamp an electrical circuit
- State the direction in which current is taken to flow in a circuit
- Distinguish an open circuit from a closed one and relate each to a switch position
- Draw a circuit diagram using the standard symbols for cell, battery, lamp, LED and switch
- Read a cell symbol and an LED symbol correctly from a circuit diagram
Key points
A lamp glows only when each end of the lamp reaches a different end of the cell.
Such an arrangement is an electrical circuit, and it gives current a complete path.
The lamp glows only while current passes through the circuit.
The direction of current is taken to be from the positive terminal to the negative terminal of the cell.
With the switch on, the circuit is closed and the lamp glows.
With the switch off, the circuit is open and the lamp does not glow.
A switch has one job only, which is to join the path up or cut it, and it may be placed anywhere in it.
A drawing of a circuit made with standard symbols is called its circuit diagram.
In the symbol for a cell the long line is the positive terminal and the short line the negative.
Full explanation
Six Arrangements, Two That Work
Suppose you lay out a cell, a lamp and some wire in six different arrangements and try each one. Only two of the six light the lamp. That is a surprising result if you expected the cell simply to make light happen. Something about the arrangement itself matters, and finding out what is the point of this topic.
The Rule Behind the Two
Look at the arrangements that worked. In each one, each end of the lamp reaches a different end of the cell. Both lamp terminals and both cell terminals are in use. In the arrangements that failed, at least one of those connections is missing.
What a Circuit Is
That working arrangement has a name. It is an electrical circuit, and what it provides is a complete path for current to flow through the lamp. The word complete is doing the work. A path that goes most of the way round is not a circuit at all, and a lamp on it will not glow.
Current Is What Makes It Glow
The lamp glows only while current passes through the circuit. That is worth stating plainly, because it explains every failure you will meet. A dark lamp means no current is passing, and the job is then to find where the path is interrupted rather than to wonder what is wrong with the lamp.
Which Way Does It Go?
Current has a direction, and everyone agrees on which one. We agree to show the current leaving the cell at its plus end and returning to it at the minus end. You cannot see this happening, so it is a convention, but it is a convention the whole world uses and it lets any diagram be read the same way by anybody.
Why the Convention Matters
For an incandescent lamp the direction makes no practical difference, since the filament heats up either way. For an LED it matters a great deal, because an LED passes current one way only. So the moment your circuit has an LED in it, knowing which way the current is taken to flow becomes the thing that decides whether anything lights.
Making a Switch
Now for the thing the torch switch has been doing all along. You can build one in a moment: leave a small gap in the circuit, and bridge it with something that conducts when you want the lamp on. That is all a switch is. Bridge the gap and the lamp lights; remove the bridge and it goes dark.
Closed and Open
These two states have names. With the switch on, the path is unbroken and we say the circuit is closed, and the lamp glows. With the switch off, there is a gap, and we say the circuit is open, and the lamp does not glow. Closed means complete, which is the opposite of how a closed door behaves, and that catches people out.
What a Switch Actually Is
So a switch has just one job: joining the path up, or cutting it apart. That is the whole of it. It does not store electricity, it does not make current stronger, and it does not change the lamp. It makes or removes a gap in the path.
It Can Go Anywhere
Here is the part people find odd. A switch may be placed anywhere in a circuit and it will work just as well. It does not have to sit next to the lamp or next to the cell. Because the path has to be complete all the way round, a gap anywhere in it stops the current everywhere, so it makes no difference where you put the gap.
And That Is Why Wall Switches Work
This explains something you see every day. The switch for a ceiling light sits on a wall several metres away, and it still controls the lamp perfectly. Those switches are designed differently from the one you just built, but they work in exactly the same way: they complete or break the path.
Drawing Circuits Faster
Drawing real cells and real lamps every time is slow and the drawings come out differently each time. So each component has an agreed symbol instead. A representation of an electrical circuit made with symbols is called its circuit diagram, and once you know the symbols you can draw any circuit in seconds.
Reading the Cell Symbol
Two drawings need care. On a cell, the longer stroke stands for its plus end and the shorter stroke for its minus end. There is no plus sign to help you, so the lengths are the only clue, and getting them the wrong way round reverses your whole circuit.
Reading the LED Symbol
The LED symbol is a triangle with two arrows beside it. The triangle points in the direction in which current can flow through the LED, and the two arrows indicate that the LED emits light. So the symbol tells you both what the component does and which way round it has to go.
One Set of Symbols for the World
These symbols are not local habits. Bodies such as the IEC, ANSI and the IEEE settle one agreed drawing for each electrical and electronic part. Using the same ones everywhere means an engineer in one country can read a diagram drawn in another without any explanation at all.
Concepts and terms
An arrangement that gives current a complete path, with one terminal of the lamp joined to one terminal of the cell and the other to the other. The lamp glows only while current passes round it.
In a circuit the current is taken to flow from the positive terminal of the cell to its negative terminal. This is a convention everyone follows, and it is what lets a diagram be read the same way by anybody.
A closed circuit has an unbroken path, so current flows and the lamp glows. An open circuit has a gap somewhere, so no current flows and the lamp stays dark. A switch moves a circuit between the two.
A simple device whose only job is joining the path up or cutting it. It may be placed anywhere along the circuit and still control it, which is why the switches at home can sit on a wall far from the lamp.
A drawing of a circuit made with agreed symbols instead of pictures. It is far quicker to draw and easier to read, and bodies such as the IEC, ANSI and IEEE keep the symbols the same worldwide.
Watch out!
Myth or fact?
Real world: the model that worked on the desk and failed on the wall
Scenario:
A class builds a model room with a lamp, a cell holder and a homemade switch. On the desk it works perfectly. Then they mount the parts on a vertical board, with the switch near the bottom and the lamp at the top, joined by much longer wires. Now the lamp glows only sometimes. Farhan thinks the longer wires are the problem and that the switch must be moved up beside the lamp. Before moving anything, Sunita presses each connection in turn while the switch is on. At one of the screw terminals near the switch the lamp flickers as she presses. They tighten that screw and the lamp becomes steady.
Analysis:
Farhan reasoning sounds plausible and is wrong, and the reason it is wrong is worth holding on to. A switch will work at any point along a circuit. The path has to be complete all the way round before current can flow. A gap at one point stops the current at every point. Moving the switch nearer the lamp changes nothing. Sunita tested the real possibility instead. A loose screw is an intermittent gap. Sometimes the circuit is closed and the lamp glows. Sometimes it is open and the lamp goes dark. The flicker as she pressed is the evidence. It ties the fault to that one point, not to the length of the wires.
Activity:
Build a circuit with a cell, a lamp and two wires, then deliberately make the connection at one terminal loose. Switch it on and gently press that terminal. Write down what the lamp does, and name the state of the circuit at each moment.
Try it at home
Title:
Make your own switch
Safety:
Use only a cell or battery. Never build a switch into a home or school power supply.
What You Notice:
The lamp glows whenever the clip bridges the gap. It goes dark as soon as the clip swings away. That is the whole of what a switch does. With the clip across the circuit is closed; with it away the circuit is open.
Did you know
A switch works just as well anywhere in a circuit, which is why wall switches can sit far from the lamp they control.
The same circuit symbols are used worldwide, agreed by bodies such as the IEC, ANSI and the IEEE.
On a drawn LED the pointed shape aims the way current can flow and two arrows show that it emits light.
Memory tricks
Closed glows, open goes dark: a complete path is the only thing a lamp needs.
Long line is the plus: in a cell symbol the longer stroke marks the positive terminal.
Practice MCQs with answers (25)
- A one of its terminals joins one terminal of the cell
- B its metal case touches any part of the wire
- C both its terminals join the two terminals of the cell
- D it is simply placed close to a fresh cell
Show answer
Why:
Both terminals of the lamp and both of the cell must be in use for a complete path.
- A an electrical circuit
- B a circuit diagram
- C an electric cell
- D an insulator
Show answer
Why:
That is exactly what an electrical circuit is.
- A from the negative terminal to the positive terminal
- B first one way and then back the other way
- C different in every circuit that is built
- D from the positive terminal to the negative terminal
Show answer
Why:
By convention current flows from positive to negative.
- A closed
- B open
- C complete
- D fused
Show answer
Why:
Switch off leaves a gap, so the circuit is open and the lamp is dark.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
All three are correct. The gap can be anywhere, which is why the position of the switch does not matter, and the symbols are agreed so that any reader can follow any diagram.
- A Its one job is joining the path up or cutting it
- B It stores electricity until it is needed
- C It makes the current in the circuit stronger
- D It changes the colour of the lamp light
Show answer
Why:
Making or removing a gap in the path is the whole of what a switch does.
- A only right beside the lamp
- B only right beside the cell
- C anywhere along it
- D only between two cells
Show answer
Why:
A gap anywhere stops the current everywhere, so position makes no difference.
- A The switch sends some kind of signal through the air to the lamp
- B The lamp has a second switch hidden inside it
- C Long wires carry current in one direction only
- D Breaking the path anywhere stops current all the way round
Show answer
Why:
Because the path must be complete, a break at any point turns the lamp off.
- A an electrical circuit
- B a circuit diagram
- C a conductor test
- D a battery holder
Show answer
Why:
A representation using symbols is the circuit diagram.
- A positive terminal
- B negative terminal
- C filament of the lamp
- D switch in the circuit
Show answer
Why:
The long line is positive and the short line negative.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
Show answer
Why:
The assertion is true, but the reason is false: an incandescent lamp does have two terminals, the metal tip and the metal case. It works either way round because its filament does, not because it lacks terminals.
- A how bright the LED will become
- B the number of cells that are needed to light it
- C the direction in which current can flow
- D which particular colour of light it will give
Show answer
Why:
The triangle points the way current can pass through the LED.
- A emits light
- B needs two cells
- C has two terminals
- D works either way round
Show answer
Why:
The arrows are there to show the LED gives out light.
- A each individual school in a country
- B the IEC, ANSI and the IEEE
- C the maker of each separate component
- D the person drawing the diagram
Show answer
Why:
International organisations keep the symbols the same everywhere.
- A It makes every circuit cheaper to build
- B It stops circuits from ever going wrong
- C It means that only trained engineers are able to read a diagram
- D A diagram drawn anywhere can be read without explanation
Show answer
Why:
Shared symbols let people in different countries and industries understand each other.
- A a second lamp hidden in the circuit
- B a way to make the cell stronger
- C a gap somewhere in the path
- D the colour marking on the wires
Show answer
Why:
A lamp glows only while current passes, so an unbroken path is the thing to check.
- A four of the six cells must have been flat
- B the arrangement itself decides whether current flows
- C the lamp will only work at certain times of the day itself
- D a lamp glows whenever a cell is nearby
Show answer
Why:
Only arrangements that complete the path let current flow, which is why most fail.
- A open
- B closed
- C complete
- D conducting
Show answer
Why:
A gap makes the circuit open, and the lamp stays dark.
- A False, but only when a switch is fitted
- B True, closed means shut off to current
- C True, which is why closed lamps are dark
- D False, a closed circuit has an unbroken path
Show answer
Why:
Closed means complete, so current flows. It is the opposite of how a closed door behaves.
- A An incandescent lamp
- B An LED
- C A switch
- D A length of wire
Show answer
Why:
An LED passes current one way only, so direction decides whether it lights.
- A the only possible way to show a lamp correctly
- B needed first, before any circuit can be built
- C quicker and comes out the same every time
- D a way of making the circuit brighter
Show answer
Why:
Symbols are fast and consistent, which is the whole reason for having them.
- A negative terminal is on the left
- B cell has been drawn incorrectly
- C circuit will not work at all
- D positive terminal is on the left
Show answer
Why:
The long line always marks the positive terminal, whichever side it is drawn on.
- A It is a convention everybody agrees to follow
- B It changes from any one circuit across to another
- C Nobody at all has ever worked out the true direction
- D It depends on which brand of cell has been used in it
Show answer
Why:
You cannot see it happening, so the positive-to-negative direction is an agreed convention.
- A False, unless the wires happen to be very long indeed
- B False, a gap anywhere breaks the whole path
- C True, since otherwise the current goes round it
- D True, and that is why switches are built in
Show answer
Why:
Position is irrelevant, because the path must be complete all the way round.
- A A fresh cell placed close enough to the lamp
- B A switch fitted somewhere near the lamp
- C A complete path with current passing through it
- D A diagram drawn with the correct symbols
Show answer
Why:
The lamp needs a complete circuit and current actually flowing round it.
Important short questions (7)
It is an arrangement that provides a complete path for current to flow. One terminal of the lamp is joined to one terminal of the cell, and the other to the other.
From the positive terminal of the electric cell to its negative terminal.
A closed circuit has an unbroken path, so current flows and the lamp glows. An open circuit has a gap in it, so no current flows and the lamp stays dark.
A switch either completes or breaks the circuit. It does not store electricity, make the current stronger or change the lamp in any way.
The path has to be complete all the way round, so a gap at any point stops the current everywhere. It therefore makes no difference where the gap is made.
The longer stroke stands for the plus end and the shorter stroke for the minus end.
The triangle shows the direction in which current can flow through it. The two arrows show that the LED emits light.
Long answer questions (3)
The arrangements that work are the ones that give the current a complete path. In each of those, one end of the lamp reaches one end of the cell, and its other end reaches the cell other end. So all four terminals are in use. That arrangement is called an electrical circuit. In the four that fail, at least one connection is missing, so the path has a gap. The lamp glows only while current passes, and current cannot cross a gap. So the cell is not what decides whether the lamp lights. The arrangement is.
A switch does nothing but join the path up or cut it apart. When it is on, the path is unbroken and the circuit is closed, so the lamp glows. When it is off there is a gap, the circuit is open, and the lamp goes dark. It works anywhere in the circuit. The path has to be complete all the way round before current can flow. A gap at one point therefore stops the current at every point. So it makes no difference whether the gap is beside the lamp, beside the cell or halfway along a wire. That is why a ceiling light switch can sit metres away and still control the lamp.
Drawing real cells and lamps is slow, and the drawings come out differently each time. So each component has an agreed symbol instead. A circuit drawn with them is called a circuit diagram. International bodies such as the IEC, ANSI and the IEEE keep those symbols the same everywhere. A diagram drawn in one country can then be read in another with no explanation. Two symbols need care. A cell symbol has no plus sign: the long line is positive and the short line negative. Read the lengths wrongly and the whole circuit is reversed. In the LED symbol the triangle points the way current can flow, and the arrows show it emits light. An LED passes current one way only, so misreading that triangle leaves it dark.
Flashcards (17)
Play this topic
- Open or closed?
- Fact or myth?
- Drawing a circuit diagram
- Symbol and meaning
- Circuit sprint
Conductors and Insulators
Current flows easily through conductors such as metals and not through insulators such as plastic and rubber. Both matter: metal carries the current and the covering keeps it away from us.
What you will be able to do
- Build a tester and use it to sort objects into conductors and insulators
- Define a conductor and an insulator in terms of how current passes
- Explain why copper is used for wires although silver and gold conduct better
- Explain why both conductors and insulators are needed in the same appliance
- State why the human body makes electrical safety a matter of life and death
Key points
Current moves through some materials without difficulty, and those are conductors, or good conductors.
Current cannot get through other materials at all, and those are insulators, or poor conductors.
Metals are conductors of electricity, which is why wires are made of metal.
Silver heads the list of conductors, with copper and gold just after it.
Copper is mainly used for wires because it costs comparatively less and is abundant.
Plastic, rubber and ceramics are electrical insulators.
Wire cores, switch contacts and the pins of plugs and sockets are all conducting metal.
Insulators wrap each wire, plug body and switch so no shock reaches a person.
A human body conducts as well, so current passing through it can injure a person badly, or kill them.
Full explanation
Why Are Wires Made of Metal?
Every wire you have used in this chapter has been metal inside. That is not a habit or a tradition. Try building a circuit with a thread of cotton or a strip of paper in place of the wire and the lamp stays dark. Something about metal lets current through, and this topic is about finding out which materials share that property.
Build Yourself a Tester
You can settle the question by experiment. Connect a cell and a lamp with wires, but leave two ends free instead of joining them. Touch those two free ends together and the lamp should glow, which tells you the rest of the circuit is sound. That arrangement is a tester, and it is ready to use.
How the Tester Answers
Now touch the two free ends to the two ends of any object, keeping the wires from touching each other. If the lamp glows, current has passed through the object and completed the circuit. If the lamp stays dark, current could not get through. The object has given you a plain yes or no.
Collect a Drawer Full
Test as many things as you can find. Try a metal spoon, a coin, a key, a sewing needle and aluminium foil. Try pencil lead too. Then a cork, a rubber eraser, a glass bangle, a plastic scale, a wooden block, a candle, cardboard and paper. Write down the object, what it is made of, whether the lamp glowed, and your conclusion.
The Lamp Glows for Some Only
Your table will show a clear split. The lamp glows for some materials and not for others. So current moves through some materials and not through others. That difference belongs to the material itself, not to the shape or the size of the object.
The Two Names
Where current moves through without difficulty, the material is a good conductor, or simply a conductor. Where it cannot get through at all, the material is an insulator, or a poor conductor. Both names for each are worth knowing, because you will meet them both.
Reading Your Own Table
Look down your list of conductors and a pattern jumps out. The spoon, the coin, the key, the needle and the foil are all metal. Metals are conductors of electricity. That is the answer to the question this topic started with: wires are made of metal because metal lets the current through.
And the Insulators
Now the other column. Plastic, rubber and ceramics are electrical insulators, and so are wood, glass, paper and wax. Nothing in that list will carry current for you, which sounds like a list of useless materials until you ask what the covering on a wire is for.
The Best Conductors
Among the metals, silver conducts best of all, with copper and gold just behind it. That is a fact worth remembering, and it immediately raises a question. If silver and gold are that good, why is every wire in your house made of copper?
Why Copper Wins Anyway
Because being best is not the only thing that matters. Copper is used for making electrical wires mainly because it costs comparatively less and its supply is abundant. Silver and gold would work slightly better and would be absurdly expensive. Copper is the practical choice, not the perfect one.
Both Kinds Are Important
It would be easy to decide conductors are the useful ones and insulators are the leftovers. That is wrong, and the proof is in your hand. Wire cores, switch contacts and the pins of plugs and sockets are all conducting metal, because something has to carry the current.
What the Covering Does
And a sleeve of rubber, plastic or ceramic goes around every wire, plug body and switch, so that no shock can reach a person. So one appliance needs both: a conductor to carry the current where it is wanted, and an insulator to stop it going anywhere else, including into you.
The Reason This Is Serious
Here is the fact that makes the covering matter. A human body conducts electricity as well. Current that gets through a person can do them serious harm, or kill. The plastic on a wire is not there to keep the wire tidy. It is there because the material underneath would conduct current straight into whoever touched it.
Handling Appliances
So treat every electrical appliance with care. Never touch switches or appliances with wet hands, because water makes matters worse, and never poke anything into a socket. The rule from the start of the chapter still holds for your experiments: cells and batteries only.
Looking Back at the Chapter
You began with a torch and a switch that seemed to make light appear. You now know what was inside it. Cells with two terminals make a battery. A lamp holds a filament between two terminals. A complete path runs round, and the switch opens and closes it. Metal carries the current, and plastic keeps it where it belongs.
Concepts and terms
A material current moves through without difficulty, also called a good conductor of electricity. Metals behave this way, which is why wire cores, switch contacts and plug pins are all made of metal.
A material current cannot get through at all, also called a poor conductor. Plastic, rubber and ceramic all behave this way, and each is sleeved around wires, plug bodies and switches.
Silver heads the list, with copper and gold just after it, but copper is what wires are mainly made from. It costs comparatively less and its supply is abundant, so it is the practical choice rather than the very best one.
A cell and a lamp joined with two free wire ends. Touching both ends to an object completes the circuit only if the object conducts, so the lamp glowing is the test result.
A human body conducts electricity as well. Current passing through it may cause severe injury or even death, which is why appliances must be handled with care and switches never touched with wet hands.
Watch out!
Myth or fact?
Real world: the extension lead with a cracked sleeve
Scenario:
Zoya notices that the extension lead behind the television has a split in its plastic sleeve, with a little bare copper showing through. Her younger brother says it does not matter because the wire still works and the lamp on the shelf lights perfectly well. Zoya disagrees and tells their father, who unplugs the lead at the socket and puts it aside to be replaced. Later in class she tests samples of copper wire and of plastic sleeving with a cell-and-lamp tester. The copper lights the lamp every time. The plastic never does, however firmly she presses the wires against it.
Analysis:
Her brother is right that the lead still works. He is wrong about what matters. The copper inside is a conductor, which is why the lamp lights. The plastic around it is an insulator, which is why nobody is hurt while it is intact. Her tester results show both facts directly. The danger comes from the third fact in this topic. A human body conducts as well as well. Where the sleeve is gone, current has a path out of the copper and into whoever touches it. Current through a body can injure a person badly, or kill them. Working is not the same as being safe. Unplugging it at the socket was the right response.
Activity:
Look at three electrical items in your home without touching any socket, and for each one write down one part made of a conductor and one part made of an insulator. Then write a sentence saying what would go wrong if the insulating part were missing.
Try it at home
Title:
Sort a drawer into two groups
Safety:
Use only a cell or battery, never a wall socket. Keep the two bare wire ends from touching each other while you test.
What You Notice:
Every object that lit the lamp turns out to be metal, apart from pencil lead. So the behaviour follows the material and not the shape, which is why a flat coin and a thin needle give the same answer.
Did you know
Both silver and gold carry current better than copper does, yet wires are made of copper because it is cheaper and plentiful.
A human body conducts as well of electricity, which is why a shock can cause severe injury or even death.
Ceramics are insulators, which is why you see ceramic parts on electric poles holding the wires away from the metal.
Memory tricks
Metal carries, plastic protects: the conductor does the work and the insulator keeps you safe.
Silver, copper, gold conduct best, but copper is cheap and plentiful, so copper wins.
Practice MCQs with answers (25)
- A an insulator
- B a terminal
- C a conductor
- D a filament
Show answer
Why:
Such a material is a conductor, or good conductor, of electricity.
- A I and II only
- B II and III only
- C I and III only
- D I, II and III
Show answer
Why:
I and III are correct. II is not: rubber is an insulator, so the lamp stays dark. Pencil lead surprises most people by lighting it.
- A Plastics
- B Ceramics
- C Wax and paper
- D Metals
Show answer
Why:
Metals are conductors, which is why wires are made of them.
- A A metal spoon
- B A rubber eraser
- C An iron key
- D Aluminium foil
Show answer
Why:
Rubber is an insulator, so current cannot pass through it. The other three are metal.
- A iron, tin and lead
- B plastic, rubber and glass
- C copper, wood and wax
- D silver, then copper and gold
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Why:
Silver first, then copper and gold conduct better than other metals.
- A costs comparatively less and is abundant
- B conducts better than any other metal
- C is the only metal that conducts at all
- D takes a plastic covering more easily
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Why:
Silver and gold conduct slightly better, but copper is cheap and plentiful, so it is practical.
- A her lamp has fused
- B both her cells are flat
- C plastic is an insulator
- D her wires are too short
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Why:
A dark lamp here means current could not pass, so the plastic is an insulator.
- A They would make the cells run down much faster
- B They would melt the plastic covering of the wire
- C They would reverse the direction of the current
- D They would complete the circuit without the object
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Why:
Touching wires give a false yes, because the lamp would glow whatever the object was.
- A Count up the number of cells that are fitted in the holder
- B Touch the two free ends together and look for a glow
- C Hold the lamp up to the light and inspect it
- D Touch one free end to a metal object only
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Why:
Touching the ends together completes the circuit, so a glow shows the rest of it is sound.
- A Wire cores, switch contacts and plug pins
- B The plug tops and the plastic covering of wires
- C The ceramic fittings up on an electric pole
- D The rubber grip on a pair of pliers
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Why:
Conductors are used wherever current must be carried.
- A make the wires easier to roll up neatly
- B stop the metal inside from rusting away
- C help the current flow more quickly
- D protect people from electric shocks
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Why:
The covering is an insulator and it keeps the current from reaching anyone who touches the wire.
- A The body is an insulator, so current builds up on it
- B The body stores electricity for several hours
- C The body is a conductor, so current can pass through it
- D The body conducts only when it is completely dry
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Why:
Because we conduct, current can pass through us and cause severe injury or death.
- A severe injury or even death
- B a mild and harmless tingle
- C no effect of any kind at all
- D the skin to become an insulator
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Why:
The chapter is explicit that it can do serious harm, or kill.
- A Conductors are the useful ones and insulators are simply not
- B Both are important and one appliance often needs both
- C Insulators are useful and conductors are not
- D Neither is needed if the current is small
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Why:
The current needs something to carry it and something to stop it escaping.
- A is a conductor and carries current to the pole
- B is cheaper than any metal available
- C becomes a conductor when it rains heavily
- D is an insulator and keeps current out of the pole
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Why:
Being an insulator, ceramic holds the wire up without letting current pass into the pole.
- A the cork and the coin only
- B all three of the objects
- C the needle and the coin only
- D none of the three objects
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Why:
The needle and the coin are metal, so they conduct. Cork does not.
- A good
- B poor
- C metal
- D complete
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Why:
An insulator is also described as a poor conductor of electricity.
- A False, it is simply made of an insulator
- B False, unless the object is made of metal
- C True, every object should light the lamp
- D True, which is why the test is repeated
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Why:
A dark lamp is a genuine result. It tells you the material is an insulator.
- A Every single object of the same size behaves alike
- B Only the weight of an object is what decides it in the end
- C The shape of an object is really what matters the most
- D A spoon and a coin of the same metal both conduct
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Why:
Objects of very different shapes behave the same way if they are made of the same material.
- A an insulator of electricity
- B a conductor of electricity
- C the same material as plastic
- D unable to carry any current
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Why:
A glowing lamp means current passed through, so pencil lead conducts.
- A Both true, and the reason explains the assertion
- B Both true, but the reason does not explain it
- C The assertion is true and the reason is false
- D The assertion is false and the reason is true
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Why:
Both are true, and the reason is the whole purpose of the covering. The metal carries the current and the covering keeps it away from you.
- A test every appliance with a tester first
- B hold appliances only by their metal parts
- C use appliances only in a dark room
- D never touch switches with wet hands
Show answer
Why:
Wet hands make a shock far more likely, so switches must be touched only with dry hands.
- A It would be far too expensive for the purpose
- B It does not conduct at all well over long distances
- C It cannot be covered over with any insulator
- D It stops conducting the moment it becomes warm
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Why:
Cost and availability decide the choice, and copper is the practical compromise.
- A True, but only in very large appliances
- B True, metal carries the current and plastic contains it
- C False, because an appliance uses just one kind of material
- D False, insulators are used only outdoors
Show answer
Why:
The two do different jobs, so most appliances need both at once.
- A Only metals are of any use in electricity
- B Insulators make current flow more smoothly
- C Metal carries the current and the covering keeps it there
- D The very best conductor is always the one that gets chosen
Show answer
Why:
Conductors carry the current and insulators confine it, and both jobs are needed.
Important short questions (7)
A conductor is a material current moves through without difficulty. An insulator is a material through which current cannot pass.
The top three conductors are silver first, then copper and gold. Copper is the one mainly used for making wires.
Copper costs comparatively less than silver and its supply is abundant. It is the practical choice even though silver conducts slightly better.
The tester has a gap in its circuit. Touching both free ends to an object completes the circuit only if the object conducts, so the lamp glowing means current passed through it.
If they touch, they complete the circuit by themselves. The lamp would then glow whatever object was being tested, so every result would be a false yes.
The wires and switch inside are conductors, because something must carry the current. The covering on the wire is an insulator, so the current cannot reach anyone touching it.
Because current can therefore pass through a person and is able to maim or to kill. That is the reason wires and switches are covered with insulators.
Long answer questions (3)
First I would connect a cell and a lamp with wires, leaving two ends free. Touching those ends together checks that the lamp glows. That proves the rest of the circuit is sound. Then I would touch the two free ends to the two ends of each object in turn. I would keep the wires from touching each other. For each one I would record the object, its material, whether the lamp glowed and my conclusion. The lamp would glow for the spoon, the coin, the key, the needle and the foil. It would stay dark for the cork, the eraser, the bangle, the block and the paper. Everything in the first group is metal. So my results would show that metals are conductors, while plastic, rubber, wood, glass and paper are insulators.
A wire has to carry current from one place to another. Only a conductor can do that, so the inside of the wire is metal, usually copper. If that were the whole wire it would be dangerous. A human body conducts as well. Current could pass out of the metal into anyone who touched it, and current that gets through a body can do serious harm, or kill. So the metal is covered with plastic or rubber. Those are insulators and will not let current through. The covering confines the current to the path we want. Neither part could do the job alone. Without the metal nothing is carried, and without the covering nothing is safe.
Being the best conductor is only one thing that decides which material gets used. Silver heads the list, with copper and gold just after it. All three would carry current well. The difference is cost and supply. Copper costs comparatively less and its supply is abundant. Wiring a whole house in silver or gold would be hugely expensive for a very small gain. A house needs a great length of wire, so a small difference in price per metre adds up enormously. Copper is therefore the practical choice. It is good enough to do the job, cheap enough to use everywhere, and plentiful enough that supply is no problem.
Flashcards (17)
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- Conductor or insulator?
- Fact or myth?
- Testing a material fairly
- Material and behaviour
- Conductor sprint
Class 7 Science Chapter 3 — frequently asked questions
The positive terminal is the small protruding metal cap. The negative terminal is the flat metal disc at the other end.
Two or more cells set in a line, each plus end resting against the minus end that follows it.
Connecting more cells supplies energy for a longer time, or supplies more energy, or does both at once.
The filament is the thin wire fixed inside the glass bulb. Current passing through it makes it hot enough to glow, and that glow is the light the lamp gives.
The metal tip at the bottom of the lamp is one terminal. The metal case around it is the other.
It means the filament inside has broken. The break leaves a gap so current cannot pass through, and the lamp will not glow however good the cells are.
It is an arrangement that provides a complete path for current to flow. One terminal of the lamp is joined to one terminal of the cell, and the other to the other.
From the positive terminal of the electric cell to its negative terminal.
A closed circuit has an unbroken path, so current flows and the lamp glows. An open circuit has a gap in it, so no current flows and the lamp stays dark.
A conductor is a material current moves through without difficulty. An insulator is a material through which current cannot pass.
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