Class 8 Science Chapter 4: Electricity: Magnetic and Heating Effects

A Circuit With No Lamp

Suppose you build a circuit out of one cell, a switch and a length of wire, and you have no lamp to put in it. Is there any way to tell whether current is flowing? There is, and finding it out is how this whole chapter starts.

What the Compass Does

Lay a magnetic compass on the bench and run the wire over it, close but not touching. Let the needle settle on north. Now close the switch and watch. The needle swings away from the direction it was pointing, and it does so the moment the current starts.

And When You Switch Off

Open the switch and the needle swings back to north. Work the switch a few more times and the same thing happens every time: aside while the current flows, back when it stops. Nothing has been added to the circuit and nothing has been taken away.

Why This Is Surprising

A compass needle has only ever done this near a magnet. There is no magnet anywhere in this circuit. A cell, a switch and a piece of wire are the whole of it, and yet the needle behaves as though a magnet had been brought close.

The Needle Is a Magnet

That is the first piece of the explanation. A compass needle is itself a tiny magnet, which is why it turns whenever anything magnetic comes near it. You met that in Grade 6, in the chapter on exploring magnets.

It Works Through Things

The second piece is that a magnetic effect passes straight through material that is not itself magnetic. The compass in the activity sits under a piece of cardboard and never touches the wire, and the effect reaches it anyway. Air, card, wood and plastic make no difference at all.

What a Magnetic Field Is

The region around a magnet, or around a wire carrying a current, where the magnetic effect can be felt is called a magnetic field. You know a field is there because something magnetic put into it moves. The compass is how you find the field; it is not the field itself.

The Rule in One Sentence

Here is the finding. When an electric current flows through a conductor, it produces a magnetic field around that conductor. This is called the magnetic effect of electric current, and it is one of the most useful facts in all of physics.

The Field Needs the Current

And the other half of the rule matters just as much: stop the current and the field goes with it. That is what the needle returning to north tells you. The wire does not become a magnet and stay one. It carries a field for exactly as long as it carries a current.

No Magnet Was Involved

It is worth saying plainly, because it is the step most people skip. Nothing in the circuit was magnetic to begin with. The current made the field. Two subjects that had looked completely separate, electricity and magnetism, turn out to be linked.

Oersted, 1820

Hans Christian Oersted, a professor in Denmark, made this discovery in 1820. The story is that he was giving a demonstration and a magnetic compass happened to be lying on the bench. He noticed that its needle twitched whenever the circuit was closed or opened.

What He Did Next

What he did then is the part worth copying. He did not announce it. He investigated until he was certain that a current really does produce a magnetic field, and only then published what he had found.

Why Repeating Mattered

Other scientists read the paper and repeated the experiment to see whether they got the same result. They did, and they went on to study the connection further. A finding that only one person can produce is not yet knowledge, and Oersted’s held up precisely because other people could reproduce it.

Where You Meet It

This one effect is behind a surprising amount of equipment. Electromagnets, which the next topic builds, electric bells, motors, fans and loudspeakers all depend on a current producing a magnetic field. Anything that has to turn, pull, click or vibrate on command is likely to be using it.

A Detector You Can Build

And it answers the question this topic opened with. A compass laid under a wire is a current detector. It is not part of the circuit, it takes nothing from it, and it tells you whether current is flowing. Be careful about one thing, though. A needle that is already lying along the field has nowhere to swing, so a still needle may mean no current or may just mean a badly placed compass.

What To Carry Forward

Three sentences. A current produces a magnetic field around the conductor carrying it. The field lasts exactly as long as the current does. And a compass is how you see it, because the needle is a small magnet and the field turns it.

Scenario:

Aakarsh builds a circuit with one cell, a switch and a length of wire, and has no lamp to put in it. He wants to know whether current is flowing at all. Mohini lays a magnetic compass on the bench under the middle of the wire and tells him to work the switch while she watches. The needle swings aside each time he closes the circuit and settles back to north each time he opens it. Their neighbour tries the same thing and reports that his needle never moves, so his circuit must be broken.

Analysis:

The compass is doing the job the lamp would have done. It reports the current without being part of the circuit. The neighbour has drawn a conclusion his test does not support. A needle already lying along the field has nowhere to turn to. So a still needle has two explanations, and he has considered only one of them.

Activity:

Build the circuit with a compass instead of a lamp and find a position where the needle swings furthest.

Title:

Try it at home: a current detector

Safety:

Touch the wire to the cell for a second at a time. A wire left across a cell warms up and runs the cell flat.

What You Notice:

The swing is largest when the wire lies along the needle, and smallest when it crosses it. So the field has a direction, and is not just a push.

A compass makes a current detector that takes nothing from the circuit it is testing, which is why it works even when you have no lamp.

Oersted was not looking for the link at all; a compass happened to be lying on the bench during a demonstration about something else.

The same effect that moves the needle also drives every fan, motor and loudspeaker you have ever used.

CURRENT ON, FIELD ON — The field appears with the current and goes with it. The wire never keeps it.

THE NEEDLE IS THE DETECTOR — It is a magnet, so it turns in a field. It is not part of the circuit at all.

A Model at the Exhibition

At a school science exhibition, Mohini and Aakarsh stopped at a working model built by a senior. Where a crane would have a hook, this one had an iron nail wrapped in wire and joined to a battery. Close the circuit and the nail picked up a handful of iron clips. Open it and they dropped off again.

A Nail That Picks Up Clips

What puzzled them was that there was no magnet in the model at all. They knew from Grade 6 that iron is a magnetic material and that a magnet attracts it. Here was iron being attracted, and nothing but a circuit to do the attracting.

Wind the Wire Into a Coil

The previous topic showed that a straight wire carrying a current has a magnetic field around it. The effect is real but weak. Wind the same wire into a coil, fifty turns of it round a paper cylinder, and the fields of all those turns work together instead of separately.

The Coil Alone Is a Magnet

Put a compass at each end of the coil and connect it to a cell. Both needles deflect. Disconnect the cell and both return. The coil on its own, with nothing inside it, is behaving exactly like a magnet, which is more than a single straight wire managed.

Now Put Iron Inside It

Slide an iron nail into the paper cylinder and repeat. The deflections are much larger than before, and now the ends of the nail will hold a chain of paper clips. Nothing about the coil or the cell has changed. The iron core is the whole difference.

What To Call It

A coil that carries a current and acts as a magnet is what the word electromagnet names. Most of the ones built for real work have iron inside them, for the reason you have just seen: it turns the same coil and the same current into a considerably stronger magnet.

Switch It Off

Stop the current and the coil loses its magnetic effect at once. The clips fall. This is the feature, not a limitation. An ordinary bar magnet cannot be told to let go, and an electromagnet can.

Does It Have Poles?

A bar magnet has a north and a south. Does a coil? Label the two ends of your electromagnet A and B, bring a compass up to end A and see which pole of the needle is attracted to it.

Finding Which End Is Which

Unlike poles attract, as you learnt in Grade 6. So if the north pole of the compass is drawn towards end A, then end A must be a south pole. Test end B the same way and you will find it is the opposite. An electromagnet has two poles, exactly like a magnet.

Making It Stronger: More Current

One cell supplies only a small current, so the field is weak: the needle barely moves and the coil holds two or three clips. Use a battery of two cells, or four, and the current is larger. The field is stronger, the deflection is bigger and the coil holds many more clips.

Making It Stronger: More Turns

Keep the same cell and wind more turns on the same cylinder instead. That strengthens it too. So there are three levers: the current, the number of turns, and whether there is an iron core. You can pull one or all of them.

Reversing the Poles

Now swap the two connections to the cell so the current runs the other way round the coil. The strength does not change, but the poles trade places: the end that was a north is now a south. That is something no bar magnet will ever do for you.

Lifting Electromagnets

A lifting electromagnet is a very strong one, hung where a crane would otherwise carry a hook. The operator works a switch. Current on, and the magnet picks up iron and steel; current off, the field vanishes and the load is released wherever it needs to go.

Why a Crane Prefers One

Think about what a hook would need: somebody at the bottom to attach the load and somebody to unhook it. A switch does both jobs from the cab, which is why scrap yards and factories hang lifting electromagnets on their cranes: they shift, raise and separate heavy metal fast.

The Earth Does It Too

One last thought, on the largest possible scale. A freely hung magnet points north because the Earth itself behaves like an enormous magnet. Deep in the core, liquid iron moves, and that movement produces electric currents, which produce a magnetic field. Migrating birds, fish and other animals steer by it, and it shields us from harmful particles arriving from space.

What To Carry Forward

A coil carrying a current is a magnet. An iron core makes it a much stronger one. It has two poles, which swap when the current reverses, and its strength rises with the current and with the number of turns. Above all, it starts and stops with the switch.

Scenario:

A scrap yard replaces the hook on its crane with a lifting electromagnet. The operator closes a switch, the magnet picks up a load of steel offcuts, the crane swings across and the switch opens, dropping the load in the right bin. On a busy afternoon the loads get smaller and smaller, and the operator finds the coil warm to the touch. A new apprentice suggests fitting a bigger iron core. The foreman asks whether anything has been left switched on.

Analysis:

Two separate things are being confused: how strong the magnet can be, and whether anything is driving it. A bigger core raises the ceiling, but the loads are falling off because the supply is weakening, and the warm coil says where the energy has been going. The foreman is asking the right question.

Activity:

Build a coil round a nail, count the clips it lifts with one cell, then with two, and write both numbers down.

Title:

Try it at home: how many clips?

Safety:

A few seconds at a time, and let the wire cool between tries. A coil left across a cell gets hot and flattens the cell.

What You Notice:

Both changes help, and they add. The jump from one cell to two is usually larger than the jump from twenty turns to fifty, which is why a crane runs on a big supply rather than on an enormous coil.

An empty coil with nothing inside it already deflects a compass at each end, so the iron core makes an existing magnet stronger rather than making a magnet at all.

Reversing the current swaps the poles of an electromagnet without changing its strength, which no permanent magnet will ever do.

The Earth is itself a giant electromagnet: liquid iron moving in the core produces currents, and those produce the field every compass points along.

CURRENT, TURNS, CORE — The three levers on the strength of an electromagnet. Reversing the current is not one of them.

SWITCH OFF, LET GO — The one thing a permanent magnet can never do, and the reason a crane uses an electromagnet.

A Wire You Can Feel

While winding the coil in the last topic you may have noticed something on the side: the ends of the wire got warm. That was not a fault in the equipment. It is the second thing a current does, and this topic is about it.

Before the Switch

Tie a short length of nichrome wire between two nails about five centimetres apart on a card, and wire it to a cell through a switch left open. Touch the wire. It feels like any other piece of wire, which is to say cool.

Thirty Seconds Later

Close the switch for about thirty seconds, open it again, and touch the wire for a moment only. It is noticeably warm. Do it twice more and the same thing happens, so the warmth is not a trick of the fingers.

Why That Happens

A current does not pass through a wire entirely freely. The material opposes the flow to some extent, and the name for that opposition is resistance. Where there is resistance, some of the electrical energy arriving at the wire leaves it as heat instead of travelling on.

Resistance, Plainly

It helps to be clear about the word. Resistance is not a defect in a badly made wire. Every conductor has some, because every conductor is made of something, and it is exactly what the heat in this activity comes out of.

Not All Conductors Alike

Different conductors oppose the current to different degrees. Take two wires of the same thickness and the same length, one of copper and one of nichrome, and the nichrome offers the greater opposition by a long way.

Why Nichrome Was Chosen

That is why the activity specifies nichrome. For the same current it produces more heat than copper would, so the effect is large enough to feel after half a minute rather than after an hour. It is the same reason a heater element is made of it and the wires feeding the heater are not.

The Rule in One Sentence

Here is the finding. Any conductor carrying a current warms up, and the name for that is the heating effect. A copper connecting wire warms as well; it simply warms far less than nichrome does.

Two Cells Instead of One

Now repeat the activity with a battery of two cells, under a teacher’s eye. For the same half minute the wire comes out hotter. A larger current makes more heat, which is the first of the things that matter.

Four More Things That Matter

The current is not the whole story, though. The heat produced in a wire also depends on what the wire is made of, how thick it is, how long it is, and how long the current runs. Change any one of those and you change the result.

The Heating Element

Every appliance that uses this effect holds a short rod, or a wound coil, put there for the purpose, and that part is called the heating element. In some appliances you can see it, and when it is working hard it glows red.

A Lamp You Already Knew

You met one of these in Grade 7 without naming it. The filament of an incandescent lamp is heated by the current until it gives out light, and that is why the torch bulb in the earlier activity felt warm afterwards.

Where It Is Wanted

The list of appliances built on this one effect is long. An electric iron, a kettle, a room heater, a stove, an immersion rod for heating water and a hair dryer all work the same way. Industry uses it on a much larger scale: a high-temperature electric furnace melts scrap steel so that it can be made into usable steel again.

Where It Is Not

The same effect causes trouble where nobody asked for it. Long wires carrying power across the country warm up, and that warmth is energy lost on the way. An appliance or a connection that overheats can damage a plug or a socket, melt the plastic parts, and in the worst case start a fire.

Rated for the Current

Two sensible precautions follow. Household circuits carry safety devices whose job is to cut in before an overheating circuit becomes dangerous. And the wires, plugs and sockets used on a switchboard have to be rated for the current they will actually carry, because an undersized one is where the overheating starts.

What To Carry Forward

A current warms every conductor it flows through, because the conductor resists it and the resistance turns electrical energy into heat. How much heat depends on the current, the material, the thickness, the length and the time. The effect boils your kettle and it melts your plug, and the only difference between those two is whether you wanted it.

Scenario:

A class is given two cards. Each has a cell, a switch and a wire tied between two nails five centimetres apart. On the first card the wire is nichrome, thin and short; on the second it is copper of the same thickness and length. Both switches are closed for thirty seconds. The nichrome wire is clearly warm when touched briefly; the copper is barely changed. One group writes that copper does not have the heating effect. Another group repeats the nichrome card with two cells and finds it hotter still, and writes that the heat depends on the current alone.

Analysis:

Both groups have over-read a real observation. The first has turned a difference of degree into a difference of kind, and the second has taken the one variable it changed to be the only one that matters. The chapter names four more.

Activity:

Set up one card and change exactly one thing at a time, writing down what you changed and what you felt.

Title:

Try it at home: find the warm spot

Safety:

A moment only, with the back of a finger, and never on a mains appliance. Only a battery torch for this.

What You Notice:

The warmth is strongest at the bulb, because the filament resists far more than the wires feeding it. Your test changed the time and nothing else, which is the only way to say what the time did.

A copper connecting wire warms up as well; it simply warms far less than nichrome of the same thickness and length.

A high-temperature electric furnace uses this effect to melt scrap steel so that it can be made into usable steel again.

The torch bulb that felt warm in an earlier activity was showing you this effect before the chapter had named it.

RESIST, AND IT HEATS — The opposition is not a fault. It is where the heat comes from, in a kettle and in a plug alike.

CURRENT, MATERIAL, THICK, LONG, TIME — The five things the heat depends on. Change one at a time or you have measured nothing.

What Is Inside a Cell?

So far the cell has been the thing you connect to and not something you look at. It lit a lamp, it made a nail into a magnet and it warmed a wire. Now turn the question round. What is inside it, and where does the electricity actually come from?

The Oldest Design

Start with one of the earliest electric cells ever built, the Voltaic cell. It is also called a Galvanic cell, and the two names come from two Italian scientists you will meet further down this page.

Three Parts, and No More

Picture a container of glass or plastic. Standing in it are two rods, and between them is a liquid that they are partly dipped into. That is the whole cell: two rods, a liquid and something to hold it.

The Electrodes

The two rods are called the electrodes, and the important thing about them is that they are made of different metals. Copper with iron will do it. Zinc with copper is another pair. Two rods of the same metal will not work at all, however carefully you set them up.

The Electrolyte

The liquid is called the electrolyte, and it is usually a weak acid or a solution of salt. Lemon juice serves, and so does salty water. What will not serve is pure water, which does not carry the charge well enough to be of any use.

Where the Current Comes From

Now the part that matters. A chemical reaction takes place between the rods and the liquid, and that reaction is what produces the electricity. It does not come out of the metals on their own, and it does not come out of the liquid on its own. It takes the pair of them together.

Which Way It Flows

Connect the cell into a circuit and current flows out of the positive terminal, round the circuit and back in at the negative terminal. Put a lamp in the way and it lights, which is how you know the reaction is doing its job.

Why a Cell Goes Dead

The reaction uses up the chemicals as it goes. Sooner or later they are spent, the reaction stops and the cell can supply no more electricity at all. A cell in that state is called dead, and resting it will not bring it back, because there is nothing left to recover.

A Frog, and a Disagreement

The story of how all this was worked out is worth knowing, because it is really a story about testing an idea. In the late 1700s Luigi Galvani noticed that the leg of a dead frog kicked when he touched it with iron and copper, two metals that differ.

Galvani’s Idea

It was already known that electricity could make a muscle move, so the kick itself was not the puzzle. The puzzle was where the electricity had come from, and Galvani concluded that it had come from the frog.

Volta’s Idea

Alessandro Volta disagreed. He thought the electricity came from the two metals rather than from the animal, which was a bold thing to say about an experiment somebody else had done and explained.

The Test That Settled It

And here is the move that matters. Volta did not argue; he removed the frog. He put paper soaked in salt water where the leg had been, touched it with the two metals, and still got a current. The frog had not been necessary, so the frog had not been the source. A combination of two metals and a liquid was, and that finding led straight to the first battery.

A Cell From a Lemon

You can build the same thing from fruit. Push a copper wire and an iron nail into a lemon, a short distance apart, and the lemon is a cell: the copper and the iron are the electrodes and the juice is the electrolyte. One lemon is usually too feeble to light an LED, so join five or six in a row, copper of one to nail of the next, and try the LED across the two free ends.

If the LED Stays Dark

An LED only passes current one way round, as you learnt earlier, so its longer leg has to go to the positive end. If nothing happens, swap the two legs over before you conclude that the cell has failed. That is one explanation ruled out for the price of a second attempt.

Which Pairs of Metals

Several pairs work: zinc with copper, zinc with silver, aluminium with copper, iron with copper, magnesium with copper, lead with copper. Copper tends to act as the positive electrode and zinc as the negative one, because of the chemical properties of each. You will take that further in later grades.

What To Carry Forward

A cell holds chemicals, not electricity. Two different metals in a liquid that conducts, a reaction between them, and a current for as long as the chemicals last. When they are gone the cell is dead, and the next topic asks what can be done about that.

Scenario:

Mohini sets up two jars. In the first she stands a copper strip and an iron nail in lemon juice; in the second she stands the same pair in pure water. She wires each in turn to an LED. The first jar lights the LED faintly; the second does nothing. She then tries the first jar again with two copper strips instead of the copper and the nail, and the LED stays dark. Her partner says the second jar failed because the LED was round the wrong way.

Analysis:

Three set-ups and three different reasons. That is why changing one thing at a time matters. The jar of pure water has no conducting liquid. The jar with two copper strips has no metal pair. And the partner is proposing a fourth explanation that is cheap to rule out.

Activity:

Build a lemon cell with copper and iron, then join several lemons in line and see how many it takes to light an LED.

Title:

Try it at home: a lemon cell

Safety:

Lemon juice stings cut skin and eyes. Wash your hands after, and do not eat the lemons once nails have been in them.

What You Notice:

One lemon on its own is usually not enough. Each lemon is a cell and joining them in a row adds their effect, which is exactly what a battery of cells does.

Volta settled a scientific disagreement by removing the part he doubted rather than by arguing about it, and the first battery came out of that one decision.

A lemon with a copper wire and an iron nail in it is a genuine Voltaic cell, with the juice doing the work of the electrolyte.

Copper tends to act as the positive electrode and zinc as the negative one, because of their chemical properties.

TWO METALS, ONE LIQUID — And they must be different metals. Two rods alike give no current at all.

CHEMICALS IN, CURRENT OUT — A cell stores chemicals rather than electricity. When they are spent it is dead.

A Cell You Can Carry

The previous topic left a working cell standing on a bench in an open jar of acid. It was an important discovery and it is hopeless for everyday life. You cannot put it in a torch, a clock or a pocket, and tipping it over would be a small disaster.

Why the Jar Had To Go

So the liquid had to go. What replaced it is the cell you have been using all along without looking inside. The dry cell is in more devices today than any other kind.

Dry, But Not Quite Dry

The name is slightly misleading, and the misunderstanding is worth heading off. A dry cell is called dry because its electrolyte is not a liquid but a thick moist paste. There is moisture in there. What there is not is anything that can pour out.

The Zinc Container

Look at the structure. The outside of the cell is a container made of zinc, and that container is not packaging. The zinc itself is the negative terminal of the cell, which is why the outside of a cell is one of its working parts.

The Carbon Rod

Down the middle of the cell runs a rod of carbon, and the top of it is covered with a metal cap. That capped rod is the positive terminal, and the cap is the part you press against a spring in the torch.

What Sits Between Them

Around the carbon rod sits the paste that does the work of the electrolyte. So the cell has the same three ingredients as the Voltaic cell in the last topic: two unlike materials to act as terminals, and something between them that conducts.

One Use Only

A dry cell serves once and once only. The chemicals inside it are used up as it works, exactly as in a Voltaic cell, and when they are spent there is no way to put them back. The cell has to be disposed of rather than revived.

And Then It Is Waste

Which is a real cost, and not only in money. Every dry cell ever sold has eventually become something to throw away, and that is the main reason rechargeable batteries are now used instead for most purposes.

A Battery You Charge Again

A rechargeable battery can be charged and used over and over. Nothing has to be bought and discarded each time it runs down: it goes back on charge and comes back ready.

Where You Find Them

They come in every size the job needs. Small ones run watches and phones. Larger ones run laptops, tablets and cameras. Bigger ones again run an inverter in a house or drive an electric vehicle along a road.

What That Saves

Two things at once. There is far less waste, because one battery does the work of a long stream of cells. And there is less money spent, when you count it over the years rather than over a week.

They Still Wear Out

But they do not last for ever, and this is where people get the wrong idea. After being charged and used many times a rechargeable battery slowly wears out. It still works; it simply holds less than it did when it was new.

The Sign In Your Pocket

You have probably seen this happen. A phone a year or two old needs charging more often than it did when it arrived, and the reason is not that anybody has been using it carelessly. Many charge cycles have gradually worn the battery down.

Lithium, and Where It Comes From

Today the commonest rechargeable battery is the lithium-ion battery, found in very nearly everything that takes a battery at all. It depends on particular metals, lithium and cobalt among them, and those are mined and processed in only a few parts of the world. So countries are competing to secure supplies, recycling old batteries, and looking for alternatives.

What Comes Next

Scientists are working on solid-state batteries, which would replace the liquid or paste electrolyte with a solid material. The hope is a battery that is safer, charges faster and lasts longer than the ones we have. That matters because the world is moving towards cleaner ways of generating electrical power, and a cleaner supply needs somewhere good to store it.

What To Carry Forward

A dry cell traded the liquid for a paste and became portable, and it is used once. A rechargeable battery goes back on charge instead, saving waste and money, and it wears out slowly rather than suddenly. And the question of what a battery is made from has become a question about the whole world, not just about what is in your torch.

Scenario:

Aakarsh has had his phone for two years. It now needs charging twice a day, where it used to last from morning to night. His cousin says the battery must be dead and should be thrown away. His uncle says all batteries behave like this and nothing can be done. Mohini points at the torch in the same drawer. It takes ordinary dry cells, and those have never been charged once. The two cases are not the same kind of thing at all.

Analysis:

Three claims and one useful observation. The battery is not dead but worn, which is what many charge cycles do. And Mohini has spotted the real distinction. It lies between a cell that can be charged at all and one that cannot.

Activity:

Count how many devices in your house take dry cells and how many take a rechargeable battery, and write the two numbers down.

Title:

Try it at home: count the cells

Safety:

Do not open any battery or device. A cell that has leaked should be handled by an adult.

What You Notice:

The devices that work hardest almost always have a battery you charge, and the ones that sip current slowly still take dry cells. The choice follows the work, not the price.

The outside of a torch cell is one of its working parts: the zinc container is the negative terminal rather than a wrapper round the cell.

A phone battery that needs charging twice a day is not dead but worn, because a dead one would not take a charge at all.

Lithium-ion batteries have made battery metals a matter of international competition, since only a few places mine and process them.

DRY MEANS NO SPILL — The paste inside a dry cell is moist. Dry describes what cannot pour out, not what is not there.

ZINC OUT, CARBON IN — The zinc case is the negative terminal and the capped carbon rod at the centre is the positive one.