Electricity — Class 10 NCERT: 30 flashcards
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1. Define electric current.
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Electric current is the rate of flow of electric charge: $I = Q/t$.
2. What is the SI unit of electric current?
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The ampere (A), where $1\ \text{A} = 1\ \text{C/s}$.
3. How much charge does one coulomb represent in terms of electrons?
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One coulomb is the charge on about $6.25 \times 10^{18}$ electrons.
4. How is an ammeter connected in a circuit and why?
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An ammeter is connected in series so that the same current flows through it as through the component being measured.
5. Define potential difference between two points.
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Potential difference is the work done to move a unit charge from one point to the other: $V = W/Q$.
6. What is the SI unit of potential difference?
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The volt (V), where $1\ \text{V} = 1\ \text{J/C}$.
7. How is a voltmeter connected and why?
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A voltmeter is connected in parallel across the two points whose potential difference is to be measured.
8. State Ohm's law.
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For a conductor at constant temperature, $V = IR$, i.e. current is directly proportional to potential difference.
9. What is resistance and what is its SI unit?
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Resistance is the property of a conductor that opposes the flow of charge; its SI unit is the ohm ($\Omega$).
10. How does the resistance of a wire depend on its length and area of cross-section?
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Resistance increases with length ($R \propto l$) and decreases with area of cross-section ($R \propto 1/A$).
11. What is resistivity and on what does it depend?
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Resistivity $\rho$ is a property of the material of the conductor, not its shape; it depends on the material and temperature.
12. Write the formula for equivalent resistance in series.
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$R_s = R_1 + R_2 + R_3 + \dots$
13. Write the formula for equivalent resistance in parallel.
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$\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \dots$
14. In a series circuit, what is the same and what divides?
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The current is the same through each resistor; the voltage divides across them.
15. In a parallel circuit, what is the same and what divides?
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The voltage is the same across each resistor; the current divides among the branches.
16. How does the equivalent resistance in series compare with the largest resistor?
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The equivalent resistance in series is always greater than the largest individual resistor.
17. How does the equivalent resistance in parallel compare with the smallest resistor?
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The equivalent resistance in parallel is always less than the smallest individual resistor.
18. Write the three forms of the electrical power formula.
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$P = VI = I^2R = \frac{V^2}{R}$, measured in watts (W).
19. What is the heating effect of current and where is it used?
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When current flows through a resistor, electrical energy converts to heat; it is used in heaters, irons, toasters and fuses.
20. What is a fuse and how does it work?
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A fuse is a safety device with a thin wire that melts when current exceeds a safe value, breaking the circuit.
21. Why are household appliances connected in parallel?
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So that each appliance gets the full supply voltage and can be switched on or off independently.
22. What is a rheostat?
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A rheostat is a variable resistor used to control the current in a circuit.
23. What is the commercial unit of electrical energy?
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The kilowatt-hour (kWh), which is the energy consumed by a 1 kW appliance in 1 hour.
24. Is current 'used up' in a circuit? Explain.
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No, current is not used up; the same current enters and leaves a resistor. What is converted is energy, into heat or light.
25. What is a resistor and why is it used?
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A resistor is a component with a known fixed resistance, used to control current in a circuit.
26. State the relation between current and charge.
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$I = Q/t$, where $Q$ is charge in coulombs and $t$ is time in seconds.
27. What is the relation between potential difference, work and charge?
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$V = W/Q$, where $W$ is work done in joules and $Q$ is charge in coulombs.
28. How does temperature affect the resistance of a metal wire?
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For metals, resistance usually increases with temperature.
29. What is the formula connecting resistance, resistivity, length and area?
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$R = \rho \, l / A$, where $\rho$ is resistivity, $l$ is length and $A$ is area of cross-section.
30. Why do old fairy lights all go dark when one bulb fails?
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Because they are connected in series, so breaking one path stops the current through the whole circuit.
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