Question Bank — Electricity

Section A — MCQs (10 questions, 1 mark each)

1. The SI unit of electric resistivity is:
(a) ohm
(b) ohm-metre
(c) ampere
(d) volt-metre

2. In a metallic conductor, the direction of conventional electric current is taken as:
(a) same as the direction of flow of electrons
(b) opposite to the direction of flow of electrons
(c) perpendicular to the flow of electrons
(d) independent of electron flow

3. If the potential difference across a conductor is doubled while keeping its resistance constant, the current through it becomes:
(a) half
(b) double
(c) four times
(d) remains the same

4. Which of the following is the correct expression for electric power?
(a) P = I²R
(b) P = V/I
(c) P = I/V
(d) P = V²I

5. Assertion-Reason

Assertion (A): The resistance of a conductor is directly proportional to its length and inversely proportional to its area of cross-section.
Reason (R): Resistivity is a characteristic property of the material of the conductor and depends on its length and area of cross-section.

6. Assertion-Reason

Assertion (A): In a series combination of resistors, the total resistance is always greater than the largest individual resistance.
Reason (R): In series, the same current flows through each resistor and the equivalent resistance is the sum of individual resistances.

7. An ammeter is always connected in a circuit:
(a) in parallel with the component
(b) in series with the component
(c) either in series or parallel
(d) only across the battery

8. The commercial unit of electrical energy is:
(a) watt
(b) joule
(c) kilowatt-hour
(d) ampere-hour

9. Tungsten is used for the filament of an electric bulb because:
(a) it has low resistivity
(b) it has a high melting point and does not oxidise easily
(c) it is a good insulator
(d) it has low resistance

10. When three resistors are connected in parallel, the reciprocal of the equivalent resistance is equal to:
(a) the sum of the individual resistances
(b) the sum of the reciprocals of the individual resistances
(c) the product of the individual resistances
(d) the difference of the individual resistances

Section B — Very Short Answer (6 questions, 2 marks each)

11. Define the term ‘electric current’. State its SI unit and write the relation between current, charge and time.

12. What is meant by saying that the potential difference between two points is 1 V? How is potential difference related to the work done in moving a charge?

13. State Ohm’s law. Write the mathematical expression for it and name the physical quantity represented by the constant of proportionality.

14. A wire of given material is stretched to double its length. What happens to its resistance? Give reason.

15. Why is a voltmeter always connected in parallel across the points between which potential difference is to be measured?

16. Distinguish between resistance and resistivity. Give one example where resistivity is useful in selecting a material.

Section C — Short Answer (5 questions, 3 marks each)

17. On what factors does the resistance of a metallic conductor depend? Explain with the help of the formula for resistance.

18. Derive the expression for the equivalent resistance of three resistors connected in series.

19. Three resistors of 5 Ω, 10 Ω and 15 Ω are connected in parallel to a 12 V battery. Calculate the total current drawn from the battery.

20. Explain, with reason, why the cord of an electric heater does not glow while the heating element glows brightly when connected to the same source.

21. A current of 2 A flows through a conductor when a potential difference of 10 V is applied across its ends. If the potential difference is increased to 30 V, find the new current. What assumption is made in your calculation?

Section D — Long Answer (3 questions, 5 marks each)

22. (a) State Joule’s law of heating and write its mathematical form.
(b) An electric heater rated 2 kW is used for 3 hours daily. Calculate the energy consumed in 30 days in kWh and the cost if 1 kWh costs ₹4.
(c) Why are fuses rated for specific currents used in household circuits?

23. Draw a labelled circuit diagram showing a battery, ammeter, voltmeter and three resistors connected in parallel. Explain how you would experimentally verify that the total current is the sum of currents through each resistor. State the observations and conclusion.

24. A wire of resistance 10 Ω is bent into a circle. Find its equivalent resistance between two diametrically opposite points. If the same wire is now cut into four equal parts and these parts are connected in parallel, calculate the new equivalent resistance. Show all steps.

Section E — Case/Source-Based (2 questions, 4 marks each)

25. Case: In many households, multiple electrical appliances such as bulbs, fans and heaters are used simultaneously. These appliances are designed to operate at the same voltage but draw different currents. Electricians prefer to connect them in a specific manner so that each appliance gets the required current independently and if one appliance fails, others continue to work.

(a) Which type of combination (series or parallel) is used for connecting household appliances? Why?
(b) If two appliances of resistances 20 Ω and 30 Ω are connected across a 220 V supply in the preferred combination, calculate the total current drawn.
(c) What happens to the total resistance of the circuit when more appliances are added in this combination?
(d) Why is a fuse placed in series with the main supply line?

26. Case: An electric bulb has a filament made of a metal with very high melting point. When connected to a power source, the filament becomes white hot and emits light, while the connecting wires remain relatively cool.

(a) Name the metal commonly used for the filament and state one property that makes it suitable.
(b) Why is the bulb filled with inert gases like argon or nitrogen?
(c) If the resistance of the filament is 440 Ω and it is connected to 220 V, calculate the power consumed by the bulb.
(d) What would happen if the filament material had a low melting point?

Section F — HOTS and Application (4 questions, 3 marks each)

27. Predict-and-justify: What would happen to the brightness of a bulb connected in series with a variable resistor (rheostat) if the resistance of the rheostat is gradually increased? Justify your answer using the relevant law.

28. Analyse anomalous observation: In an experiment to verify Ohm’s law using a nichrome wire, the student observes that the V–I graph is a straight line passing through the origin only up to a certain current and then deviates. Analyse the possible reason for this deviation.

29. Apply to unfamiliar real-life situation: In a remote village, the power supply line has high resistance due to the use of thin aluminium wires over a long distance. Suggest two practical ways to reduce power loss during transmission and explain how each method works.

30. Compare two situations: Two identical bulbs are connected to the same battery. In situation 1 they are joined in series; in situation 2 they are joined in parallel. In which situation will the bulbs glow brighter? Justify by comparing the current and power in both cases.