Electricity – Short Answer Type Questions
50 Short Answer Questions — Electricity
CBSE Board Examination Focus:
- Concise, exam‑standard answers for short‑answer questions (2–4 marks).
- Topic‑wise organization covering definitions, formulas, practicals and numericals.
- Emphasis on clear steps, units and diagram pointers where relevant.
Instructions: Each question below has a focused short answer. Use the set for timed practice and quick revision before exams.
Short Answer Questions (1–50)
1. What is electric current? Give its SI unit.
Answer: Electric current is the rate of flow of charge; I = Q/t. SI unit is ampere (A).
2. Define potential difference.
Answer: Potential difference between two points is work done per unit charge to move a charge between them. Unit volt (V).
3. State Ohm's law.
Answer: For an ohmic conductor at constant temperature, the current through it is directly proportional to the potential difference across it: V = IR.
4. What is resistance? How is it related to resistivity?
Answer: Resistance R opposes current; R = V/I. For a uniform wire R = ρ(l/A), where ρ is resistivity, l length, A area.
5. Explain why resistance increases with length and decreases with cross‑sectional area.
Answer: Longer wires offer more collisions to charge carriers (more obstruction) so R ∝ l. Larger area provides more parallel paths for carriers so R ∝ 1/A.
6. Define resistivity and give its unit.
Answer: Resistivity (ρ) is intrinsic property indicating how strongly a material resists current; unit Ω·m.
7. What does the slope of a V–I graph represent for a resistor?
Answer: For V vs I the slope (ΔV/ΔI) gives the resistance R. For an ohmic conductor the graph is straight and passes through origin.
8. What is internal resistance of a cell? How does it affect terminal voltage?
Answer: Internal resistance r is resistance within a cell. When current I flows, terminal voltage V = E − Ir (E is EMF), so internal resistance reduces terminal voltage under load.
9. Write the relation between EMF, terminal voltage and internal resistance.
Answer: V = E − Ir (when current flows from the cell), where E is EMF, r internal resistance, I current.
10. Explain the difference between EMF and terminal voltage.
Answer: EMF (E) is the open‑circuit voltage of the source (no current). Terminal voltage is actual voltage across terminals under load: V = E − Ir.
11. How do resistances add in series? Provide formula and reason.
Answer: In series R_total = R_1 + R_2 + … because current is same through each and potential differences add across each resistor.
12. How do resistances add in parallel? Provide formula and explain.
Answer: In parallel 1/R_total = 1/R_1 + 1/R_2 + … because potential difference across each branch is same and currents add, making equivalent resistance smaller.
13. Define electric power and give three equivalent expressions.
Answer: Power P is rate of energy conversion: P = VI = I²R = V²/R. Unit watt (W).
14. How is electrical energy related to power and time? Give unit used in billing.
Answer: Energy E = P t = V I t. Billing uses kilowatt‑hour (kWh): 1 kWh = 3.6 × 10⁶ J.
15. What is Joule heating and write the expression for heat produced.
Answer: Joule heating is heat produced in a resistor: H = I² R t in time t.
16. What is a short circuit and why is it dangerous?
Answer: A short circuit is an unintended low‑resistance path causing excessive current, which can damage equipment or cause fire; protection devices are necessary.
17. Explain the working principle of a fuse.
Answer: A fuse contains a thin wire that melts when current exceeds rated value, opening circuit and preventing damage from overcurrent.
18. Why is earthing important in electrical installations?
Answer: Earthing provides a low‑resistance path to ground for fault currents, preventing electric shocks and protecting appliances.
19. Describe connection of ammeter and voltmeter in a circuit.
Answer: Ammeter is connected in series to measure current; voltmeter is connected in parallel across the component to measure potential difference.
20. State two differences between conductor and insulator.
Answer: Conductors have free charge carriers and low resistivity (e.g., metals). Insulators lack free carriers and have high resistivity (e.g., rubber). Conductors conduct current easily; insulators do not.
21. What is meant by 'ohmic' and 'non‑ohmic' behaviours? Give examples.
Answer: Ohmic: V ∝ I (linear), e.g., metallic resistor. Non‑ohmic: V–I not linear, e.g., filament lamp, diode.
22. A wire of length l and radius r has resistance R. If radius is doubled, what is new resistance?
Answer: Area A ∝ r² so area quadruples; resistance becomes R/4.
23. Define current density.
Answer: Current density J is current per unit cross‑sectional area: J = I/A, vector in direction of conventional current.
24. Explain drift velocity qualitatively.
Answer: Drift velocity is the slow average velocity of charge carriers under an electric field, superimposed on random thermal motion, producing macroscopic current.
25. How does temperature affect resistance of metals and semiconductors?
Answer: For metals resistance increases with temperature; for intrinsic semiconductors resistance decreases as temperature increases (more carriers).
26. What is the equivalent resistance of two equal resistors R connected in parallel?
Answer: R_eq = R/2.
27. Two resistors 4 Ω and 6 Ω are connected in series across 20 V. Calculate current and potential drop across each resistor.
Answer: R_total = 10 Ω, I = V/R = 2 A. V_4Ω = I×4 = 8 V; V_6Ω = I×6 = 12 V.
28. Explain how a potentiometer can be used to compare EMFs.
Answer: Potentiometer provides a variable potential difference; by balancing the EMF against a section of potentiometer wire (no current through the cell), one can compare EMFs accurately without drawing current from cells.
29. What is Kirchhoff's first law and how is it applied?
Answer: KCL: sum of currents entering a junction equals sum leaving. Used to write current relations at circuit nodes for solving circuit currents.
30. What is Kirchhoff's second law and its significance?
Answer: KVL: sum of potential differences around any closed loop is zero. It enforces energy conservation and helps write loop equations to find unknown currents/voltages.
31. A 60 W bulb is connected to 230 V mains. Find the current drawn and resistance of the bulb when lit.
Answer: P = V I ⇒ I = P/V = 60/230 ≈ 0.261 A. R = V/I ≈ 230/0.261 ≈ 882 Ω (approx.).
32. Explain why incandescent bulbs have higher resistance when hot than cold.
Answer: Filament temperature increases when lit, increasing lattice vibrations and scattering, raising resistivity and thus resistance; hence non‑linear V–I curve.
33. What is meant by 'rating' of an electrical appliance? Give example.
Answer: Rating indicates designed operating power and voltage, e.g., 60 W, 230 V for a bulb; helps choose appropriate supply and estimate energy use.
34. How much energy is consumed by a 2 kW heater running for 3 hours? Express in kWh and J.
Answer: Energy = P t = 2 kW × 3 h = 6 kWh = 6 × 3.6×10⁶ J = 2.16×10⁷ J.
35. Why are household circuits wired in parallel rather than series?
Answer: Parallel ensures each appliance gets full supply voltage and can operate independently; series would make operation interdependent and change voltages when devices are switched.
36. A cell of EMF 12 V and internal resistance 1 Ω supplies current 2 A. What is terminal voltage?
Answer: V = E − Ir = 12 − (2×1) = 10 V.
37. What are semiconductors used for in electrical devices? Give one application.
Answer: Semiconductors control conductivity (diodes, transistors). Application: rectifiers, switching, sensors, integrated circuits.
38. Describe in brief how you would experimentally verify Ohm's law for a metallic conductor.
Answer: Connect conductor with ammeter in series and voltmeter in parallel; vary voltage using a variable source, record V and I, plot V vs I. Linear relationship through origin verifies Ohm's law; slope = R.
39. What is the effect of connecting two identical cells in series? How does EMF change?
Answer: EMFs add in series; two identical cells E each give total EMF 2E; internal resistances also add.
40. What is the effect of connecting two identical cells in parallel (ideal case)?
Answer: EMF remains E (same), but equivalent internal resistance decreases (if ideal identical, current capacity increases), providing more current without increasing voltage.
41. A 100 Ω resistor carries 0.2 A. Find power dissipated.
Answer: P = I²R = (0.2)² × 100 = 0.04 × 100 = 4 W.
42. Explain with one line why a voltmeter should have high internal resistance.
Answer: High internal resistance ensures it draws negligible current and does not alter the circuit voltage being measured.
43. Explain with one line why an ammeter should have low internal resistance.
Answer: Low internal resistance minimizes voltage drop across ammeter and avoids changing circuit current being measured.
44. How does a circuit breaker differ from a fuse?
Answer: Circuit breaker is a reusable protective switch that trips on overcurrent; fuse melts once and must be replaced after operation.
45. Two resistors 3 Ω and 6 Ω are connected in parallel across 12 V. Find currents through each and total current drawn from supply.
Answer: I_3 = 12/3 = 4 A; I_6 = 12/6 = 2 A; I_total = 6 A.
46. What is the typical direction of conventional current relative to electron flow?
Answer: Conventional current is opposite to electron flow (conventional: positive to negative; electrons: negative to positive).
47. A resistor draws 0.5 A at 12 V. Calculate its resistance.
Answer: R = V/I = 12/0.5 = 24 Ω.
48. What safety measures should be followed while performing electrical experiments in lab?
Answer: Use low voltages, check connections, keep hands dry, switch off before modifying, use insulated leads, avoid short circuits, follow teacher instructions.
49. Explain why two bulbs in series share the supply voltage but may have different brightness.
Answer: In series same current flows through both, but voltage drop across each depends on resistance; bulbs with higher resistance dissipate more power and may be brighter (depends on R and P = I²R).
50. Summarise three key points students must remember while solving circuit numericals in exams.
Answer: (1) Convert units to SI consistently; (2) Label currents and polarities; (3) Simplify circuit stepwise (series/parallel) and check limiting cases for plausibility.