Electricity: Class 10 Important Questions with Answers

Formula sheet + numerical method

5-step method for any circuit numerical

1. Write the data with units; convert to SI (mm → m, mA → A, min → s).
2. Find the equivalent resistance: reduce each parallel block first, then add series parts.
3. Total current: I = V/R_total.
4. For a part: series part → V_part = I × R_part; parallel branch → I_branch = V_block/R_branch.
5. Check: series voltages add to the battery voltage; branch currents add to the total current. Write the final unit.

Case-based question 1 (4 marks)

(i) What is "1 unit"? (1) (ii) Energy used per day. (1) (iii) Bill for 30 days, and the current drawn by the geyser — can it run on a 5 A circuit? (2)

(i) Resistance of P. (1) (ii) Currents through Q and S. (1) (iii) If P is stretched to twice its length, what is its new resistance and does its resistivity change? (2)

(i) Resistance of one bulb. (1) (ii) Current through each bulb in the parallel set. (1) (iii) One bulb fuses in each set — what happens? Which set glows brighter? (2)

(i) Resistance of the resistor. (1) (ii) Shape of its V–I graph. (1) (iii) Why does the torch bulb not follow Ohm's law exactly? Why must the ammeter be in series and the voltmeter in parallel? (2)

20 MCQs (1 mark each)

1. SI unit of charge:
   (a) ampere (b) coulomb (c) volt (d) ohm
2. 1 A equals
   (a) 1 J/s (b) 1 C/s (c) 1 V/C (d) 1 J/C
3. Electrons in 2 C:
   (a) 6.25 × 10¹⁸ (b) 1.25 × 10¹⁹ (c) 3.2 × 10⁻¹⁹ (d) 1.6 × 10¹⁹
4. Work to move 3 C through 4 V:
   (a) 0.75 J (b) 1.33 J (c) 7 J (d) 12 J
5. A voltmeter is connected
   (a) in series (b) in parallel (c) either way (d) to the key only
6. Slope of a V–I graph gives
   (a) current (b) power (c) resistance (d) charge
7. Unit of resistivity:
   (a) Ω (b) Ω/m (c) Ω m (d) Ω m²
8. Wire stretched to twice its length: R becomes
   (a) 2R (b) R/2 (c) 4R (d) R/4
9. Best conductor in Table 11.2:
   (a) copper (b) aluminium (c) silver (d) tungsten
10. Heating elements are made of
    (a) copper (b) tungsten (c) nichrome (d) silver
11. 2 Ω, 3 Ω, 5 Ω in series give
    (a) 0.97 Ω (b) 5 Ω (c) 10 Ω (d) 30 Ω
12. Two 4 Ω in parallel:
    (a) 8 Ω (b) 4 Ω (c) 2 Ω (d) 1 Ω
13. In a parallel circuit, the same for all branches is
    (a) current (b) p.d. (c) resistance (d) power
14. Heat produced H =
    (a) IRt (b) I²Rt (c) IR²t (d) VIR
15. Doubling the current (same R, t) makes heat
    (a) 2× (b) 4× (c) ½ (d) same
16. Fuse wire should have
    (a) high m.p. (b) low m.p. (c) low ρ only (d) high m.p. and low ρ
17. 1 kWh =
    (a) 3.6 × 10³ J (b) 3.6 × 10⁵ J (c) 3.6 × 10⁶ J (d) 36 J
18. Power of a 60 W, 220 V lamp on 110 V:
    (a) 60 W (b) 30 W (c) 15 W (d) 120 W
19. Current drawn by a 2.2 kW heater on 220 V:
    (a) 1 A (b) 10 A (c) 0.1 A (d) 100 A
20. Filament of a bulb is made of tungsten because of its
    (a) low m.p. (b) high m.p. (c) low cost (d) magnetism

Answer key

Why: 3 → 2 × 6.25 × 10¹⁸ · 4 → W = VQ = 12 J · 18 → P ∝ V², ¼ of 60 W · 19 → I = 2200/220.

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: An ammeter is connected in series. R: Current is the same at all points of a series circuit. → (a)
2. A: Resistivity of a wire doubles when its length is doubled. R: Resistance is directly proportional to length. → (d)
3. A: Alloys are used in heating appliances. R: Alloys have high resistivity and do not oxidise easily at high temperatures. → (a)
4. A: Household appliances are connected in parallel. R: In parallel, each appliance gets the same voltage. → (a)
5. A: The equivalent resistance of resistors in parallel is less than the smallest one. R: In parallel the reciprocals add. → (a)
6. A: A thick wire has more resistance than a thin wire of the same length. R: R ∝ 1/A. → (d)
7. A: Tungsten is used for bulb filaments. R: Tungsten has a low melting point. → (c)
8. A: The cord of a heater does not glow. R: The cord has very low resistance, so little heat is produced in it. → (a)
9. A: kWh is a unit of power. R: 1 kWh = 3.6 × 10⁶ J. → (d)
10. A: A fuse is connected in series with the appliance. R: A fuse works on the heating effect of current. → (b)

Give reasons (2 marks each)

1. A voltmeter has high resistance.

It is connected in parallel; high resistance means it draws almost no current and does not disturb the circuit. 2

2. Bulbs in a house glow with full brightness even when others are switched off.

They are in parallel; each gets the full 220 V independently. 2

3. Copper wires are used for connections, not nichrome.

Copper has very low resistivity, so little energy is lost as heat; nichrome would heat up. 2

4. Bulbs are filled with nitrogen or argon.

These are chemically inactive gases; they prevent the hot filament from oxidising, prolonging its life. 2

5. The V–I graph of a filament bulb is not a straight line.

Its temperature rises with current and resistance increases with temperature, so V/I is not constant. 2