CBSE Class 10 Physics Electricity MCQs Set 17

Answer: (c) electrons move from one point to another in a definite direction only when a potential difference is applied across the conductor

Explanation:
1. Without a potential difference the electrons move randomly in all directions, so there is no net flow.
2. A potential difference gives them a drift in one direction, which constitutes current.

Teacher's Note:
1. Random motion gives zero net current.
2. Drift is a small steady motion superposed on random motion.

Answer: (c) H ∝ I²

Explanation:
1. Joule's law gives H = I²Rt.
2. For fixed R and t, H is proportional to I².

Teacher's Note:
1. Double the current gives four times the heat.
2. The relation is quadratic.

Answer: (b) heating effect of current

Explanation:
1. The iron has a resistance coil that gets hot when current passes through it.
2. This is the heating effect of current.

Teacher's Note:
1. Heater, toaster and iron use the heating effect.
2. Electric bell uses the magnetic effect.

Answer: (b) electrons

Explanation:
1. Only the loosely held outer electrons can be transferred by rubbing.
2. Losing electrons leaves the body with excess positive charge.

Teacher's Note:
1. Protons stay in the nucleus.
2. Positive charging means loss of electrons.

Answer: (c) earth wire

Explanation:
1. Green insulation is used for the earth wire.

Teacher's Note:
1. Earth wire protects against shock by carrying leaked current to the ground.

Answer: (d) 1:4

Explanation:
1. With the same voltage, H = V²t/R.
2. Series resistance is 2R and parallel resistance is R/2.
3. Ratio = (1/2R) : (2/R) = 1 : 4.

Teacher's Note:
1. At fixed voltage, smaller resistance gives more heat.
2. Parallel gives 4 times the heat of series.

Answer: (a) live wire

Explanation:
1. In household wiring the live wire has red insulation, the neutral wire black, and the earth wire green.

Teacher's Note:
1. Red = live, black = neutral, green = earth.

Answer: (a) 10

Explanation:
1. For n parts, each has 100/n Ω, and the parallel resistance is 100/n².
2. Set 100/n² = 1, so n = 10.

Teacher's Note:
1. R' = R/n².
2. Take the square root of 100.

Answer: (d) earth wire

Explanation:
1. The top, larger hole of the socket is connected to the earth wire, and the metal casing of the appliance is joined to it through the longer, thicker pin.

Teacher's Note:
1. The earth pin is longer and thicker.
2. It makes contact first and breaks last.

Answer: (c) live wire

Explanation:
1. The switch is placed in the live wire so that the appliance is disconnected from the high potential when it is off.
2. This prevents shock when we touch the appliance.

Teacher's Note:
1. Fuse and switch go in the live wire.
2. The earth wire is never switched.

Answer: (d) 25

Explanation:
1. Each part has resistance R/5.
2. Five equal resistances of R/5 in parallel give R' = (R/5)/5 = R/25.
3. Therefore R/R' = 25.

Teacher's Note:
1. n equal parts of a wire in parallel give R/n².
2. Here n = 5.

Answer: (c) Wire cut into four equal parts, connected in parallel across the battery

Explanation:
1. Energy per second = V²/R, so it is largest when R is smallest.
2. Four equal parts in parallel give R/16, which is smaller than R, R/2 and R/4 of the other cases.

Teacher's Note:
1. Smallest resistance gives the largest power at fixed voltage.
2. n parts in parallel give R/n².

Answer: (c) I²R

Explanation:
1. From Joule's law, H = I²Rt.
2. Rate of heat = H/t = I²R.

Teacher's Note:
1. Heating power = I²R = VI = V²/R.
2. Check units: A²·Ω = W.

Answer: (c) It/e

Explanation:
1. Charge passed Q = It.
2. Number of electrons n = Q/e = It/e.

Teacher's Note:
1. n = Q/e.
2. Check units: (A·s)/C is a pure number.

Answer: (b) will be more in the thinner wire

Explanation:
1. The thinner wire has a larger resistance.
2. In series the current is the same, and P = I²R, so the thinner wire dissipates more power.

Teacher's Note:
1. In series, larger resistance gets more power.
2. In parallel, smaller resistance gets more power.

Answer: (a) the body has acquired an excess of electrons

Explanation:
1. Only electrons move from one body to another during rubbing, because protons are bound inside the nucleus.
2. A body becomes negative when it gains electrons.

Teacher's Note:
1. Negative charge means excess electrons.
2. Positive charge means deficiency of electrons.

Answer: (b) Joule

Explanation:
1. The heating effect of current, H = I²Rt, is known as Joule's law of heating.

Teacher's Note:
1. Joule gave the law of heating.
2. Ohm gave V = IR.

Answer: (d) 25 W

Explanation:
1. Resistance R = V²/P = 220²/100 = 484 Ω.
2. New power = 110²/484 = 25 W.

Teacher's Note:
1. At fixed R, power is proportional to V².
2. Halving the voltage gives one-fourth power.

Answer: (b) repulsion

Explanation:
1. Attraction can also occur between a charged and a neutral body.
2. Repulsion occurs only between two bodies with charge of the same sign, so it is the sure test.

Teacher's Note:
1. Attraction is not a sure test.
2. Repulsion is.

Answer: (a) 10⁶ µA

Explanation:
1. 1 µA = 10^–6 A.
2. So 1 A = 10⁶ µA.

Teacher's Note:
1. Micro means 10^–6.
2. Milli means 10^–3.

Answer: (c) a rheostat (variable resistance)

Explanation:
1. A zig-zag line is the symbol of a resistance.
2. An arrow across it shows that its value can be varied, so it is a rheostat.

Teacher's Note:
1. Arrow means variable.
2. A galvanometer is shown by a circle with G.

Answer: (d) One coulomb is the charge of one electron

Explanation:
1. The charge of one electron is only 1.6 × 10^–19 C.
2. One coulomb is the charge of about 6.25 × 10¹⁸ electrons, so statement (d) is false.

Teacher's Note:
1. Charge is a scalar and is quantised.
2. Do not confuse 1 C with the charge of an electron.