CBSE Class 7 Science Advanced Physics Electricity MCQs Set 04

Answer: (b) Dynamo

Explanation:
1. A dynamo is a generator: it turns mechanical energy into electrical energy.
2. A motor does the reverse and a cell converts chemical energy.
3. The answer is dynamo.

Teacher's Note:
1. Generator = dynamo.
2. Motor: electrical to mechanical.

Answer: (a)

Explanation:
1. The fuse symbol is a wavy S-shaped wire drawn between two terminals.
2. (b) is the symbol of a switch, and (c) and (d) are other symbols.
3. The answer is (a).

Teacher's Note:
1. A fuse wire melts when too much current flows.
2. The wavy line shows the thin fuse wire.

Answer: (b) 4 V

Explanation:
1. As in the book, the emf of cells joined in parallel is taken as that of the cell with the highest emf.
2. The highest emf here is 4 V: option (b).

Teacher's Note:
1. This follows the book's rule for this question.
2. In practice only identical cells are joined in parallel.

Answer: (b) Polarization

Explanation:
1. Hydrogen bubbles collecting on the carbon rod cause polarization.
2. Manganese dioxide, a depolarizer, oxidizes the hydrogen to water.
3. So it prevents polarization: option (b).

Teacher's Note:
1. A depolarizer prevents polarization.
2. MnO₂ is used in the Leclanche cell.

Answer: (a) Electro typing

Explanation:
1. Electric motors, generators and telegraphs use electromagnetism.
2. Electrotyping uses the chemical effect of current.
3. The answer is electrotyping.

Teacher's Note:
1. Electrotyping is the chemical effect.
2. Motor and generator both use magnets.

Answer: (d) A → r; B → p; C → q

Explanation:
1. A zigzag line is the symbol of a resistor: A → r.
2. A zigzag line with a sliding arrow is a rheostat (variable resistor): B → p.
3. A circle with a loop is an electric bulb: C → q.
4. This is option (d).

Teacher's Note:
1. The arrow shows the sliding contact of a variable resistance.
2. A rheostat can change the current.

Answer: (d) QRP

Explanation:
1. State the main parts: porous pot and zinc rod in ammonium chloride solution: (Q).
2. The pot holds powdered carbon and manganese dioxide with a carbon rod: (R).
3. Name the plates: zinc is negative and carbon is positive: (P).
4. The order is Q, R, P: option (d).

Teacher's Note:
1. Describe the cell from outside to inside.
2. Polarity of the plates is stated last.

Answer: (b) Manganin

Explanation:
1. Copper, aluminium and silver are good conductors with low resistance.
2. Manganin is an alloy with high resistance, so it conducts less.
3. The answer is manganin.

Teacher's Note:
1. Alloys like manganin and nichrome have high resistance.
2. Less conduction means more resistance.

Answer: (d) Large and positive

Explanation:
1. Lightning occurs when a charged cloud induces the opposite charge on the ground.
2. The base of the cloud is negative, so the Earth below becomes large and positive.
3. The answer is large and positive.

Teacher's Note:
1. Induction gives the ground the opposite charge.
2. The discharge is the lightning flash.

Answer: (c) Heating effect of current

Explanation:
1. A heater has a coil of high-resistance wire that becomes hot when current flows.
2. This is the heating effect of current: (c).

Teacher's Note:
1. Heater, geyser and iron use the heating effect.
2. The other effects are not involved.

Answer: (d) Both (b) and (c)

Explanation:
1. (a) is correct.
2. (b) is incorrect: bulbs in series share the emf, so they glow dimmer.
3. (c) is incorrect: bulbs in parallel each get the full emf, so they glow brighter.
4. Both (b) and (c) are incorrect: option (d).

Teacher's Note:
1. Series is dim, parallel is bright.
2. Pick the statements that are wrong.

Answer: (d) All the above.

Explanation:
1. The positively charged Q attracts electrons from P.
2. P loses electrons and becomes positive: (a) and (b) are true.
3. Now both bodies are positive, so they repel each other: (c) is true.
4. All are correct: (d).

Teacher's Note:
1. Only electrons move in a solid.
2. Like charges repel.

Answer: (b)

Explanation:
1. (a) 1.5 + 1.5 + 2 = 5 V.
2. (b) 3 + 1.5 + 1.5 = 6 V.
3. (c) 6 + 1.5 + 1.5 = 9 V.
4. (d) The cells of 1.5 V, 1.5 V and 3 V are in parallel and give about 3 V at most.
5. Only (b) gives 6 V: option (b).

Teacher's Note:
1. Add the emf of cells in series.
2. Cells in parallel do not add.

Answer: (d)

Explanation:
1. The strength of an electromagnet increases with the number of turns of the coil.
2. (b) uses silk thread, an insulator, so no current flows through the coil.
3. (a) has very few turns, and (c) has fewer turns than (d), so (d) has the most turns of copper wire.
4. The strongest electromagnet is (d).

Teacher's Note:
1. More turns, stronger magnet.
2. The coil must be made of a conductor such as copper wire.

Answer: (c)

Explanation:
1. (b) has silk thread (an insulator) in the circuit, so no current flows and the bulbs do not glow.
2. (d) has a bulb without a filament, so it will not glow.
3. (c) has all bulbs working and joined in parallel, so each bulb gets the full emf and glows brightly; a fused bulb does not put the others out.
4. This is the circuit used for decoration: (c).

Teacher's Note:
1. Check each diagram for insulators and missing filaments.
2. Parallel connection lets every bulb glow fully.

Answer: (b) Electric motor

Explanation:
1. An electric motor turns electrical energy into rotation.
2. A generator does the reverse, and the two cells convert chemical energy.
3. The answer is electric motor.

Teacher's Note:
1. Motor: electrical to mechanical.
2. Cells: chemical to electrical.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: in parallel each appliance gets the full supply voltage.
2. R is true: each appliance is joined directly to the supply.
3. R explains A: (a).

Teacher's Note:
1. Household wiring is in parallel.
2. Each branch gets the full voltage.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. 500 W × 2 h = 1000 Wh = 1 unit, and 100 W × 10 h = 1000 Wh = 1 unit, so A is true.
2. R is true: units = power (kW) × time (h).
3. R explains A: (a).

Teacher's Note:
1. Units = kWh.
2. Both appliances give 1000 Wh.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: with more bulbs in series, each bulb is dimmer.
2. R is true: the energy from the battery is shared among the bulbs.
3. R explains A: (a).

Teacher's Note:
1. Series bulbs share the voltage.
2. Each gets less than the full emf.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: 100 W × 100 h = 10,000 Wh = 10 kWh = 10 units.
2. R is false: 1 unit = 1 kWh, not 1000 kWh.
3. So A is true and R is false: (c).

Teacher's Note:
1. 1 unit = 1 kWh.
2. Always convert W to kW.