CBSE Class 10 Physics Electricity MCQs Set 16

Answer: (b) nichrome

Explanation:
1. Nichrome, an alloy of nickel and chromium, has high resistance and a high melting point and does not rust easily.
2. Tungsten is used for lamp filaments, while copper and aluminium conduct too well.

Teacher's Note:
1. A heating element must be hot without melting or oxidising.
2. High resistance gives more heat for the same current.

Answer: (c) steps up or steps down an alternating voltage

Explanation:
1. A transformer works on electromagnetic induction between two coils on an iron core.
2. It needs a changing current, so it works only on alternating voltage.

Teacher's Note:
1. A rectifier changes a.c. to d.c. and an inverter does the opposite.
2. A transformer does not change the power, apart from small losses.

Answer: (a) voltameter

Explanation:
1. A voltameter is a vessel with two electrodes and an electrolyte in which electrolysis is carried out.
2. The other instruments measure electrical quantities.

Teacher's Note:
1. Do not confuse a voltameter with a voltmeter.
2. The mass deposited at an electrode is measured with a balance.

Answer: (a) oxidation of the filament

Explanation:
1. A tungsten filament glows at a very high temperature and would burn in air.
2. A vacuum or an inert gas such as argon protects it from oxidation.

Teacher's Note:
1. Modern lamps are filled with argon or nitrogen at low pressure.
2. Tungsten has a high melting point of about 3400 °C.

Answer: (a) joule

Explanation:
1. Energy is measured in joule in the SI system.
2. Watt is the unit of power, volt of potential difference and ampere of current.
3. The commercial unit is the kilowatt hour.

Teacher's Note:
1. 1 kWh = 3.6 × 10⁶ J.
2. Energy = power × time.

Answer: (d) four times

Explanation:
1. By Joule's law H = I²Rt.
2. With I replaced by 2I, H becomes 4 times (2I)² = 4I².

Teacher's Note:
1. Heat is proportional to the square of the current.
2. Doubling the time only doubles the heat.

Answer: (c) Oersted

Explanation:
1. In 1820 Oersted showed that a current in a wire deflects a compass needle placed near it.
2. So a current produces a magnetic field.

Teacher's Note:
1. Faraday discovered electromagnetic induction.
2. Joule studied the heating effect and Ohm studied resistance.

Answer: (a) soft iron

Explanation:
1. Soft iron is magnetised when the current flows and loses its magnetism when the current is switched off.
2. Steel would remain a permanent magnet.

Teacher's Note:
1. This is why electromagnets in cranes release the load when the current is off.
2. Steel is used for permanent magnets.

Answer: (b) the square of the current

Explanation:
1. H = I²Rt.
2. H varies as the square of the current, directly as the resistance and as the time.

Teacher's Note:
1. So for a fixed resistance and time, doubling the current makes the heat four times.
2. Heat is not directly proportional to the current.

Answer: (d) the kilowatt hour

Explanation:
1. Electrical energy is billed in kilowatt hours (units).
2. One kilowatt hour is the energy used when a power of 1 kW is kept up for 1 hour.

Teacher's Note:
1. A kilowatt is a unit of power, not of energy.
2. 1 kWh = 3.6 × 10⁶ J.

Answer: (c) mass of the element liberated at an electrode by a charge of one coulomb

Explanation:
1. By Faraday's first law m = zIt.
2. With I t = 1 coulomb, m = z.
3. So z is the mass liberated by one coulomb (one ampere for one second).

Teacher's Note:
1. The unit of z is kg/C.
2. Atomic mass divided by valency is the chemical equivalent.

Answer: (d) electromagnetic induction

Explanation:
1. When a coil rotates in a magnetic field the number of lines of force linked with it changes, and an emf is induced.
2. That is electromagnetic induction, discovered by Faraday.

Teacher's Note:
1. A dynamo works on the reverse principle to a motor.
2. It converts mechanical energy into electrical energy.

Answer: (a) coulomb, joule and watt

Explanation:
1. Charge is measured in coulomb.
2. Energy in joule.
3. Power (energy per second) in watt.

Teacher's Note:
1. Ampere is a unit of current, not charge.
2. 1 A = 1 C/s.

Answer: (d) tungsten and nichrome

Explanation:
1. A lamp filament needs a very high melting point, so tungsten is used.
2. A heater element needs high resistance and resistance to oxidation, so nichrome is used.

Teacher's Note:
1. Tungsten melts at about 3400 °C.
2. Nichrome is an alloy of nickel and chromium.

Answer: (c) anode; cathode

Explanation:
1. Current enters the electrolyte through the anode (positive electrode).
2. It leaves through the cathode (negative electrode).

Teacher's Note:
1. Metals and hydrogen are deposited at the cathode.
2. Non-metals appear at the anode.

Answer: (d) electrochemical equivalent

Explanation:
1. Faraday's first law, m = zIt, defines z as the mass liberated by one coulomb.
2. z is the electrochemical equivalent.

Teacher's Note:
1. The chemical equivalent is atomic mass divided by valency.
2. z has the unit kg per coulomb.

Answer: (a) right hand rule

Explanation:
1. The right hand (dynamo) rule gives the direction of the induced current in a moving conductor.
2. The left hand (motor) rule gives the direction of force on a current-carrying conductor.

Teacher's Note:
1. Remember: right hand for a generator, left hand for a motor.
2. The thumb gives the direction of motion.

Answer: (d) secondary; primary

Explanation:
1. Vs / Vp = Ns / Np.
2. For a step-up transformer Vs > Vp, so Ns > Np.
3. The secondary has more turns.

Teacher's Note:
1. In a step-up transformer the output current is smaller than the input current.
2. Power input equals power output for an ideal transformer.

Answer: (d) 900 kJ

Explanation:
1. Power = 0.5 kW = 500 W = 500 J/s.
2. Time = 30 × 60 = 1800 s.
3. Energy = 500 × 1800 = 9 × 10⁵ J = 900 kJ.

Teacher's Note:
1. This is the same as 0.25 kWh.
2. Always convert minutes to seconds when using watts and joules.

Answer: (b) double

Explanation:
1. H1 = I²Rt.
2. H2 = (2I)² (R/2) t = 4I² (R/2) t = 2I²Rt.
3. H2 = 2 H1, so the heating effect is doubled.

Teacher's Note:
1. The square of the current wins over the halving of the resistance.
2. Check with numbers: I = 1, R = 2 gives 2t and 4 × 1 × t = 4t.