Quiz Chapter 4: Electromagnetism

1. Which change will NOT increase the speed of rotation of an electric motor?

1. Increasing the current supplied to the coil
2. Using stronger permanent magnets
3. Reversing the polarity of the power supply
4. Increasing the number of turns of the rectangular coil

2. What is the primary function of a split-ring commutator in a direct current (d.c.) motor?

1. To increase the speed of rotation of the coil
2. To reverse the direction of current in the coil every half-revolution
3. To switch off the circuit automatically when overloaded
4. To reduce friction during high-speed rotation

3. Which factor causes energy loss due to hysteresis in a transformer?

1. Resistance of secondary winding copper wire
2. Repeated magnetisation and demagnetisation of the core material
3. Induced circulation currents within the solid core body
4. Leakage of magnetic flux into surrounding air

4. A copper rod falls vertically through a uniform horizontal magnetic field. What happens across the ends of the rod?

1. An electric potential difference is induced across its ends
2. The magnetic field vanishes instantly
3. The copper rod loses its gravitational mass
4. A steady direct current flows in a closed circle within the rod

5. An ideal transformer has 800 turns in the primary coil and 100 turns in the secondary coil. If the primary voltage is 240 V a.c., what is the secondary voltage?

1. 30 V
2. 1920 V
3. 120 V
4. 60 V

6. An ideal transformer steps down voltage from 240 V to 12 V. If the secondary current is 10 A, what is the current in the primary coil?

1. 0.5 A
2. 20 A
3. 2.0 A
4. 5.0 A

7. A step-up transformer has a turns ratio Ns : Np of 15:1. If 12 V a.c. is supplied to the primary coil, what is the output secondary voltage?

1. 0.8 V
2. 180 V
3. 120 V
4. 240 V

8. According to Lenz's Law, the direction of an induced current is such that it always:

1. Aids the motion or change producing it
2. Opposes the motion or change producing it
3. Aligns parallel to the magnetic field lines
4. Flows from negative to positive potential externally

9. A step-down transformer connected to 240 V a.c. mains provides 12 V to an appliance. If the secondary current is 4.0 A and the transformer is 80% efficient, what is the primary current?

1. 0.25 A
2. 0.20 A
3. 1.25 A
4. 2.50 A

10. A power line transmits 50 kW of electrical power at a voltage of 10 kV through cables with total resistance 4 Ω. What is the power lost as heat in the cables?

1. 100 W
2. 200 W
3. 2500 W
4. 500 W