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. 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

3. Why can transformers NOT operate when connected to a continuous steady Direct Current (d.c.) supply?

1. Direct current has too high internal resistance
2. Direct current creates a constant magnetic field, causing zero rate of change of flux linkage
3. Direct current melts the soft iron core
4. Direct current flows only through the secondary coil

4. 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

5. 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

6. Faraday's Law of Electromagnetic Induction states that the magnitude of induced e.m.f. is directly proportional to the:

1. Resistance of the conductor coil
2. Rate of cutting or rate of change of magnetic flux linkage
3. Temperature of the magnetic core
4. Total length of connecting copper lead wires

7. 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

8. Which rule determines the direction of induced current in a straight conductor moving perpendicular to a magnetic field?

1. Fleming's Right-Hand Rule
2. Fleming's Left-Hand Rule
3. Right-Hand Grip Rule
4. Maxwell's Screw Rule

9. When a straight wire carrying current towards the reader (out of the page) is placed in a magnetic field directed to the right, in which direction is the magnetic force on the wire?

1. Upward
2. Downward
3. To the left
4. To the right

10. 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