MAGNETISM AND
ELECTRO-MAGNETISM
1. The magnetic field due to a current-carrying wire decreases as the distance from the wire:
a) Increases
b) Decreases
c) Remains constant
d) None of the above
2. A wire carries a current of 4 A. The magnetic field at a distance of 2 meters from the wire is 0.6 T. What will be the magnetic field at a distance of 4 meters?
a) 0.15 T
b) 0.3 T
c) 0.6 T
d) 1.2 T
3. A conductor of length 0.5 m carrying a current of 3 A experiences a magnetic force of 2 N. What is the strength of the magnetic field?
a) 0.5 T
b) 1 T
c) 1.5 T
d) 2 T
4. A rectangular coil of dimensions 4 cm x 6 cm carries a current of 5 A. If the coil is placed in a magnetic field of 0.8 T, what is the torque experienced by the coil?
a) 0.96 Nm
b) 1.6 Nm
c) 2.4 Nm
d) 3.2 Nm
5. The magnetic flux through a coil is given by Φ = 5 Tm². If the area of the coil is 2 m², what is the magnetic flux density?
a) 2.5 T
b) 5 T
c) 10 T
d) 25 T
6. A charged particle moves with a velocity of 2 m/s in a magnetic field of 0.5 T. If the force experienced by the particle is 4 N, what is the charge of the particle?
a) 2 C
b) 4 C
c) 8 C
d) 16 C
7. A particle with a charge of 4 C and a mass of 2 kg moves in a magnetic field with a velocity of 6 m/s. What is the magnitude of the magnetic field required to produce a radius of curvature of 3 m?
a) 4 T
b) 6 T
c) 8 T
d) 12 T
8. According to Ampere's law, the line integral of the magnetic field around a closed loop is equal to:
a) The electric current passing through the loop
b) The magnetic flux through the loop
c) The magnetic field at the center of the loop
d) Zero
9. A solenoid of length 10 cm and 1000 turns carries a current of 2 A. What is the magnetic field inside the solenoid?
a) 0.001 T
b) 0.02 T
c) 0.2 T
d) 2 T
10. The magnetic field inside a toroid with 500 turns is 0.4 T. What is the magnetic flux through the toroid?
a) 200 Tm²
b) 500 Tm²
c) 800 Tm²
d) 2000 Tm²
11. Electromagnetic induction is based on the principle of:
a) Magnetic field due to a current
b) Force on a current-carrying conductor
c) Changing magnetic field
d) Torque on a current-carrying coil
12. According to Faraday's law of electromagnetic induction, the magnitude of the induced electromotive force (emf) is directly proportional to:
a) The magnetic field strength
b) The number of turns in the coil
c) The rate of change of magnetic flux
d) The resistance of the coil
13. A coil with 200 turns experiences a change in magnetic flux of 0.05 Tm²/s. What is the induced emf in the coil?
a) 10 V
b) 20 V
c) 50 V
d) 100 V
14. According to Lenz's law, the direction of the induced current in a circuit is such that it:
a) Opposes the change in magnetic field
b) Supports the change in magnetic field
c) Creates a stronger magnetic field
d) Creates a weaker magnetic field
15. When the magnetic field through a coil is halved, the induced emf:
a) Doubles
b) Halves
c) Remains unchanged
d) Becomes zero
16. Mutual induction occurs when:
a) A changing magnetic field induces a current in the same circuit
b) A changing magnetic field induces a current in a neighboring circuit
c) A constant magnetic field induces a current in the same circuit
d) A constant magnetic field induces a current in a neighboring circuit
17. A primary coil with 500 turns is connected to a battery. A secondary coil with 2000 turns is placed nearby. If the current in the primary coil changes at a rate of 2 A/s, what is the induced emf in the secondary coil?
a) 0.4 V
b) 0.8 V
c) 1.6 V
d) 3.2 V
18. Non-inductive winding is used in:
a) Transformers
b) Motors
c) Inductors
d) Resistors
19. A conductor moves at a speed of 10 m/s perpendicular to a magnetic field of 0.5 T. What is the induced emf across the conductor?
a) 5 V
b) 10 V
c) 25 V
d) 50 V
20. In an alternating current generator (dynamo), the output current is:
a) Direct current (DC)
b) Alternating current (AC)
c) Pulsating DC
d) None of the above
21. A DC generator converts:
a) Mechanical energy into electrical energy
b) Electrical energy into mechanical energy
c) DC into AC
d) AC into DC
22. In an electric motor, the input is:
a) AC
b) DC
c) Both AC and DC
d) None of the above
23. A transformer is a device used to:
a) Increase electrical resistance
b) Convert AC to DC
c) Step up or step down voltage
d) Generate electromagnetic waves
24. The primary coil of a transformer has 200 turns and the secondary coil has 1000 turns. If the primary voltage is 120 V, what is the secondary voltage?
a) 24 V
b) 120 V
c) 240 V
d) 600 V
25. Sources of power loss in a transformer include:
a) Resistance of the windings
b) Eddy currents
c) Hysteresis losses
d) All of the above
26. Power transmission is more efficient at:
a) Low voltages and high currents
b) High voltages and low currents
c) Equal voltages and currents
d) It does not depend on voltage or current
27. The primary coil of a transformer has 100 turns and the secondary coil has 200 turns. If the primary voltage is 240 V, what is the secondary voltage?
a) 120 V
b) 240 V
c) 480 V
d) 960 V
28. A transformer has a turns ratio of 1:10 and an input power of 1000 W. What is the output power?
a) 10 W
b) 100 W
c) 1000 W
d) 10,000 W
29. A transformer with a step-up ratio of 1:5 has an input current of 5 A. What is the output current?
a) 1 A
b) 5 A
c) 10 A
d) 25 A
30. The efficiency of a transformer is given by:
a) (Output power / Input power) x 100%
b) (Input power / Output power) x 100%
c) (Output voltage / Input voltage) x 100%
d) (Input voltage / Output voltage) x 100%
31. The direction of the induced current in a coil can be determined using:
a) Ampere's law
b) Faraday's law
c) Lenz's law
d) Ohm's law
32. The back emf in a motor is a result of:
a) Induced current in the armature
b) Induced current in the field coils
c) Induced current in the brushes
d) None of the above
33. The speed of a DC motor is directly proportional to:
a) Armature resistance
b) Armature current
c) Field strength
d) Back emf
34. The frequency of the alternating current generated by a generator is determined by:
a) The speed of the generator
b) The number of turns in the coil
c) The strength of the magnetic field
d) The resistance of the load
35. A transformer is more efficient when the:
a) Turns ratio is high
b) Turns ratio is low
c) Input voltage is high
d) Input voltage is low
36. The power loss in a transformer is primarily due to:
a) Resistance of the windings
b) Eddy currents
c) Hysteresis losses
d) Core losses
37. A transformer has a step-down ratio of 1:4 and an input voltage of 400 V. What is the output voltage?
a) 100 V
b) 200 V
c) 800 V
d) 1600 V
38. The efficiency of a transformer is maximum when:
a) The resistance of the windings is maximum
b) The resistance of the windings is minimum
c) The load power factor is maximum
d) The load power factor is minimum
39. A transformer has an input power factor of 0.8 lagging. What is the output power factor?
a) 0.8 lagging
b) 0.8 leading
c) Unity (1)
d) It cannot be determined without additional information
40. The voltage induced in a coil with a magnetic flux of 0.02 Wb and 500 turns is:
a) 10 V
b) 20 V
c) 40 V
d) 100 V
41. The emf induced in a coil is 24 V when the magnetic flux changes at a rate of 50 T/s. What is the number of turns in the coil?
a) 12
b) 24
c) 50
d) 1200
42. An alternating current generator produces an output of 240 V with a frequency of 50 Hz. What is the peak voltage of the generated waveform?
a) 120 V
b) 169.7 V
c) 240 V
d) 339.4 V
43. In a DC generator, the commutator is responsible for:
a) Changing AC to DC
b) Controlling the speed of the generator
c) Switching the direction of current in the external circuit
d) Regulating the voltage output
44. The main advantage of AC over DC for long-distance power transmission is:
a) Lower power losses
b) Higher voltage levels
c) Better compatibility with appliances
d) Improved safety
45. In a step-up transformer, the number of turns in the secondary coil is:
a) Greater than the number of turns in the primary coil
b) Less than the number of turns in the primary coil
c) Equal to the number of turns in the primary coil
d) It depends on the input voltage
46. The power transmitted through a power line can be calculated using the formula:
a) P = IV
b) P = V^2/R
c) P = VI cos(θ)
d) P = V^2 cos(θ)/R
47. The voltage across the secondary coil of a transformer is 120 V when the primary voltage is 240 V. What is the turns ratio of the transformer?
a) 1:1
b) 1:2
c) 2:1
d) 2:3
48. The primary coil of a transformer has 400 turns and the secondary coil has 100 turns. If the primary voltage is 240 V, what is the secondary voltage?
a) 60 V
b) 96 V
c) 240 V
d) 960 V
49. The back emf in a motor:
a) Helps in reducing power loss
b) Helps in maintaining a constant speed
c) Helps in regulating the voltage supply
d) Helps in cooling the motor
50. The frequency of the alternating current in a power transmission line is typically:
a) 50 Hz
b) 60 Hz
c) 50-60 Hz
d) It varies depending on the location
ANSWER KEY:
1. c) Magnetic field strength
2. b) Magnetic field strength
3. d) Cross-product of the current and magnetic field vectors
4. a) Magnetic field and current direction are parallel
5. c) qvB sinθ
6. a) Perpendicular to both the velocity and magnetic field vectors
7. c) Directly proportional to the magnitude of the magnetic field and the velocity of the charged particle
8. a) qB/m
9. c) Zero
10. b) qvB sinθ
11. d) The ratio of the charge to the mass of an electron
12. c) The rate of change of magnetic flux
13. b) 20 V
14. a) Opposes the change in magnetic field
15. a) Doubles
16. b) A changing magnetic field induces a current in a neighboring circuit
17. d) 3.2 V
18. c) Inductors
19. c) 25 V
20. b) Alternating current (AC)
21. a) Mechanical energy into electrical energy
22. b) DC
23. c) Step up or step down voltage
24. c) 240 V
25. d) All of the above
26. b) High voltages and low currents
27. b) 240 V
28. c) 1000 W
29. b) 5 A
30. a) (Output power / Input power) x 100%
31. c) Lenz's law
32. a) Induced current in the armature
33. d) Back emf
34. a) The speed of the generator
35. b) Turns ratio is low
36. d) Core losses
37. b) 200 V
38. c) The load power factor is maximum
39. c) Unity (1)
40. a) 10 V
41. d) 1200
42. b) 169.7 V
43. c) Switching the direction of current in the external circuit
44. a) Lower power losses
45. a) Greater than the number of turns in the primary coil
46. a) P = IV
47. c) 2:1
48. b) 96 V
49. b) Helps in maintaining a constant speed
50. c) 50-60 Hz
