Electromagnetic EffectsCopy
1. What is electromagnetic induction?
A. Production of current by rubbing materials
B. Production of voltage by chemical reaction
C. Production of e.m.f. by changing magnetic field
D. Production of heat by electric current
2. Which of the following is required to induce an e.m.f. in a wire?
A. A battery
B. A moving magnet
C. A stationary resistor
D. A heater
3. Which factor increases the magnitude of induced e.m.f.?
A. Using plastic insulation
B. Decreasing number of turns
C. Moving the magnet more slowly
D. Increasing the speed of motion
4. The e.m.f. induced in a coil increases when:
A. The wire becomes thicker
B. The number of turns increases
C. The coil is made of wood
D. The circuit is broken
5. What is the principle behind Lenz’s law?
A. Like poles repel
B. The induced current always supports the magnetic change
C. The induced current opposes the change that caused it
D. Opposite poles repel
6. Lenz’s law demonstrates:
A. Conservation of mass
B. Newton’s second law
C. Conservation of energy
D. Conservation of temperature
7. What happens when a magnet is pulled out of a coil?
A. Current is induced in one direction
B. Current flows permanently
C. The coil repels the magnet without current
D. No current is induced
8. What is the purpose of slip rings in an a.c. generator?
A. To store charge
B. To reverse current
C. To provide continuous contact
D. To increase resistance
9. In an a.c. generator, what do brushes do?
A. Induce current
B. Rotate the coil
C. Keep electrical contact with rotating coil
D. Store energy
10. Which of the following is a correct feature of an a.c. output graph?
A. Constant value
B. Linear increase
C. Sinusoidal waveform
D. Exponential growth
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Physics Full Scale Course
11. What happens to the e.m.f. in a generator when the coil is vertical in a magnetic field?
A. Maximum
B. Zero
C. Constant
D. Increasing
12. What does reversing the direction of rotation in an a.c. generator do?
A. Stops current
B. Increases resistance
C. Reverses direction of induced current
D. Removes e.m.f.
13. What is the magnetic field pattern around a straight current-carrying wire?
A. Straight lines
B. Parallel lines
C. Circular loops
D. Irregular curves
14. The direction of the magnetic field around a current-carrying wire can be found using:
A. Left-hand rule
B. Newton’s third law
C. Right-hand grip rule
D. Ohm’s law
15. In a solenoid, increasing the current will:
A. Decrease magnetic field strength
B. Make field weaker
C. Make field stronger
D. Have no effect
16. Which device uses the magnetic effect of current?
A. Ammeter
B. Relay
C. Thermometer
D. Barometer
17. How is magnetic effect used in loudspeakers?
A. Converts light to sound
B. Moves diaphragm by varying current in coil
C. Converts magnetic energy to heat
D. Reflects sound waves
18. In an experiment with a wire and magnetic field, when does the wire experience a force?
A. When no current flows
B. When current is parallel to the field
C. When current is perpendicular to field
D. When the circuit is open
19. Reversing the current in a wire placed in a magnetic field causes:
A. No change in force
B. Increase in voltage
C. Direction of force to reverse
D. Field strength to increase
20. Reversing the magnetic field direction causes:
A. No change in current
B. Reversal of the force direction
C. Resistance to fall
D. Increase in induced voltage
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Physics Full Scale Course
21. What rule shows the relative directions of force, magnetic field, and current?
A. Fleming’s left-hand rule
B. Fleming’s right-hand rule
C. Right-hand grip rule
D. Ampere’s rule
22. What is the pattern of magnetic field between two parallel conductors with current in same direction?
A. Cancelled field
B. Repelling lines
C. Attracting field
D. Circular inward field
23. In a d.c. motor, what causes rotation?
A. Magnetic repulsion
B. Magnetic attraction
C. Force on current-carrying coil
D. Electric repulsion
24. The turning effect in a motor increases when:
A. Current decreases
B. Field strength is weaker
C. Number of turns increases
D. Resistance increases
25. Which part of the d.c. motor reverses current direction every half turn?
A. Brush
B. Armature
C. Coil
D. Split-ring commutator
26. What is the role of brushes in a d.c. motor?
A. Prevent rotation
B. Increase current
C. Maintain contact with the coil
D. Store charge
27. A transformer works on which principle?
A. Electrostatics
B. Thermodynamics
C. Electromagnetic induction
D. Gravity
28. Which of the following best describes a transformer?
A. Copper coil around wooden rod
B. Two coils wrapped on iron core
C. Iron coil on copper rod
D. Magnet wrapped with wire
29. What is the equation used for an ideal transformer?
A. Vp × Vs = Np × Ns
B. Vp/Vs = Np/Ns
C. Vp + Vs = Np × Ns
D. Vp = Ns/Np
30. A step-up transformer has:
A. More turns in the primary coil
B. More turns in the secondary coil
C. Equal turns in both coils
D. No coils at all
31. Why is electricity transmitted at high voltages?
A. To reduce current and power loss
B. To increase energy
C. To avoid transformers
D. To protect power stations
32. Power loss in cables is reduced when:
A. Voltage is low
B. Resistance is high
C. Current is high
D. Voltage is high
Answer key and explanations
1. Correct Answer: C
Explanation: Electromagnetic induction refers to generating e.m.f. (voltage) by changing magnetic fields
2. Correct Answer: B
Explanation: Moving a magnet relative to a coil or wire induces a current
3. Correct Answer: D
Explanation: Faster motion = more rapid field cutting = greater e.m.f.
4. Correct Answer: B
Explanation: More turns in a coil → more total field lines cut → greater induced e.m.f.
5. Correct Answer: C
Explanation: Lenz’s law: induced current always opposes the change causing it
6. Correct Answer: C
Explanation: Lenz’s law is based on conservation of energy
7. Correct Answer: A
Explanation: When magnet moves out, current is induced in the opposite direction
8. Correct Answer: C
Explanation: Slip rings ensure smooth contact between spinning coil and circuit
9. Correct Answer: C
Explanation: Brushes make sliding contact with rotating slip rings
10. Correct Answer: C
Explanation: A.C. output is sinusoidal (sine wave) due to continuous coil rotation
11. Correct Answer: B
Explanation: When the coil is vertical, rate of field line cutting is zero → e.m.f. = 0
12. Correct Answer: C
Explanation: Reversing rotation = reverses direction of e.m.f.
13. Correct Answer: C
Explanation: Magnetic field around a current-carrying wire is made of concentric circles
14. Correct Answer: C
Explanation: Right-hand grip rule shows direction of circular magnetic field
15. Correct Answer: C
Explanation: Stronger current → stronger magnetic field in solenoid
16. Correct Answer: B
Explanation: Relays use electromagnets to switch circuits remotely
17. Correct Answer: B
Explanation: Loudspeakers use coils and magnets to vibrate and create sound
18. Correct Answer: C
Explanation: The maximum force on the wire occurs when current is perpendicular to the magnetic field
19. Correct Answer: C
Explanation: Reversing current reverses force direction (Fleming’s left-hand rule)
20. Correct Answer: B
Explanation: Reversing magnetic field reverses direction of magnetic force
21. Correct Answer: A
Explanation: Fleming’s left-hand rule gives direction of force (thumb), field (first finger), and current (second finger)
22. Correct Answer: C
Explanation: Parallel currents attract each other due to overlapping field interaction
23. Correct Answer: C
Explanation: Force on a coil in a magnetic field causes rotation (turning effect)
24. Correct Answer: C
Explanation: More turns = more magnetic interaction = stronger turning force
25. Correct Answer: D
Explanation: Split-ring commutator reverses current every half turn to maintain rotation in same direction
26. Correct Answer: C
Explanation: Brushes keep electrical contact with the spinning commutator
27. Correct Answer: C
Explanation: Transformers work via electromagnetic induction (changing magnetic field induces voltage)
28. Correct Answer: B
Explanation: A transformer has two coils (primary and secondary) wrapped on an iron core
29. Correct Answer: B
Explanation: Ideal transformer formula: Vp/Vs = Np/Ns
30. Correct Answer: B
Explanation: Step-up transformer → more turns in secondary than primary
31. Correct Answer: A
Explanation: High voltage = low current = reduced I²R losses in transmission cables
32. Correct Answer: D
Explanation: Power loss = I²R, so reducing current (by increasing voltage) reduces loss