1. A magnetic field is produced by:
A) Electric charges at rest
B) Moving electric charges
C) Gravitational forces
D) Heat energy
Answer: B) Moving electric charges
Explanation: Magnetic fields arise from motion of charges (current) or changing electric fields.
2. The SI unit of magnetic field (magnetic flux density, B) is:
A) Tesla (T)
B) Weber (Wb)
C) Ampere (A)
D) Gauss
Answer: A) Tesla (T)
Explanation: 1 T = 1 N/(A·m).
3. The direction of the magnetic field around a straight current-carrying wire is given by:
A) Fleming’s left-hand rule
B) Right-hand thumb rule
C) Ampere’s law
D) Coulomb’s law
Answer: B) Right-hand thumb rule
Explanation: Thumb points along current, fingers curl along magnetic field direction.
4. Magnetic field lines:
A) Start from south pole and end at north pole
B) Start from north pole and end at south pole
C) Are straight only
D) Are imaginary and random
Answer: B) Start from north pole and end at south pole
Explanation: Magnetic field lines always form closed loops outside and inside the magnet.
5. The strength of magnetic field around a long straight wire at distance r is:
A) B=μ_0 I/2πr
B) B=μ_0 I/4πr^2
C) B=μ_0 I/r
D) B=I/r^2
Answer: A) B=μ_0 I/2πr
Explanation: Biot–Savart law for a long straight wire.
6. Unit of magnetic flux (Φ) is:
A) Tesla (T)
B) Weber (Wb)
C) Ampere (A)
D) Henry (H)
Answer: B) Weber (Wb)
Explanation: Magnetic flux = B·A, unit is Wb.
7. Fleming’s left-hand rule is used to find:
A) Direction of induced current
B) Force on current-carrying conductor in magnetic field
C) Magnetic field around wire
D) Electric field direction
Answer: B) Force on current-carrying conductor in magnetic field
Explanation: Thumb = force, forefinger = field, middle finger = current.
8. Magnitude of magnetic force on a charge q moving with velocity v in magnetic field B at angle θ is:
A) F=qvB
B) F=qvBsinθ
C) F=qvBcosθ
D) F=qv^2 B
Answer: B) F=qvBsinθ
Explanation: Force is maximum when velocity is perpendicular to field.
9. A charge moving parallel to magnetic field experiences:
A) Maximum force
B) Zero force
C) Half force
D) Depends on speed
Answer: B) Zero force
Explanation: F = qvB sin θ → θ = 0° → F = 0.
10. Unit of magnetic moment (μ) is:
A) A·m²
B) N·m
C) Tesla
D) Weber
Answer: A) A·m²
Explanation: μ = I·A, current × area.
11. Torque on a current-carrying loop in uniform magnetic field is:
A) τ = B I A sin θ
B) τ = B I A cos θ
C) τ = B I A
D) τ = 0
Answer: A) τ = B I A sin θ
Explanation: θ = angle between plane of loop and field.
12. A compass needle aligns along:
A) Electric field
B) Magnetic field
C) Current direction
D) Gravity
Answer: B) Magnetic field
Explanation: Needle aligns along local magnetic field lines.
13. A solenoid behaves like:
A) Permanent magnet
B) Electromagnet
C) Capacitor
D) Resistor
Answer: B) Electromagnet
Explanation: Current through coil produces magnetic field similar to bar magnet.
14. Magnetic field at the center of a circular current loop of radius R is:
A) B=μ_0 I/2R
B) B=μ_0 I/2πR
C) B=μ_0 I/4πR^2
D) B=μ_0 I/R^2
Answer: A) B=μ_0 I/2R
Explanation: From Biot–Savart law for a circular loop.
15. Magnetic lines of force are:
A) Closed curves
B) Open curves
C) Straight only
D) Random
Answer: A) Closed curves
Explanation: They start from N pole and end at S pole outside magnet, forming loops inside.
16. Magnetic field inside a long solenoid of n turns/m and current I is:
A) B = μ₀ n I
B) B = μ₀ I / 2π r
C) B = μ₀ I n²
D) B = μ₀ I / L
Answer: A) B = μ₀ n I
Explanation: Field inside solenoid is uniform.
17. Direction of magnetic field inside solenoid is determined by:
A) Fleming’s left-hand rule
B) Right-hand screw rule
C) Biot–Savart law
D) Coulomb’s law
Answer: B) Right-hand screw rule
Explanation: Curl fingers along current, thumb gives field direction.
18. Two parallel wires carrying current in same direction:
A) Repel
B) Attract
C) No force
D) Depends on current
Answer: B) Attract
Explanation: Magnetic field produced by each wire exerts force on the other.
19. Two parallel wires carrying current in opposite direction:
A) Attract
B) Repel
C) Neutral
D) None
Answer: B) Repel
Explanation: Currents in opposite direction produce opposing fields → repulsion.
20. Ampere’s force between two parallel wires:
A) F/L=(μ_0 I_1 I_2)/2πd
B) F/L=(μ_0 I_1 I_2)/d^2
C) F/L=μ_0 I_1 I_2
D) F = 0
Answer: A) F/L=(μ_0 I_1 I_2)/2πd
Explanation: Standard formula for force per unit length.
21. Magnetic field at point due to infinite straight wire decreases with:
A) 1/r
B) 1/r²
C) r²
D) r
Answer: A) 1/r
Explanation: Biot–Savart law for straight wire: B ∝ 1/r.
22. SI unit of magnetic dipole moment:
A) A·m²
B) N·m
C) J/T
D) Both A & C
Answer: D) Both A & C
Explanation: μ = I·A = torque/field, both units equivalent.
23. A charge moving in a magnetic field in circular path has:
A) Constant speed
B) Zero acceleration
C) Zero force
D) Increasing speed
Answer: A) Constant speed
Explanation: Magnetic force is perpendicular to velocity → centripetal motion, speed constant.
24. Radius of circular path of charge q, mass m, velocity v in perpendicular B:
A) r = mv/qB
B) r = qB/mv
C) r = mv²/qB
D) r = qBv/m
Answer: A) r = mv/qB
Explanation: Centripetal force mv²/r = qvB → r = mv/qB.
25. Time period of circular motion of charge in uniform magnetic field:
A) T=2πm/qB
B) T=2πq/mB
C) T=2πm/q
D) T=q/mB
Answer: A) T=2πm/qB
Explanation: T = 2πr/v = 2π m/qB.
26. The magnetic field at a point due to a small current element I” ” dlis given by:
A) Ampere’s law
B) Biot–Savart law
C) Faraday’s law
D) Coulomb’s law
Answer: B) Biot–Savart law
Explanation: Biot–Savart law relates a small current element to the magnetic field it produces at a point.
27. Magnetic field due to a small current element Idlat distance r is:
A) dB∝Idl/r
B) dB∝Idl/r^2
C) dB∝Idl/r^3
D) dB∝Idl
Answer: B) dB∝Idl/r^2
Explanation: Biot–Savart law: dB ⃗=(μ_0/4π)(I” ” dl ⃗×r ̂)/r^2.
28. Direction of magnetic field due to current element is:
A) Along current
B) Along radius
C) Perpendicular to plane of current element and radius vector
D) Opposite to current
Answer: C) Perpendicular to plane of current element and radius vector
Explanation: Cross product in Biot–Savart law gives direction.
29. Magnetic field at center of circular loop of radius R carrying current I:
A) B=μ_0 I/2πR
B) B=μ_0 I/2R
C) B=μ_0 I/4πR^2
D) B=μ_0 I/R^2
Answer: B) B=μ_0 I/2R
Explanation: Biot–Savart integration over circular loop.
30. Field at the center of a semicircular wire of radius R carrying current I:
A) B=μ_0 I/4R
B) B=μ_0 I/2R
C) B=μ_0 I/4πR
D) B=μ_0 I/8R
Answer: A) B=μ_0 I/4R
Explanation: B = (μ₀ I / 4R) for semicircle (half of full circle).
31. Magnetic field at the axis of a solenoid (length L >> radius) with n turns/m and current I:
A) B=μ_0 nI
B) B=μ_0 I/2R
C) B=μ_0 In^2
D) B=μ_0 IL
Answer: A) B=μ_0 nI
Explanation: Field inside long solenoid uniform.
32. Magnetic field inside a toroid of mean radius R and N turns carrying current I:
A) B=μ_0 NI/2πR
B) B=μ_0 NI/R
C) B=μ_0 I/2R
D) B=μ_0 I/2πR^2
Answer: A) B=μ_0 NI/2πR
Explanation: Field confined inside toroid, derived via Ampere’s law.
33. Ampere’s circuital law states:
A) Magnetic field along path = μ₀ × total current enclosed
B) ∮B ⃗⋅dl ⃗=μ_0 I_enclosed
C) Magnetic field due to charges only
D) Both A & B
Answer: D) Both A & B
Explanation: Ampere’s law relates line integral of B to current enclosed.
34. For a straight wire, Ampere’s law gives:
A) B=μ_0 I/2πr
B) B=μ_0 I/2R
C) B=μ_0 I/R^2
D) B=μ_0 I/4πR^2
Answer: A) B=μ_0 I/2πr
Explanation: Same as Biot–Savart for long straight wire.
35. Magnetic field inside an ideal solenoid is:
A) Non-uniform
B) Uniform and along axis
C) Zero
D) Radial
Answer: B) Uniform and along axis
Explanation: Superposition of fields from all turns produces uniform field.
36. For circular current loop, magnetic moment μ =
A) I × circumference
B) I × area of loop
C) B × area
D) I² × area
Answer: B) I × area of loop
Explanation: μ = I·A, defines strength of current loop.
37. Unit of magnetic moment in SI:
A) Tesla
B) Ampere·m²
C) Weber
D) Henry
Answer: B) Ampere·m²
Explanation: μ = I × A → unit A·m².
38. Torque on current-carrying loop in uniform B field:
A) τ = μ B sin θ
B) τ = μ B cos θ
C) τ = μ B
D) τ = 0
Answer: A) τ = μ B sin θ
Explanation: θ = angle between μ and B.
39. Potential energy of magnetic dipole in magnetic field:
A) U = – μ · B
B) U = μ · B
C) U = μ / B
D) U = 0
Answer: A) U = – μ · B
Explanation: Minimum energy when μ aligns with B.
40. Right-hand screw rule determines:
A) Current direction from B
B) B direction around current
C) Force on conductor
D) Magnetic flux
Answer: B) B direction around current
Explanation: Curl fingers along rotation, thumb shows current.
41. Two parallel wires carrying currents in same direction experience:
A) Repulsion
B) Attraction
C) No force
D) Varies with distance
Answer: B) Attraction
Explanation: Like currents attract; basis of defining ampere.
42. Unit force per unit length between parallel wires:
A) 2×10⁻⁷ N/m per 1 A current separated by 1 m
B) 10⁻⁷ N/m
C) 1 N/m
D) 10⁻³ N/m
Answer: A) 2×10⁻⁷ N/m per 1 A current separated by 1 m
Explanation: Defines 1 Ampere.
43. Magnetic field at distance r from infinite wire decreases as:
A) 1/r²
B) 1/r
C) r²
D) r
Answer: B) 1/r
Explanation: Biot–Savart law: B ∝ 1/r.
44. Magnetic field outside a very long solenoid is:
A) Uniform
B) Zero
C) Maximum
D) Same as inside
Answer: B) Zero
Explanation: Fields from turns cancel outside.
45. Magnetic field inside toroid:
A) Uniform along circular path
B) Radial
C) Zero
D) Tangential but varying
Answer: A) Uniform along circular path
Explanation: Symmetry → B constant along each circular loop.
46. A current-carrying loop produces field:
A) Only at center
B) Along axis
C) Uniform everywhere
D) None
Answer: B) Along axis
Explanation: B strongest at center, decreases along axis.
47. Magnetic field at axis of circular coil of N turns:
A) B=μ_0 NI/2R
B) B=μ_0 I/2R
C) B=μ_0 I/R
D) B=μ_0 NI/R
Answer: A) B=μ_0 NI/2R
Explanation: Multiply single loop field by N turns.
48. Magnetic field at point on axis of short solenoid:
A) Uniform
B) Non-uniform
C) Zero
D) Radial
Answer: B) Non-uniform
Explanation: Only long solenoid approximates uniform field.
49. For long straight wire, magnetic field at 1 m from 1 A current:
A) 2×10^(-7) T
B) 10^(-7) T
C) 4×10^(-7) T
D) 8×10^(-7) T
Answer: A) 2×10^(-7) T
Explanation: B = μ₀ I / 2πr → μ₀ = 4π×10⁻⁷.
50. Magnetic field of coil increases by:
A) Increasing current
B) Increasing number of turns
C) Reducing coil radius
D) All of above
Answer: D) All of above
Explanation: B ∝ N I / R.
51. Electromagnetic induction is the phenomenon of:
A) Generation of magnetic field by current
B) Induction of current in a conductor due to changing magnetic field
C) Attraction between magnets
D) Magnetic field due to moving charges
Answer: B) Induction of current in a conductor due to changing magnetic field
Explanation: Faraday’s experiments showed changing flux induces current.
52. Faraday’s law states:
A) Induced emf ∝ magnetic field
B) Induced emf ∝ rate of change of magnetic flux
C) Induced emf ∝ current
D) Induced emf ∝ resistance
Answer: B) Induced emf ∝ rate of change of magnetic flux
Explanation: E=-dΦ_B/dt
53. Lenz’s law gives:
A) Direction of induced current
B) Magnitude of induced current
C) Both direction and magnitude
D) None
Answer: A) Direction of induced current
Explanation: Induced current opposes change in flux (conservation of energy).
54. Unit of magnetic flux (Φ) is:
A) Tesla
B) Weber
C) Ampere·turn
D) Henry
Answer: B) Weber (Wb)
Explanation: Flux = B·A, 1 Wb = 1 T·m².
55. Unit of induced emf is:
A) Tesla
B) Volt
C) Weber
D) Ampere
Answer: B) Volt
Explanation: EMF is potential difference induced.
56. A conductor moving in uniform magnetic field experiences:
A) Constant magnetic flux
B) Induced emf
C) Zero current
D) Zero voltage
Answer: B) Induced emf
Explanation: Motion changes flux through conductor → Faraday’s law.
57. Induced emf in a coil rotating in uniform B-field is maximum when:
A) Plane of coil parallel to field
B) Plane of coil perpendicular to field
C) Coil stationary
D) Coil axis perpendicular to field
Answer: B) Plane of coil perpendicular to field
Explanation: Flux changes fastest → max emf.
58. Induced emf in coil:
A) Zero if flux constant
B) Max if flux changes fastest
C) Always constant
D) Depends on coil material
Answer: A) Zero if flux constant
Explanation: No change in flux → no induced emf.
59. EMF induced in a coil of N turns:
A) E=NdΦ/dt
B) E=dΦ/dt
C) E=NΦ
D) E=NI
Answer: A) E=NdΦ/dt
Explanation: Faraday’s law generalized for N turns.
60. A bar magnet moving toward a coil induces:
A) Current in same direction as flux change
B) Current opposing flux change
C) Zero current
D) Infinite current
Answer: B) Current opposing flux change
Explanation: Lenz’s law ensures induced current opposes motion.
61. Magnetic flux through coil of area A in uniform B:
A) Φ=B/A
B) Φ=B⋅Acosθ
C) Φ=B+A
D) Φ=BA/θ
Answer: B) Φ=B⋅Acosθ
Explanation: θ = angle between B and normal to plane of coil.
62. Motional emf is generated due to:
A) Changing B-field
B) Motion of conductor in B-field
C) Resistance
D) Temperature
Answer: B) Motion of conductor in B-field
Explanation: Conductor cuts magnetic lines → induced emf.
63. EMF induced in rod of length l moving with velocity v perpendicular to B:
A) E=Blv
B) E=Bl/v
C) E=Blv^2
D) E=B/lv
Answer: A) E=Blv
Explanation: Motional emf formula: ε = B l v.
64. Eddy currents are induced in:
A) Insulators
B) Conductors
C) Non-conductors
D) Vacuum
Answer: B) Conductors
Explanation: Loop currents induced in bulk of conducting material.
65. Lenz’s law ensures:
A) Energy conservation
B) Maximum current
C) Minimum resistance
D) None
Answer: A) Energy conservation
Explanation: Induced current always opposes change → work done = energy.
66. Direction of induced current in moving magnet-coil system given by:
A) Right-hand rule
B) Left-hand rule
C) Fleming’s right-hand rule
D) Fleming’s left-hand rule
Answer: C) Fleming’s right-hand rule
Explanation: Right-hand rule for generators → direction of induced current.
67. Changing area of loop in uniform B produces:
A) No EMF
B) Induced EMF
C) Only flux
D) Only torque
Answer: B) Induced EMF
Explanation: Change in flux (B·A) → EMF induced.
68. Self-induction occurs due to:
A) Mutual induction
B) Changing current in same coil
C) Static magnetic field
D) Electrostatics
Answer: B) Changing current in same coil
Explanation: Time-varying current induces EMF in same coil → Lenz’s law.
69. Inductance unit:
A) Weber
B) Henry
C) Tesla
D) Volt
Answer: B) Henry (H)
Explanation: L = EMF / (dI/dt).
70. Mutual induction occurs when:
A) EMF induced in one coil due to current change in another coil
B) EMF induced in same coil
C) Magnetic field is uniform
D) Current constant
Answer: A) EMF induced in one coil due to current change in another coil
Explanation: Basis for transformers.
71. Transformer works on principle of:
A) Self-induction only
B) Mutual induction
C) Static electricity
D) Magnetic monopoles
Answer: B) Mutual induction
Explanation: Changing current in primary → changing flux → induces EMF in secondary.
72. Induced EMF in secondary coil:
A) Directly proportional to turns of secondary
B) Inversely proportional to primary turns
C) Independent of turns
D) Zero
Answer: A) Directly proportional to turns of secondary
Explanation: Faraday’s law: ε ∝ N dΦ/dt.
73. Energy stored in inductor L carrying current I:
A) U=1/2 LI^2
B) U=LI
C) U=1/2 I^2/L
D) U=I/L
Answer: A) U=1/2 LI^2
Explanation: Energy stored in magnetic field of inductor.
74. Back EMF in motor opposes:
A) Applied voltage
B) Magnetic field
C) Current flow
D) Resistance
Answer: C) Current flow
Explanation: Lenz’s law: induced EMF opposes cause (current) in coil.
75. EMF induced is maximum when:
A) Magnetic flux changes fastest
B) Flux constant
C) Coil stationary
D) Coil parallel to field
Answer: A) Magnetic flux changes fastest
Explanation: Rate of change of flux determines EMF magnitude.
76. The device which converts electrical energy into mechanical energy is:
A) Generator
B) Motor
C) Transformer
D) Dynamo
Answer: B) Motor
Explanation: Electric motor works on principle of force on current-carrying conductor in magnetic field.
77. The device which converts mechanical energy into electrical energy is:
A) Motor
B) Generator
C) Transformer
D) Battery
Answer: B) Generator
Explanation: Electromagnetic induction principle: rotating coil in B-field produces EMF.
78. The principle of DC motor is:
A) Electromagnetic induction
B) Force on current-carrying conductor in magnetic field
C) Mutual induction
D) Self-induction
Answer: B) Force on current-carrying conductor in magnetic field
Explanation: Torque on loop → rotation.
79. Torque on rectangular coil in motor is maximum when plane of coil is:
A) Parallel to magnetic field
B) Perpendicular to magnetic field
C) At 45° to field
D) Any orientation
Answer: B) Perpendicular to magnetic field
Explanation: Torque τ = μB sin θ → maximum at θ = 90°.
80. Back EMF in motor is produced due to:
A) Motion of coil in magnetic field
B) Resistance of coil
C) Supply voltage
D) Magnetic flux
Answer: A) Motion of coil in magnetic field
Explanation: EMF induced opposes applied voltage, limiting current.
81. Transformer operates on:
A) DC supply
B) AC supply
C) Constant current
D) None
Answer: B) AC supply
Explanation: Only changing flux (AC) induces EMF in secondary.
82. Step-up transformer increases:
A) Current
B) Voltage
C) Resistance
D) Power
Answer: B) Voltage
Explanation: V₂/V₁ = N₂/N₁ → N₂ > N₁ → voltage increases.
83. Step-down transformer decreases:
A) Voltage
B) Current
C) Resistance
D) Power
Answer: A) Voltage
Explanation: N₂ < N₁ → V₂ < V₁.
84. Efficiency of ideal transformer:
A) 50%
B) 75%
C) 100%
D) 0%
Answer: C) 100%
Explanation: Ideal → negligible losses, power in = power out.
85. Energy loss in transformer occurs due to:
A) Eddy currents
B) Hysteresis
C) Resistance of coil
D) All of the above
Answer: D) All of the above
Explanation: Real transformers have losses due to these factors.
86. Induced EMF in rotating coil of generator is maximum when plane of coil is:
A) Parallel to B-field
B) Perpendicular to B-field
C) Stationary
D) At 45°
Answer: A) Parallel to B-field
Explanation: Rate of change of flux maximum when plane parallel (axis perpendicular).
87. Frequency of AC induced in generator depends on:
A) Number of turns only
B) Speed of rotation only
C) Both speed and number of poles
D) Voltage applied
Answer: C) Both speed and number of poles
Explanation: f = (np)/120 for n rpm, p poles.
88. In AC generator, slip rings:
A) Connect coil to external circuit continuously
B) Reverse current direction
C) Act as brushes
D) None
Answer: A) Connect coil to external circuit continuously
Explanation: Allow AC output; contrast with commutator in DC.
89. In DC generator, commutator:
A) Produces AC
B) Converts AC to DC
C) Increases voltage
D) Reduces resistance
Answer: B) Converts AC to DC
Explanation: Segmented commutator reverses current direction in coil → DC output.
90. RMS value of AC:
A) Maximum value
B) Average value
C) V_rms=V_0/√2
D) Twice max value
Answer: C) V_rms=V_0/√2
Explanation: Standard relation for sinusoidal AC.
91. Peak value of AC current I₀ related to RMS I:
A) I_0=I_rms
B) I_0=√2 I_rms
C) I_0=I_rms/2
D) I_0=2I_rms
Answer: B) I_0=√2 I_rms
Explanation: RMS is effective value: I_rms=I_0/√2.
92. Induced EMF in rod moving in uniform B-field:
A) E=Blv
B) E=B/lv
C) E=Bv/l
D) E=Bv^2 l
Answer: A) E=Blv
Explanation: Motional EMF formula.
93. Induced current in short-circuited coil opposes:
A) Change in flux
B) Voltage supply
C) Resistance
D) None
Answer: A) Change in flux
Explanation: Lenz’s law.
94. A loop rotating in B-field experiences torque:
A) τ = μB sin θ
B) τ = μB cos θ
C) τ = μB
D) τ = 0
Answer: A) τ = μB sin θ
Explanation: Standard torque formula for magnetic dipole.
95. Hall effect helps measure:
A) Magnetic field
B) Charge carrier density
C) Current type (electrons/holes)
D) All of the above
Answer: D) All of the above
Explanation: Hall voltage depends on B, carrier density, and charge sign.
96. Direction of Hall voltage determined by:
A) Fleming’s left-hand rule
B) Right-hand rule
C) Lenz’s law
D) Biot–Savart law
Answer: B) Right-hand rule
Explanation: For conventional current and magnetic field, right-hand gives sign.
97. In magnetic braking, eddy currents:
A) Slow motion of conductor
B) Accelerate motion
C) Generate power only
D) Heat nothing
Answer: A) Slow motion of conductor
Explanation: Lenz’s law → currents oppose motion → braking effect.
98. Magnetic flux linkage in coil:
A) Φ × N
B) B × A
C) μ₀ I
D) None
Answer: A) Φ × N
Explanation: Flux through N turns → total flux linkage.
99. Energy stored in inductor:
A) U = ½ L I²
B) U = L I
C) U = LI²
D) U = ½ I²/L
Answer: A) U = ½ L I²
Explanation: Magnetic energy stored in inductor field.
100. Which of the following is NOT an application of electromagnetic induction?
A) Transformer
B) AC generator
C) Electric motor
D) Electric heater
Answer: D) Electric heater
Explanation: Heater works by resistive heating, not induction.