CBSE Class 12th Physics Moving Charges and Magnetism 100 MCQs

Physics – Moving Charges and Magnetism

Class 12 Physics (Chapter 4) • 100 Multiple Choice Questions

Read Chapter Notes

Question 1

The SI unit of magnetic field (B) is:

Correct Answer: Tesla (T)

Question 2

According to Biot-Savart's law, the magnetic field dB due to a current element Idl is directly proportional to:

Correct Answer: Current (I)

Question 3

Magnetic field at the center of a circular current-carrying loop of radius R is given by:

Correct Answer: B = μ₀I / (2R)

Question 4

Magnetic field due to a long straight current-carrying conductor at a distance r is:

Correct Answer: Inversely proportional to r

Question 5

The force F on a charge q moving with velocity v in a magnetic field B is given by:

Correct Answer: F = q(v × B)

Question 6

When a charged particle enters a uniform magnetic field perpendicular to its direction of motion, its path is:

Correct Answer: Circle

Question 7

Work done by the magnetic force on a moving charge is:

Correct Answer: Zero

Question 8

Lorentz force is the sum of:

Correct Answer: Electric and Magnetic force

Question 9

The frequency of revolution of a charged particle in a magnetic field is independent of its:

Correct Answer: Speed (or kinetic energy)

Question 10

A cyclotron is used to accelerate:

Correct Answer: Positive ions (Protons, Alpha particles)

Question 11

The value of permeability of free space (μ₀) is:

Correct Answer: 4π × 10⁻⁷ T·m/A

Question 12

Two parallel wires carrying currents in the same direction:

Correct Answer: Attract each other

Question 13

Ampere's Circuital Law states that the line integral of magnetic field B around any closed path is equal to:

Correct Answer: μ₀ times the net current enclosed

Question 14

The magnetic field inside a long ideal solenoid carrying current I with n turns per unit length is:

Correct Answer: B = μ₀nI

Question 15

Magnetic field outside a closely wound long solenoid is practically:

Correct Answer: Zero

Question 16

The force on a straight conductor of length L carrying current I placed in a uniform magnetic field B at an angle θ is:

Correct Answer: F = I L B sin θ

Question 17

A moving coil galvanometer can be converted into an ammeter by connecting a:

Correct Answer: Low resistance in parallel (Shunt)

Question 18

To convert a moving coil galvanometer into a voltmeter, we connect a:

Correct Answer: High resistance in series

Question 19

Magnetic dipole moment (m) of a current loop is given by:

Correct Answer: m = N I A

Question 20

Torque τ acting on a current loop placed in a uniform magnetic field B is:

Correct Answer: τ = m × B

Question 21

A moving coil galvanometer works on the principle that:

Correct Answer: A current-carrying coil experiences torque in a magnetic field

Question 22

The unit of magnetic dipole moment is:

Correct Answer: Ampere·meter² (A·m²) (and J/T is also correct, but A·m² is standard SI)

Question 23

Current sensitivity of a galvanometer can be increased by:

Correct Answer: Increasing the magnetic field (B)

Question 24

Increasing the current sensitivity of a galvanometer:

Correct Answer: May not necessarily increase voltage sensitivity

Question 25

The ratio of magnetic dipole moment to angular momentum (m/L) of a revolving electron is called:

Correct Answer: Gyromagnetic ratio

Question 26

The value of Gyromagnetic ratio of an electron is approximately:

Correct Answer: 8.8 × 10¹⁰ C/kg

Question 27

Minimum value of magnetic dipole moment of an atomic electron is called:

Correct Answer: Bohr Magneton

Question 28

A charge moving with constant velocity parallel to a uniform magnetic field will:

Correct Answer: Continue moving in a straight line

Question 29

If a charged particle enters a magnetic field at an acute angle to the field direction, its path is:

Correct Answer: Helical

Question 30

The radial magnetic field in a moving coil galvanometer is achieved by using:

Correct Answer: Concave cylindrical poles and soft iron core

Question 31

The pitch of a helical path of a charged particle moving in a magnetic field depends on:

Correct Answer: Velocity component parallel to B

Question 32

Biot-Savart Law is analogous to which law in electrostatics?

Correct Answer: Coulomb's Law

Question 33

For a given perimeter, which shape of a current carrying loop will have the maximum magnetic dipole moment?

Correct Answer: Circle

Question 34

A neutron enters a uniform magnetic field perpendicularly. What will be its path?

Correct Answer: Straight line (undeflected)

Question 35

The relation between μ₀, ε₀ and speed of light c in vacuum is:

Correct Answer: c = 1 / √(μ₀ ε₀)

Question 36

When a galvanometer is converted into an ammeter, its effective resistance:

Correct Answer: Decreases

Question 37

An ideal ammeter has a resistance of:

Correct Answer: Zero

Question 38

An ideal voltmeter has a resistance of:

Correct Answer: Infinity

Question 39

Magnetic field at a point on the axis of a circular loop at distance x from center is proportional to:

Correct Answer: (R² + x²)^(-3/2)

Question 40

Ampere's Circuital Law is valid only for:

Correct Answer: Steady currents

Question 41

What is the magnetic field inside a hollow pipe carrying direct current?

Correct Answer: Zero

Question 42

Force between two long parallel wires carrying current I₁ and I₂ separated by distance d is:

Correct Answer: F/L = (μ₀ I₁ I₂) / (2πd)

Question 43

One ampere is defined as that constant current which produces a force of ______ per meter of length between two infinitely long parallel wires 1 m apart in vacuum.

Correct Answer: 2 × 10⁻⁷ N

Question 44

Magnetic field inside a Toroid having N turns, current I, and mean radius r is:

Correct Answer: B = μ₀NI / (2πr)

Question 45

Two parallel wires carrying currents in opposite directions:

Correct Answer: Repel each other

Question 46

A proton and an alpha particle enter a uniform magnetic field with the same velocity. The ratio of their radii (r_p : r_α) is:

Correct Answer: 1 : 2

Question 47

For a galvanometer, the restoring torque per unit twist of suspension is k. The deflection θ for current I is given by:

Correct Answer: θ = (NAB / k) I

Question 48

A charge Q is moving v velocity along the direction of a magnetic field B. The magnetic force acting on it is:

Correct Answer: Zero

Question 49

If a uniform magnetic field is directed out of the page and an electron moves rightward, what is the direction of the magnetic force?

Correct Answer: Upward

Question 50

To increase the range of an ammeter, we must:

Correct Answer: Decrease the value of shunt resistance

Question 51

What is the dimensional formula for magnetic field (B)?

Correct Answer: [M T⁻² A⁻¹]

Question 52

Work done by the Lorentz magnetic force is always zero because:

Correct Answer: Force is perpendicular to velocity

Question 53

The kinetic energy of a charged particle moving in a uniform magnetic field:

Correct Answer: Remains constant

Question 54

Does a cyclotron accelerate electrons?

Correct Answer: No, because their mass is too small (relativistic mass increases quickly)

Question 55

The cyclotron frequency ν is given by:

Correct Answer: ν = qB / (2πm)

Question 56

Magnetic field at the ends of a long solenoid is:

Correct Answer: μ₀nI / 2

Question 57

Does the magnetic field of a long ideal solenoid depend on its radius?

Correct Answer: No, it is independent of radius

Question 58

A current carrying loop behaves as a:

Correct Answer: Magnetic dipole

Question 59

Torque on a current loop is zero when the angle between the area vector and magnetic field is:

Correct Answer: 0° or 180°

Question 60

In a moving coil galvanometer, a phosphor-bronze strip is used for suspension because it has:

Correct Answer: Low restoring torque per unit twist

Question 61

The value of shunt resistance (S) required to convert a galvanometer of resistance G into an ammeter of range I is:

Correct Answer: S = (I_g / (I - I_g)) G

Question 62

The value of series resistance (R) required to convert a galvanometer (resistance G) to a voltmeter of range V is:

Correct Answer: R = (V / I_g) - G

Question 63

A magnetic field can be produced by:

Correct Answer: Both moving charge and changing electric field

Question 64

The direction of magnetic field produced by a linear current is given by:

Correct Answer: Right Hand Thumb Rule

Question 65

Magnetic field at the center of a circular coil of N turns and radius R carrying current I is:

Correct Answer: B = μ₀NI / (2R)

Question 66

If you look at a current loop and the current is anti-clockwise, that face behaves as a:

Correct Answer: North pole

Question 67

If the current in a circular loop is clockwise, the face behaves as a:

Correct Answer: South pole

Question 68

Force on a closed current loop in a uniform magnetic field is:

Correct Answer: Zero

Question 69

Can a magnetic field change the speed of a charged particle?

Correct Answer: No, speed remains constant

Question 70

Can a magnetic field change the velocity of a charged particle?

Correct Answer: Yes, by changing its direction

Question 71

An electron and a proton have the same momentum. They enter a perpendicular magnetic field. The ratio of their radii is:

Correct Answer: 1:1 (r = p / qB; both have same p and same magnitude of q)

Question 72

An electron and proton have the same kinetic energy. They enter a perpendicular B field. Which will have a larger radius?

Correct Answer: Proton (r = √(2mK) / qB; proton has larger mass)

Question 73

Time period of a charged particle in a magnetic field is given by:

Correct Answer: T = 2πm / (qB)

Question 74

Maximum kinetic energy of a particle in a cyclotron of radius R is:

Correct Answer: K = q²B²R² / (2m)

Question 75

For two infinite parallel wires with equal currents in the same direction, the magnetic field exactly midway between them is:

Correct Answer: Zero

Question 76

The soft iron core in a moving coil galvanometer is used to:

Correct Answer: Increase the magnetic field and make it radial

Question 77

Voltage sensitivity of a galvanometer is defined as:

Correct Answer: Deflection per unit voltage

Question 78

Voltage sensitivity (V_s) and Current sensitivity (I_s) are related by:

Correct Answer: V_s = I_s / R

Question 79

Gyromagnetic ratio for an electron is equal to:

Correct Answer: e / 2m

Question 80

Value of Bohr Magneton (μ_B) is approximately:

Correct Answer: 9.27 × 10⁻²⁴ A·m²

Question 81

Magnetic field at the center of a square loop of side 'a' carrying current I is:

Correct Answer: B = 2√2 μ₀I / (πa)

Question 82

Dimension of permeability of free space (μ₀) is:

Correct Answer: [M L T⁻² A⁻²]

Question 83

If a flexible spring is passed a current, it will:

Correct Answer: Compress (adjacent coils carry current in same direction and attract)

Question 84

Magnetic force between two moving charges is basically due to:

Correct Answer: Interaction of their magnetic fields

Question 85

In a moving coil galvanometer, the restoring torque is provided by:

Correct Answer: Suspension wire (and hair spring)

Question 86

The magnetic field generated by a steady current is:

Correct Answer: Time-independent

Question 87

A charge 'e' moves in a circle of radius 'r' with uniform speed 'v'. The magnetic field at the center is:

Correct Answer: B = μ₀ev / (4πr²)

Question 88

An electron beam is moving horizontally from South to North. In a downward magnetic field, it will be deflected towards:

Correct Answer: West

Question 89

Lorentz force is given by the formula:

Correct Answer: F = q(E + v × B)

Question 90

If a charged particle moves undeflected through orthogonal E and B fields, its velocity is:

Correct Answer: v = E / B

Question 91

A device based on crossed E and B fields used to select particles of a specific velocity is called:

Correct Answer: Velocity selector

Question 92

Biot-Savart Law in vector form is:

Correct Answer: dB = (μ₀/4π) I(dl × r) / r³

Question 93

Magnetic field inside a thick copper wire of radius R carrying current I at a distance r < R from center is:

Correct Answer: B = μ₀Ir / (2πR²)

Question 94

The magnetic field generated by a toroid is confined entirely:

Correct Answer: Inside the core of the toroid

Question 95

According to Ampere’s law, if no current is enclosed by a loop, the line integral of B is:

Correct Answer: Zero

Question 96

What is the angle between velocity and magnetic force on a moving charge?

Correct Answer: 90°

Question 97

If a proton is accelerated through a potential difference V and enters a magnetic field, its radius is proportional to:

Correct Answer: √V (since K = qV and r ∝ √K)

Question 98

Two parallel wires carrying currents I₁ and I₂ exert a force F on each other. If current in both is doubled, the new force is:

Correct Answer: 4F

Question 99

A long solenoid has n turns per unit length. If it is cut into two equal halves, the magnetic field at the center of one half will:

Correct Answer: Remain unchanged (if considered highly long, B depends on n, not total length)

Question 100

The magnetic moment of an electron revolving in the nth Bohr orbit is proportional to:

Correct Answer: n