# Physics Questions and Answers – Motion in a Magnetic Field

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Motion in a Magnetic Field”.

1. A positive charge is moving vertically upwards. When it enters a region of magnetic field directed towards north, what is the direction of the force on the charge?
a) Up
b) Down
c) Left
d) Right

Explanation: According to Fleming’s left-hand rule, the magnetic force will act towards left. When the positive charge enters a region of magnetic field directed towards north, the magnetic force will act towards left.

2. An electron moving with a velocity of 15 ms-1 enters a uniform magnetic field of 0.2 T, along a direction parallel to the field. What would be its trajectory in this field?
a) Elliptical
b) Straight path
c) Helical
d) Circular

Explanation: The electron will continue to follow its straight path because a parallel magnetic field does not exert any force on the electron. So, there won’t be any change in its trajectory when the electron enters a uniform magnetic field.

3. In a certain arrangement, a proton does not get deflected while moving through a magnetic field region. Under what condition is it possible?
a) F = 0
b) F = 180 N
c) F = -180 N
d) F = 3600 N

Explanation: Magnetic force on a proton is Fm = qvBsin(θ).
A proton moving parallel or antiparallel to a magnetic field does not experience any force. Hence, the proton does not get deflected while moving through a magnetic field region.

4. Which of the following will experience a maximum force, when projected with the same velocity perpendicular to the magnetic field : (i) α-particle, and (ii) β-particle?
a) Both α-particle and β-particle
b) None
c) β-particle
d) α-particle

Explanation: F = qvB.
For α-particle, q = 2e, Fα = 2evB
For β-particle, q = e, Fβ = evB.
Thus, the α-particle will experience maximum force.

5. A charged particle in a plasma trapped in a magnetic bottle leaks out after a millisecond. What is the total work done by the magnetic field during the time the particle is trapped?
a) Maximum
b) Minimum
c) Zero
d) Depends on the strength of the magnetic field

Explanation: Work done is zero. Since a magnetic field exerts a force perpendicular to the direction of motion of the charged particle, no work is done by it on the charged particle. Therefore, the total work done by the magnetic field during the time the particle is trapped is zero.
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6. Force on a charge ‘q’ moving with a velocity ‘v’ in a magnetic field is given by F = qvB. State true or false.
a) True
b) False

Explanation: Force on a charge ‘q’ moving with a velocity ‘v’ in a magnetic field is given by:
F = qvB sinθ.
The direction of the force is given by Fleming’s left-hand rule.

7. A proton enters a magnetic field of flux density 5 T with a velocity of 5 × 107 ms-1 at an angle of 30o with the field. Find the force on the proton.
a) 0.2 × 10-11 N
b) 2 × 10-11 N
c) 20 × 10-11 N
d) 200 × 10-11 N

Explanation: F = qvB sinθ.
F = 1.6 × 10-19 × 5 × 107 × 5 × sin30o
F = 2 × 10-11 N.

Sanfoundry Global Education & Learning Series – Physics – Class 12.

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