Physics Questions and Answers – Torque on Current Loop & Magnetic Dipole

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Torque on Current Loop & Magnetic Dipole”.

1. What torque acts on a 50 turn coil of 200 cm2 area carrying a current of 20 A held with its axis at right angles to a uniform magnetic field of 0.2T?
a) 8 Nm
b) 0.8 Nm
c) 4 Nm
d) 0.8 Nm
View Answer

Answer: c
Explanation: Torque = NIBA sinθ.
Torque = 50 × 20 × 0.2 × 200 × 10-4 × sin (90)
Torque = 4Nm.
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2. What is the maximum torque on a rectangular coil of area 4 cm × 80 cm of 400 turns, when carrying a current of 103 A in a magnetic field of 0.2 T?
a) 1250 Nm
b) 2560 Nm
c) 3600 Nm
d) 1985 Nm
View Answer

Answer: b
Explanation: Torque (Max) = NIBA.
Τmax = 400 × 103 × 0.2 × 320 × 10-4
Tmax = 2560 Nm.

3. Under which of these conditions, the torque acting on the current carrying loop is minimum?
a) θ = 0o
b) θ = 90o
c) θ = 270o
d) θ = 450o
View Answer

Answer: a
Explanation: When θ = 0o ➔ τ = 0.
The torque is minimum when the plane of the loop is perpendicular to the magnetic field, so the angle θ should be 0o.
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4. Identify the factor on which the torque on a planar current loop does not depend.
a) Current
b) Strength of magnetic field
c) Area of the loop
d) The shape of the loop
View Answer

Answer: d
Explanation: Torque on a planar current loop depends upon current, the strength of the magnetic field, area of the loop and the orientation of the loop in the magnetic field. It is independent of the shape of the loop.

5. Give the SI unit of magnetic pole strength.
a) Tm
b) Am2
c) Am
d) Tm2
View Answer

Answer: c
Explanation: The SI unit of magnetic pole strength is ampere meter (Am).
The strength of a magnetic pole is said to be one-ampere meter if it repels an equal and similar pole with a force of 10-7 N when placed in vacuum at a distance of one meter from it.
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6. The factor \((\frac {eh}{4\pi m_e})\) is the least value of the magnetic moment. This natural unit of magnetic moment is called Planck’s constant.
a) True
b) False
View Answer

Answer: b
Explanation: The factor \((\frac {eh}{4\pi m_e})\) is the least value of the magnetic moment. This natural unit of magnetic moment is called Bohr magneton. The value of Bohr magneton is 9.27 × 10-24 Am2. So, that makes the above statement false.

7. ‘X’ is the product of the pole strength of either magnetic pole and the magnetic length of the magnetic dipole. Identify X.
a) Magnetic dipole
b) Magnetic dipole moment
c) Magnetic pole strength
d) Magnetic dipole length
View Answer

Answer: b
Explanation: The product of the pole strength of either magnetic pole and the magnetic length of the magnetic dipole is called its magnetic dipole moment. The SI unit of magnetic dipole moment is Am2.
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8. Under which of these conditions, the torque acting on the current-carrying loop is minimum?
a) θ = 90o
b) θ = 0o
c) θ = 180o
d) θ = 360o
View Answer

Answer: a
Explanation: When θ = 90o ➔ τ = NIBA.
The torque is maximum when the plane of the loop is parallel to the magnetic field. So the angle θ has to be 90o.

9. Calculate the torque on a 500 turn rectangular coil of length 20 cm and breadth 10 cm, carrying a current of 30 A, when placed making an angle of 60o with a magnetic field of 0.5 T?
a) 625 Nm
b) 125 Nm
c) 75 Nm
d) 25 Nm
View Answer

Answer: c
Explanation: Torque = NIBA cosθ [Angle between field and plane of coil].
Torque = 500 × 30 × 0.5 × 200 × 10-4 × cos (60)
Torque =75 Nm.
Therefore, the torque is 75 Nm.
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10. Give the SI unit of magnetic dipole moment.
a) Am
b) Am2
c) Tm2
d) Tm
View Answer

Answer: b
Explanation: Magnetic dipole moment = Current × Area.
SI unit of current = Ampere (A)
SI unit of area = meter2 (m2)
So, the SI unit of magnetic dipole moment = A × m2 = Am2

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