Torque on a current loop in a uniform field — practice questions

Question 1 · difficulty L1 · recall

The torque magnitude on a single-turn current loop of area A in uniform field B is

• A. IAB cosθ
• B. IB/A
• C. IAB sinθ
• D. IA/B

Question 2 · difficulty L1 · understanding

For an N-turn coil, the magnetic dipole moment magnitude is

• A. IA/N
• B. NIB
• C. N²IA
• D. NIA

Question 3 · difficulty L2 · understanding

Torque on a current loop is maximum when its magnetic moment is

• A. perpendicular to B
• B. parallel to B
• C. antiparallel to B
• D. zero but area finite

Question 4 · difficulty L2 · application

A loop has magnetic moment 0.30 A m² in a 0.20 T field at 30°. Its torque is

• A. 0.060 N m
• B. 0.030 N m
• C. 0.015 N m
• D. 0.12 N m

Question 5 · difficulty L3 · application

If the number of turns and current are both doubled at fixed area and field orientation, torque becomes

• A. twice
• B. half
• C. four times
• D. unchanged

Question 6 · difficulty L3 · application

A rectangular loop in a uniform magnetic field can experience zero net force but nonzero torque because

• A. all segment forces point the same way
• B. magnetic field does work around the loop
• C. current vanishes on two sides
• D. opposite forces form a couple

Question 7 · difficulty L4 · analysis

A current loop is initially aligned with its magnetic moment parallel to B. A small rotation away from this position produces a torque that tends to

• A. restore the loop toward parallel alignment
• B. drive it toward 90° indefinitely
• C. reverse the current automatically
• D. increase its area

Question 8 · difficulty L4 · analysis

A loop's area is halved while its current is doubled, with N, B and angle unchanged. The torque

• A. doubles
• B. remains unchanged
• C. halves
• D. becomes one-fourth

Question 9 · difficulty L5 · analysis

A coil of magnetic moment μ is rotated from θ=30° to θ=60° in the same field. The ratio τ60/τ30 is

• A. 2
• B. 1/sqrt(3)
• C. sqrt(3)
• D. 1

Question 10 · difficulty L5 · analysis

A current loop is placed in a uniform field with μ antiparallel to B. Its instantaneous torque is zero, but this orientation is

• A. stable equilibrium
• B. a state of maximum restoring torque
• C. impossible for a current loop
• D. unstable equilibrium