Physics

Two students are discussing the advantages of magnetic brakes.

Student A says that because the magnetic force is proportional to velocity they provide a smoother braking action.

Student B says that because magnetic fields do no work, the brakes produce no heat.

Which of the students, if any, is/are correct?
A. Student A only
B. Student B only
C. Both student A and student B
D. Neither student A nor student B

A coil of wire is rotating about a vertical axis in a magnetic field that is directed $45^\circ$ below the horizontal:

Which set of changes will result in the largest reading on the voltmeter?
A. Double $r$ and halve $l$.
B. Increase both $l$ and $r$ by $50\%$.
C. Rotate the magnetic field to vertical and double $r$.
D. Rotate the magnetic field to horizontal and double $l$.

A magnet is swinging as a pendulum. Close below it is an aluminium (non-ferromagnetic) can. The can is free to spin around a fixed axis as shown.

Analyse the motion and energy transformations of both the can and the magnet.

Which of the following statements about AC induction motors is correct?
(A) There is no current produced in induction motors.
(B) Induction motors need slip rings to operate.
(C) The magnetic field rotates faster than the rotor.
(D) The rotor rotates faster than the magnetic field.

A rectangular coil has dimensions $3.0\ \mathrm{cm}\times4.0\ \mathrm{cm}$ and carries a current of $15.0\ \mathrm{mA}$ in a uniform magnetic field of $3.0\ \mathrm{T}$. The coil has 20 turns of wire and is free to rotate about the axis as shown.

If the coil is rotated by $25^\circ$, what is the torque acting on the coil?
(A) $9.79\times10^{-4}\ \mathrm{N\,m}$ clockwise
(B) $9.79\times {10}^{}$ anticlockwise (C) clockwise (D) anticlockwise