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Electric and Magnetic Fields · Paper 2 — Free Physics SL Practice Questions

1MasterySAQ-SApplications in motors and electromagnets7 marksPaper 2~11 min

A loudspeaker uses an electromagnet to produce sound. A coil of

80

turns is placed in a radial magnetic field of strength

0.15\,\text{T}

. The length of wire in the magnetic field per turn is

0.040\,\text{m}

. A current of

0.50\,\text{A}

flows through the coil, exerting a force on the coil and moving the speaker cone.

(a)

Explain how the force on the coil changes when the current is reversed. [2 marks]

(b)

Calculate the total force on the coil when the current is

0.50\,\text{A}

. [2 marks]

(c)

In a loudspeaker, the current alternates at the frequency of the sound being produced. Explain why a radial magnetic field, rather than a uniform field with fixed orientation, is essential for the loudspeaker to reproduce sound accurately at all amplitudes. [3 marks]

Solutions

2MasterySAQ-SMagnetic fields and forces5 marksPaper 2~8 min

A solenoid used in an MRI machine has

N = 500

turns and a length of

L = 0.40\,\text{m}

. It carries a current of

I = 2.0\,\text{A}

. Permeability of free space:

\mu_0 = 4\pi \times 10^{-7}\,\text{T\,m\,A}^{-1}

(a)

State one characteristic of the magnetic field inside a long solenoid. - [1 mark]

(b)

Calculate the magnitude of the magnetic field inside the solenoid. - [2 marks]

(c)

A second solenoid has the same length and carries the same current, but has

1000

turns. Deduce the ratio of the magnetic field in the second solenoid to that in the first, and explain what physical change inside the solenoid produces this difference. [2 marks]

Solutions

3ChallengeSAQ-LApplications in motors and electromagnets7 marksPaper 2~11 min

A simple electric motor in a power tool consists of a rectangular coil of wire with

50

turns, each of area

8.0 \times 10^{-4}\,\text{m}^2

, placed in a uniform magnetic field of strength

0.40\,\text{T}

. The coil rotates about an axis perpendicular to the field. The motor is connected to a battery of emf

12\,\text{V}

, and the coil has a total resistance of

2.0\,\Omega

(a)

State the formula for the torque on a current-carrying coil in a magnetic field. [1 mark]

(b)

The coil carries a current of

3.0\,\text{A}

. Calculate the maximum torque on the coil. [2 marks]

(c)

When the motor runs at a constant speed, a back-emf of

4.0\,\text{V}

is induced in the coil. Calculate the current in the coil at this operating speed. [2 marks]

(d)

At a particular instant the plane of the coil makes angle of

60°

to the magnetic field. Explain why the torque at this instant differs from the maximum torque. [2 marks]

Solutions

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