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Edexcel | Physics WPH0 | W 18 4 1 | 1 hour 30 minutes

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18

The photograph shows a demonstration electric motor.

A coil of wire is mounted on an axle so that it can rotate freely. Magnets are placed

either side of the coil to provide a magnetic field.

When there is a current in the coil, the coil rotates.

The diagrams represent the coil and magnets viewed from the end and from above

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(a)

2 Mark s

The coil rotates clockwise as seen from the end AB in the diagram.

Explain the direction of the current in the coil

Correctly describes direction

e.g. current from C to A

Or current flows CABD (could start with any letter)

Or current flows anticlockwise

Or current is out of the page at A

Or current is into the page at B

By (Fleming’s) left hand rule Or by FLHR/LHR

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(b)

2 Mark s

Use of F = BIl

F = 0.033 N

Example of calculation

F= 0.074 T × 0.29 A × 0.048 m × 32

F = 0.033 N

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(c)

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(c) (I)

3 Mark s

(QWC – Work must be clear and organised in a logical manner using technical

wording where appropriate)

Change in flux linkage as coil turns

Or wires in coil cut lines of magnetic flux/field

This induces an e.m.f.

By Lenz’s law, the induced e.m.f. is in the opposite direction to

the applied potential difference

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(c) (II)

3 Mark s

A wire of length l is moving at speed v through a magnetic field of magnetic flux

density B, as shown.

Show that the e.m.f. ? produced is given by

See φ = BA ( accept ε = BA/t or ε = BAN/t)

Produces correct expression for area Use ε = dφ/dt leading to ε = Blv

Examples of derivation

φ = BA

φ = Bls ε = dφ/dt = Bls/t ε = Blv

φ = BA

s = vt

so A = lvt

so BA = Blvt ε = dφ/dt = Blvt/t = Blv

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(c) (III)

3 Mark s

The coil of the motor rotates at 9.0 rotations per second.

Calculate the maximum e.m.f. produced across the side AC of the coil.

width AB of coil = 2.4 cm

Maximum e.m.f. =

Use of ε = Blv

Use of v = 2πr/t

Or use of v = ωr and ω = 2π/t

ε = 0.077 V

(Use of ε = BAN/t leading to an answer of ε = 0.025 V gains one mark)

Example of calculation

v = 2π × 0.012 m × 9 / 1 s

= 0.077 V

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(c) (IV)

2 Mark s

The current in the coil varies as the coil rotates because the back e.m.f. is not constant.

Suggest why the back e.m.f. is not constant.

Angle of wire’s velocity to field varies (from 90° to zero) as it rotates

Or Coil is out of the magnetic field for part of the cycle

Or The angle of the coil to the magnetic field varies as it rotates

Or The wire cuts the magnetic flux in the opposite direction (every half turn)

Rate of change of flux varies (from maximum to zero)

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(d)

2 Mark s

Use of a data logger

Connected to a current sensor in series

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