Magnetic fields and forcesEdexcel A-Level Physics: Subtopic test

1. 1
   A 0.12 m length of straight copper wire carries a steady current of 5.0 A. It lies horizontally at right angles to a uniform horizontal magnetic field of flux density 0.30 T between the flat poles of a large magnet.

   (a)

   Calculate the magnitude of the force on the wire.

   [1 mark]

   • A0.0072 N
   • B0.50 N
   • C0.18 N
   • D0.036 N

   (b)

   The field points towards the north and the conventional current flows towards the east. In which direction is the force on the wire?

   [1 mark]

   • AVertically upwards
   • BVertically downwards
   • CTowards the east
   • DTowards the north

   (c)

   The wire is turned, in the horizontal plane, so that it makes an angle of 30° with the field lines. The current is unchanged. Calculate the force on the wire.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A proton enters a region of uniform magnetic field of flux density 0.050 T with a speed of 2.4 × 10⁶ m s⁻¹, moving at right angles to the field lines. The charge on a proton is 1.60 × 10⁻¹⁹ C and its mass is 1.67 × 10⁻²⁷ kg.

   (a)

   Which of the following is the magnitude of the magnetic force on the proton as it enters the field?

   [1 mark]

   • A2.0 × 10⁻²² N
   • B1.2 × 10⁵ N
   • C3.8 × 10⁻¹⁴ N
   • D1.9 × 10⁻¹⁴ N

   (b)

   Which statement correctly describes what happens to the speed of the proton while it is in the field, and why?

   [1 mark]

   • AIt increases, because the force acts along the direction of motion
   • BIt stays constant, because the force is always perpendicular to the velocity and does no work on the proton
   • CIt decreases, because the magnetic force opposes the motion
   • DIt stays constant, because the magnetic force on a moving charge is zero

   (c)

   Calculate the magnitude of the acceleration of the proton as it enters the field.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A flat rectangular coil of 250 turns measures 8.0 cm by 5.0 cm. It is placed in a uniform magnetic field of flux density 0.18 T, with the plane of the coil initially perpendicular to the field lines.

   (a)

   Calculate the magnetic flux through the coil and the flux linkage of the coil, and state the unit of flux linkage.

   [3 marks]

   (b)

   The coil is now rotated so that the normal to its plane makes an angle of 60° with the field lines. Calculate the new flux linkage and explain, in terms of field lines, why it has changed. State the angle between the normal and the field at which the flux linkage would be zero.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A single-turn rectangular coil PQRS is mounted on a horizontal axle so that it can rotate freely between the poles of two magnets, which produce a uniform horizontal field of flux density 0.25 T. The long sides PQ and RS are each 0.080 m long, parallel to the axle and 0.060 m apart, on opposite sides of it. The short sides QR and SP join them. At the start the plane of the coil is parallel to the field lines, and a steady current of 3.0 A is then switched on.

   (a)

   Explain why the coil begins to turn when the current is switched on. Include a calculation of the force on side PQ and of the turning effect (moment) of the pair of forces about the axle.

   [6 marks]

   (b)

   The student wants to double the turning effect on the coil. Evaluate three ways of achieving this, and conclude which is best.

   [6 marks]

   Total for question 4: 12 marks

End of questions