Magnetic flux density and magnetic forcesEdexcel International A Level Physics: Subtopic test

1. 1
   A proton (charge +1.60 × 10⁻¹⁹ C) moves at 2.0 × 10⁶ m s⁻¹ and enters a region of uniform magnetic field of flux density 0.40 T. Its velocity is perpendicular to the field direction.

   (a)

   The magnitude of the force on the proton as it enters the field is

   [1 mark]

   • A1.3 × 10⁻¹³ N
   • B3.2 × 10⁻¹³ N
   • C8.0 × 10⁻²⁶ N
   • D1.3 × 10⁻¹⁹ N

   (b)

   Which statement correctly describes the direction of the force on the proton?

   [1 mark]

   • AIn the same direction as the velocity
   • BPerpendicular to both the velocity and the field
   • CIn the opposite direction to the velocity
   • DIn the same direction as the field

   (c)

   Explain why the proton follows a circular path in the field while its kinetic energy stays constant.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A circular coil of 150 turns and radius 2.0 cm is placed in a uniform magnetic field of flux density 0.30 T. In its starting position the plane of the coil is perpendicular to the field lines.

   (a)

   The magnetic flux through the coil in its starting position is

   [1 mark]

   • A5.7 × 10⁻² Wb
   • B1.3 × 10⁻³ Wb
   • C3.8 × 10⁻⁴ Wb
   • D1.9 × 10⁻⁴ Wb

   (b)

   The coil is turned so that the normal to its plane makes an angle of 60° with the field. The flux linkage is now

   [1 mark]

   • A4.9 × 10⁻² Wb turns
   • B5.7 × 10⁻² Wb turns
   • C1.9 × 10⁻⁴ Wb turns
   • D2.8 × 10⁻² Wb turns

   (c)

   State what is meant by flux linkage and explain why the flux linkage is zero when the plane of the coil is parallel to the field.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A straight horizontal copper wire of length 0.12 m and mass 3.0 g carries a current of 8.5 A in a uniform horizontal magnetic field of flux density 0.25 T. The field is directed due north and the current flows due east, so the wire is perpendicular to the field.

   (a)

   Calculate the magnitude of the magnetic force on the wire, and the magnitude of the force if the wire were turned in the horizontal plane to make an angle of 30° with the field.

   [3 marks]

   (b)

   Use Fleming's left-hand rule to deduce the direction of the force on the wire when it is perpendicular to the field, and deduce what happens to the wire if it is free to move.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student uses a top-pan balance to measure the flux density between the poles of a U-shaped magnet. The magnet rests on the balance. A rigid horizontal wire is clamped so that 6.0 cm of it lies perpendicular to the uniform field between the poles, without touching the magnet. With no current the balance reads 245.0 g. When a current of 4.0 A flows in the wire the reading rises to 256.0 g.

   (a)

   The reading on the balance rises when the current flows. Explain why, and use the readings to determine the flux density between the poles.

   [6 marks]

   (b)

   The student turns the clamp in the horizontal plane so that the wire makes an angle of 30° with the field direction. The current is still 4.0 A. Predict the new balance reading, showing your working, and suggest how the student could make the value of B obtained from the experiment more reliable. Justify your suggestions.

   [6 marks]

   Total for question 4: 12 marks

End of questions