1. 1
   A technician suspends a straight, current-carrying wire horizontally between the poles of a strong horseshoe magnet, so that the wire runs perpendicular to the magnetic field lines. When the current is switched on, the wire is pushed sharply upwards.

   (a)

   Which effect explains this observation?

   [1 mark]

   • AThe motor effect: a current-carrying conductor in a magnetic field experiences a force
   • BThe generator effect: a changing magnetic field induces a potential difference
   • CInduced magnetism, causing the wire itself to become permanently magnetised
   • DStatic discharge between the wire and the magnet

   (b)

   The technician then rotates the wire so that it lies parallel to the magnetic field lines between the magnet's poles, and switches the current on again at the same size. What force does the wire experience in this new orientation?

   [1 mark]

   • AA force twice as large as before, in the same direction
   • BNo force at all, since the wire is not cutting through the magnetic field lines
   • CA force in the opposite direction to before, of the same size
   • DA force that increases continuously over time

   (c)

   Describe two changes the technician could make to increase the size of the force on the wire when it is back in its original orientation, perpendicular to the field.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An engineer designs a simple loudspeaker in which a coil of wire, carrying a varying current, sits inside the magnetic field of a permanent magnet attached to a paper cone.

   (a)

   Which rule can the engineer use to predict the direction of the force on the current-carrying coil for a given current direction and field direction?

   [1 mark]

   • AThe principle of moments
   • BThe law of reflection
   • CNewton's Third Law
   • DFleming's left-hand rule

   (b)

   When the audio signal reverses the direction of the current in the coil, what happens to the direction of the force on the coil (and therefore the paper cone), assuming the magnetic field direction stays the same?

   [1 mark]

   • AThe force disappears completely
   • BThe direction of the force stays exactly the same
   • CThe direction of the force reverses
   • DThe force becomes twice as strong but in the same direction

   (c)

   Explain how the coil repeatedly reversing direction, pushed back and forth by the varying current in the magnetic field, allows the loudspeaker's paper cone to produce sound.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A single rectangular coil of wire is mounted on an axle between the poles of a magnet, and a direct current is passed through it, causing the coil to begin rotating.

   (a)

   Explain, in terms of the forces on the two long sides of the coil, why the coil experiences a turning effect (rotates) rather than simply moving in a straight line.

   [3 marks]

   (b)

   Explain what would happen to the direction of rotation of the coil if a split-ring commutator were used to reverse the current in the coil every half turn, and why this component is necessary for the coil to keep rotating continuously in the same direction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineer is designing a small electric motor for a cordless drill, consisting of a coil of wire wound many times around a soft iron core, mounted on an axle between the poles of a permanent magnet, with a split-ring commutator connecting the coil to a battery.

   (a)

   Explain, from the moment the battery is connected, how the drill's motor produces continuous rotation, referring to the motor effect, Fleming's left-hand rule, and the role of the commutator.

   [6 marks]

   (b)

   The engineer wants to increase the turning force (torque) produced by the drill's motor without changing the size of the battery. Explain three separate design changes the engineer could make to the motor to increase its turning force, justifying each using the factors that affect the size of the force produced by the motor effect.

   [6 marks]

   Total for question 4: 12 marks

End of questions