1. 1
   A cyclist's bicycle has a small dynamo pressed against the tyre; as the wheels turn, the dynamo's coil rotates within a magnetic field, generating the electricity that lights the bicycle's lamp.

   (a)

   A cyclist's bicycle has a small dynamo that presses a wheel against the tyre; as the bicycle moves, the dynamo's coil rotates within a magnetic field, lighting the bicycle's lamp. What is happening inside the dynamo to produce this lighting effect?

   [1 mark]

   • AAn e.m.f. is being induced in the coil because it is moving relative to the magnetic field
   • BThe coil is storing charge like a capacitor
   • CThe magnetic field is being destroyed by the moving coil
   • DThe lamp itself creates the magnetic field needed

   (b)

   The cyclist pedals faster, making the dynamo's coil rotate more quickly within the magnetic field. What effect does this have on the lamp?

   [1 mark]

   • AThe lamp becomes dimmer, since faster movement reduces the induced e.m.f.
   • BThe lamp becomes brighter, since the induced e.m.f. is larger when the coil moves faster
   • CThe lamp brightness is unaffected by the speed of pedalling
   • DThe lamp switches off completely at high speed

   (c)

   State two factors, other than the speed of rotation, that affect the magnitude of the e.m.f. induced in the dynamo's coil.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   At a hydroelectric power station, water flowing through a turbine spins a large coil within a strong magnetic field, and an engineer explains the underlying principle of electromagnetic induction to a group of visiting students using a smaller demonstration in the visitor centre.

   (a)

   Which of these is an example of the changing magnetic field method of inducing an e.m.f., rather than a conductor physically moving through a field?

   [1 mark]

   • ARotating a coil at constant speed inside a fixed magnetic field
   • BSliding a straight wire steadily between the poles of a fixed magnet
   • CMoving a bar magnet in and out of a stationary coil connected to a meter
   • DMoving a magnetic compass across a table

   (b)

   In the visitor centre demonstration, the engineer moves the N pole of a bar magnet quickly into a coil connected to a sensitive meter, and the meter deflects in one direction. According to the direction of the induced e.m.f., what happens as the magnet is then pulled back out of the coil?

   [1 mark]

   • AThe coil becomes permanently magnetised instead of showing a meter deflection
   • BThe meter deflects in exactly the same direction as before
   • CThe meter shows no deflection at all when the magnet is removed
   • DThe meter deflects in the opposite direction, since the induced e.m.f. opposes the change causing it

   (c)

   Describe how the engineer could use the bar magnet, a coil of wire and a sensitive meter to demonstrate electromagnetic induction to the visiting students.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A physics teacher pushes a straight copper rod downward through a horizontal magnetic field, with the rod connected at both ends by flexible wires to a circuit containing a sensitive meter, to demonstrate the direction of an induced current.

   (a)

   State the three quantities whose relative directions determine the direction of the induced current in the rod.

   [3 marks]

   (b)

   The teacher then pushes the rod downward through the same field twice as fast as before, and repeats the experiment using a rod of the same length but held so that a greater length of it moves through the field region at once. Explain how each of these two changes would affect the size of the induced e.m.f., and explain why reversing the direction the rod moves would affect the reading on the meter.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A wind turbine converts the kinetic energy of moving air into electrical energy using a generator, in which the wind turns a large coil (or magnet assembly) inside a strong magnetic field, and engineers must design turbines suited to different wind conditions around the world.

   (a)

   Explain, using the ideas of electromagnetic induction and the factors affecting the magnitude of an induced e.m.f., how the wind turbine produces an increasing e.m.f. as wind speed increases, and explain why turbine engineers might choose to use a stronger magnet or a coil with more turns in a turbine designed for locations with generally lower average wind speeds.

   [6 marks]

   (b)

   An engineer notices that when the wind turbine's generator is connected to the electrical grid and supplying a large current, it becomes noticeably harder for the wind to keep the turbine's blades turning at the same speed compared with when it is disconnected from the grid (producing no current). Explain, using the idea that an induced e.m.f./current always opposes the change causing it, why supplying a larger electrical current to the grid makes the turbine harder to turn, and explain the energy transfer that is taking place in this process.

   [6 marks]

   Total for question 4: 12 marks

End of questions