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Use Lenz’s law to explain whether end O or end X of the rotor is at the higher potential.

Opposing force must produce an anticlockwise moment to oppose the clockwise rotation. From Fleming’s Left Hand rule, current flows from O to X. Current flows from negative to positive inside an e.m.f. source, therefore X is at a higher potential.

9702_s25_qp_44-Q7

By reference to electromagnetic induction and to conservation of energy, explain why the oscillations are damped.

The oscillating magnet induces an e.m.f. and current in the closed circuit. The current dissipates thermal energy in the resistor, which is supplied by the mechanical energy of the oscillations.

The procedure in (a) is repeated after replacing the resistor with one of greater resistance. Suggest, with a reason, the effect of this change on the oscillations.

Greater resistance reduces the induced current, decreasing the thermal dissipation rate. Consequently, oscillations are less damped and continue longer.

9702_s25_qp_41-Q7

Explain how an electric field can be used with a magnetic field to ensure that the particle now passes through point Z.

An electrical field is applied downwards. Electric force on particle is in opposite direction to magnetic force on particle. Particle is undeflected and passes through Z when magnitudes of magnetic and electric forces are equal

9702_m25_qp_42-Q7

Use Faraday’s law to show that the variation of E with t is given by

$$E=Blat$$

where $a$ is the acceleration of the rod.

$$v=at$$ $$s=v\Delta t$$

Therefore distance moved by rod in time $\Delta t$ is

$$s=at\Delta t$$

Area of magnetic field cut in time $\Delta t$ is

$$A=lat\Delta t$$

Magnetic flux given by

$$\Phi =BA=Blat\Delta t$$

Faraday’s Law:

$$E=\frac{\Delta \Phi }{\Delta t}=\frac{Blat\Delta t}{\Delta t}=Blat$$

Catching points

• ferrous core increases magnetic flux density