A machine-monitoring vibration sensor contains a coil that moves towards a stationary bar magnet when the machine casing vibrates. This movement produces a positive pulse.
An engineer turns the magnet around so that its other pole faces the coil. The coil repeats the same movement at the same speed and produces a negative pulse.
Which statement explains the change in polarity?
Turning the magnet around leaves a negative potential difference across the coil, which stays there even while the coil is not moving.
Turning the magnet around weakens the field at the coil, and a weaker field gives a negative pulse instead of a positive one.
Turning the magnet around makes the field through the coil change faster, and a faster change gives a negative pulse instead of a positive one.
Turning the magnet around makes the field through the coil point the opposite way, so the field changes in the opposite direction during the movement.
92 exam-style questions on Edexcel GCSE Physics 13.1 Inducing a current, covering 13.1.1 Producing an electric current by induction, 13.1.2 Factors affecting the induced potential difference, 13.1.3 Alternators and dynamos, and 13.1.4 Microphones and loudspeakers. Each one has a worked solution and a mark scheme showing where the marks go.