An AC voltage is induced in a coil of wire when it is rotated in a magnetic field. This voltage can be displayed on the screen of an instrument called an oscilloscope, which shows the variation of the induced voltage on the y-axis plotted against time on the x-axis. The oscilloscope trace below is the result of rotating a coil at a constant speed in a uniform magnetic field. Which one of the following traces would result from rotating the coil at a faster constant speed, whilst keeping all other conditions unchanged? (The scales on the axes do not change.

Answer:

11) B

Increasing the rotating speed, amplitude will get larger and frequency will increase (more rotations per second).

Let us say initial amplitude is 1 (1 box up from midpoint to crest)  and frequency is 3 (3 waves shown).

A – frequency increases but amplitude remains the same. This is wrong.

B – frequency increases and amplitude increases. This is correct.

C – Amplitude remains the same. Frequency decreases. This is wrong.

D – Amplitude increases. Frequency decreases. Wrong.

E – Amplitude increases but frequency remains the same. Wrong.