The diagram shows a cathode-ray oscilloscope (c.r.o.) being used to measure the rate of rotation of a flywheel.

Question 3

The diagram shows a cathode-ray oscilloscope (c.r.o.) being used to measure the rate of rotation of a flywheel.

The flywheel has a small magnet M mounted on it. Each time the magnet passes the coil, a voltage pulse is generated, which is passed to the c.r.o. The display of the c.r.o. is 10 cm wide.

The flywheel is rotating at 3000 revolutions per minute.

Which time-base setting will display clearly separate pulses on the screen?

A 1 s cm^-1                          B 10 ms cm^-1                   C 100 μs cm^-1                               D 1 μs cm^-1

Reference: Past Exam Paper – June 2016 Paper 12 Q4

Solution:

Answer: B.

2 consecutive separate pulses represent the period of rotation of the flywheel since the period is the time taken for one complete rotation/revolution.

3000 revolutions --> 1 minute = 60 s

1 revolution --> 60 / 3000 = 0.02 s = 20 ms = 20 000 μs

Period of revolution = 0.02 s

Consider the choices:

Choice A: 1 cm --> 1s              [clearly incorrect as 2 pulses cannot be displayed]

Choice B: 1 cm --> 10 ms        [correct]

The 2 pulses are separated by 2 cm

Choice C: 1 cm --> 100 μs

The 2 pulses are separated by (20 000 / 100 =) 20 cm   [cannot be displayed on the cro]

Choice D: 1 cm --> 1 μs

The 2 pulses are separated by (20 000 / 1 =) 20 000 cm   [cannot be displayed on cro]