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]
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