Physics 9702 · Paper 2

Reading an Oscilloscope

An oscilloscope (c.r.o.) draws a voltage against time, so a sound picked up by a microphone shows as a repeating trace.
The time-base sets the time per horizontal division; the y-gain sets the voltage per vertical division.

Reading frequency off the screen

Count how many divisions one full cycle takes across the screen.
Multiply by the time-base to get the period $T$ , then use $f = 1/T$ .

A sound wave on an oscilloscope. One full cycle spans 3 divisions here; with the time-base at 1 ms per division that is a period of 3 ms, so f = 1/T ≈ 333 Hz.

A tuning fork, a microphone on a stand, and a cathode-ray oscilloscope with a blank screen, connected on a bench. — The microphone turns the sound into a voltage; the oscilloscope draws it against time so you can read the period.

Worked example

Frequency from a trace

One complete wave spans 3.0 divisions on the screen. The time-base is set to 1.0 ms per division. Find the frequency.

Period: $T = 3.0 \times 1.0\text{ ms} = 3.0\text{ ms} = 3.0\times10^{-3}\text{ s}$ .
$f = \dfrac{1}{T} = \dfrac{1}{3.0\times10^{-3}} \approx 333\text{ Hz}$ .

Answer

f \approx 330\text{ Hz}

Reading voltage and amplitude

The y-gain turns vertical divisions into volts. Count the divisions from the middle line up to a peak and multiply by the y-gain to get the peak voltage.

Common mistake

One division is not one cycle. Read where the trace repeats to find the period, and check the time-base units: a setting in microseconds per division changes the answer by a factor of a thousand.

Your turn— tap to reveal the worked answer (9702/13/M/J/25 Q26)

A wave on a c.r.o. has one cycle spanning a known number of divisions with the time-base at 300 μs per division and the y-gain at 40 V per division. The method is the same: cycle-divisions × time-base = T, then $f = 1/T$ . (9702/13/M/J/25 Q26)

Read the divisions per cycle from the screen, multiply by 300 μs, and invert for the frequency.

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