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Oscilloscopes and intensity Cambridge International AS & A Level Physics revision

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In plain words

An oscilloscope draws a graph of voltage against time on a screen, so plug in a microphone and you can see a sound wave. Two dials tell you what each square on the screen is worth: one for time across, one for volts up. Count the squares and you have the wave's period and amplitude.

Three things to know

  1. The time-base is the time shown by one division across the screen. Period = number of divisions for one complete wave × time-base, and then f = 1 ÷ T.
  2. The y-gain is the voltage shown by one division up the screen. Amplitude = number of divisions from the centre line to a peak × y-gain.
  3. The intensity of a wave is proportional to the square of its amplitude. Double the amplitude and the intensity is four times as great.

Worked example

On an oscilloscope one complete wave takes up 4.0 divisions with the time-base at 0.50 ms per division. The peaks are 3.0 divisions above the centre line with the y-gain at 2.0 V per division. Find the frequency and the amplitude.

  1. Period = 4.0 × 0.50 ms = 2.0 ms = 2.0 × 10⁻³ s.
  2. Frequency = 1 ÷ 2.0 × 10⁻³ = 500 Hz.
  3. Amplitude = 3.0 × 2.0 = 6.0 V.

Tips and tricks

  • Amplitude is measured from the centre line to a peak. Trough to peak is twice the amplitude.
  • For a better value of the period, count the divisions for several complete waves and divide.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Oscilloscopes and intensity: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. How is the intensity of a wave related to its amplitude A?
    • It is proportional to A.
    • It is proportional to A². (the answer)
    • It is proportional to 1 ÷ A.
    • It doesn't depend on A.

    Double the amplitude, four times the intensity.

  2. The time-base is 1 ms per division and one complete wave takes up 2 divisions. What is the frequency?
    • 2 Hz
    • 200 Hz
    • 500 Hz (the answer)
    • 2000 Hz

    The period is 2 ms.

  3. What does the y-gain setting of an oscilloscope tell you?
    • the voltage shown by one division up the screen (the answer)
    • the time shown by one division across the screen
    • the frequency of the signal
    • the brightness of the trace

    It sets the vertical scale.

  4. A trace measures 6 divisions from trough to peak, with the y-gain at 5 V per division. What is the amplitude?
    • 5 V
    • 15 V (the answer)
    • 30 V
    • 60 V

    Trough to peak is 30 V, and the amplitude is half of that.

  5. The intensity of a wave falls to a quarter of its value. What has happened to its amplitude?
    • It has halved. (the answer)
    • It has fallen to a quarter.
    • It has fallen to a sixteenth.
    • It has doubled.

    Intensity is proportional to amplitude squared.

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