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Cathode-Ray Oscilloscope (Cambridge (CIE) AS Physics)

Revision Note

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Cathode-ray oscilloscope

  • A cathode-ray oscilloscope is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits

  • An a.c. current on an oscilloscope is represented as a transverse wave

    • The oscilloscope can be used to determine its frequency and amplitude

  • A d.c voltage on an oscilloscope is represented as a horizontal line at the relevant voltage

  • The x-axis is the time and the y-axis is the voltage (or y-gain)

 

Cathode-ray oscilloscope display

CRO diagram, downloadable AS & A Level Physics revision notes

Diagram of cathode-ray oscilloscope display showing wavelength and time-base setting

 

  • The period of the wave can be determined from the time-base

    • Time-base is how many seconds each division represents measured commonly in s div-1 or s cm-1

  • Count the number of wavelengths shown on the screen 

  • Divide the total time by the number of wavelengths to obtain the time period T 

  • Calculate the frequency of the wave using f space equals space 1 over T

 

Worked Example

A cathode-ray oscilloscope (c.r.o.) is used to display the trace from a sound wave. The time-base is set at 7 µs mm-1.

WE - CRO question image, downloadable AS & A Level Physics revision notes

What is the frequency of the sound wave?

A. 2.4 Hz         

B. 24 Hz         

C. 2.4 kHz         

D. 24 kHz

Answer: C

Step 1: Determine the period of the wave:

  • One full wave covers three horizontal squares, which is a distance of 6 cm

  • There are 60 mm in 6 cm

    • Each mm represents a time of 7 μs

    • Time period, T = 60 × (7 × 10−6) = 4.2 × 10−4 s

Step 2: Use the frequency equation to determine frequency:

  • Recall the frequency equation

f space equals space 1 over T space equals space fraction numerator 1 over denominator 4.2 space cross times space 10 to the power of negative 4 end exponent end fraction space equals space 2380 space Hz

  • Recall that 1000 Hz = 1 kHz and the answer must be given to 2 significant figures

f space equals space 2.4 space kHz

Examiner Tips and Tricks

The time-base setting varies with units for seconds (commonly ms) and the unit length (commonly mm). Unit conversions are very important for the calculation of the time period and frequency

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    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.