All subjects · Physics · WPH16 Practical Skills in Physics II

Oscilloscopes, data loggers and sensors

Edexcel IAL Physics · WPH16 Practical Skills in Physics II

Core practicals 4, 9 and 11 use oscilloscopes, sensors and data loggers. Unit 6 tests how to read them and how to choose settings.

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Reading an oscilloscope

y-gain (volts per division): p.d. = number of vertical divisions x y-gain. Peak p.d. is measured from the centre line; peak-to-peak from top to bottom (twice as many divisions, less uncertainty in finding the centre).
Time base (time per division): time = number of horizontal divisions x time base. Measure across several whole cycles and divide.
Choose settings so the trace fills as much of the screen as possible: more divisions, smaller percentage uncertainty.

Sensors and loggers

Voltage sensor, current sensor, force sensor, light gates, temperature probe, motion sensor, GM tube with a counter.
Choose the sampling rate so there are many readings during the event: the interval must be much shorter than the time scale of the change (for capacitors, much shorter than RC).
Loggers give simultaneous readings and store data for graphing; they remove reaction-time error.
Sensitivity matters: a sensor must give a measurable output change over the range of interest.

Uncertainty in screen readings

A screen reading is uncertain to about a fraction of a division (often taken as ± 0.1 or 0.2 division). Percentage uncertainty = that fraction / number of divisions measured x 100%. Measuring 8 divisions instead of 2 cuts the percentage uncertainty by four.

Worked example

A trace has 5 complete cycles across 8.0 divisions with the time base on 0.50 ms/div, and a peak-to-peak height of 6.8 divisions at 2.0 V/div. Find the frequency and peak p.d.

Time for 5 cycles = 8.0 x 0.50 = 4.0 ms, so T = 0.80 ms
f = 1 / 0.80 x 10^-3 = 1250 Hz
Peak-to-peak p.d. = 6.8 x 2.0 = 13.6 V, so peak p.d. = 6.8 V

Answer: f = 1250 Hz, peak p.d. = 6.8 V

Key idea: p.d. = divisions x y-gain; time = divisions x time base; measure over many divisions and cycles. Sampling interval much shorter than the change. Screen readings are uncertain by a fraction of a division.

Check yourself

A trace has a peak height of 3.4 divisions with the y-gain on 5.0 V/div. Find the peak p.d. in V.
Show answer
17 V. 3.4 x 5.0 = 17 V.
Four complete cycles span 9.6 divisions with the time base on 0.20 ms/div. Find the frequency in Hz, to 3 s.f.
Show answer
2080 Hz. Time for 4 cycles = 9.6 x 0.20 = 1.92 ms; T = 0.48 ms; f = 2080 Hz.
A horizontal length of 8.0 divisions is read to ± 0.1 division. Find the percentage uncertainty, to 2 s.f.
Show answer
1.3 %. 0.1 / 8.0 x 100 = 1.3%.
A capacitor has RC = 5.0 ms. A logger should take at least 50 readings in one time constant. What minimum sampling rate is needed, in kHz?
Show answer
10 kHz. 50 readings in 0.0050 s = 10 000 per second = 10 kHz.
Why should oscilloscope settings be chosen so the trace fills most of the screen?
- The trace is brighter
- More divisions are measured, so the percentage uncertainty is smaller
- The frequency increases
- It removes the need for a time base
Show answer
More divisions are measured, so the percentage uncertainty is smaller. The reading uncertainty is a fixed fraction of a division.
Why is it better to measure the peak-to-peak height and halve it than to measure the peak height directly?
- The peak height is always zero
- Peak-to-peak gives the frequency
- It changes the y-gain
- It doubles the number of divisions measured and avoids locating the centre line
Show answer
It doubles the number of divisions measured and avoids locating the centre line. A bigger reading and no need to find the zero line both reduce uncertainty.
A collision lasts 20 ms. Which sampling rate is suitable for a force sensor?
- 1 sample per second
- 1000 samples per second
- 10 samples per second
- 1 sample per minute
Show answer
1000 samples per second. 1000 per second gives 20 readings during the collision; the others give none or one.

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Last updated 8 November 2026. Spotted a mistake? Tell us.