A heated cathode releases electrons by thermionic emission, and a high voltage makes them a beam. The oscilloscope steers that beam to draw voltage against time.
This lesson is part of SPM Physics electronics. It helps you see the signals from explaining rectification.
How does the beam form?
- A filament heats the cathode, which releases electrons by thermionic emission.
- A high voltage between the cathode and anode accelerates the electrons.
- The beam passes through the anode in a straight line, inside a vacuum.
- Deflecting plates move the beam up and down with the input voltage, and across with the time base.
- The beam hits a fluorescent screen and leaves a bright spot or line.
Worked example: reading a trace
A sinusoidal voltage is shown with the Y-gain at 2 V per division and the time base at 5 ms per division. The wave peaks 3 divisions above the centre line, and one complete cycle spans 4 divisions across.
- Peak voltage = 3 × 2 = 6 V.
- Period = 4 × 5 = 20 ms = 0.020 s.
- Frequency = 1 ÷ 0.020 = 50 Hz.
State the units at each step. The peak-to-peak height is a separate reading: 6 divisions, or 12 V.
The mistake that loses marks
A student reads the peak-to-peak height as the peak voltage and writes 12 V. Peak is measured from the centre line, not from the lowest point to the highest.
Check which one the question asks for. For peak-to-peak, use the full height. For peak, use half of it.
Check yourself
The Y-gain is 0.5 V per division and the time base is 2 ms per division. The trace is 6 divisions from the lowest to the highest point, and one cycle takes 5 divisions across. Find the peak-to-peak voltage, peak voltage and frequency.
Answer
Peak-to-peak = 6 × 0.5 = 3.0 V, so the peak voltage is 3.0 ÷ 2 = 1.5 V.
Period = 5 × 2 = 10 ms = 0.010 s. Frequency = 1 ÷ 0.010 = 100 Hz.
What to study next
Test the whole topic with the electronics practice set. If diode or transistor circuits feel weaker, return to interpreting diode behaviour.
The units and significant figure checker helps with unit conversions. If you want a teacher to go through oscilloscope readings with you, see online one-to-one Physics tuition.