4. Electricty and magnetism

4.6 Cathode – ray oscilloscopes

(a) Cathode rays

4.6 Cathode-ray oscilloscopes

Thermionic emission

Thermionic emission

Thermionic emission

The Electron gun

What is an Electron Gun?

The electron gun structure

electron gun photograph

Electron energy

• Where: e = electron charge ( 1.6 x10-19 C) V = d.c voltage (cathode – anode)

Cathode rays propertiesThese rays originate from cathode. Cathode rays travel in straight line. Cathode rays carry negative charge. Cathode rays are deflected by electric field. Cathode rays are deflected by magnetic field. These rays consist of material particles. Cathode rays consist of "electron". Produce a shadow when obstructed by objects.Cathode rays can produce fluorescence.

Deflection tube

Magnetic deflection

4.6 (b) Simple treatment of cathode-ray oscilloscope

cathode ray oscilloscope

tube photograph

tube diagram

H.T. supply

heater supply

y plates x plates

+-

phosphor screen

anode

summary

H.T. supply

heater supply

y plates x plates

+-

phosphor screen

anode

electron gun produces a beam of

electrons light produced on the screen by electron beam

a p.d. across the y plates deflects the

trace vertically

a p.d. across the x plates deflects the trace horizontally

gain andf time-base controls

peak voltage

Peak p.d. = 3 Divisions x 1.0 mV/div = 3.0 mV

period & frequency

period = 4.0 divisions x 1.0 ms/div = 4.0 ms

frequency = 1 / periodfrequency = 1 / 0.004 sfrequency = 250 Hz

self test