Thermionic emission, cathode rays and the oscilloscope: WASSCE Physics

In 1895, while experimenting with an evacuated tube, Röntgen noticed a screen glowing across the room — and then saw the bones of his wife's hand. X-rays and the cathode rays that make them opened the door to modern physics and modern medicine.

Learning objectives

By the end of this lesson you should be able to:

describe thermionic emission and the production of cathode rays
state the properties of cathode rays
describe the cathode-ray oscilloscope and its uses
explain how cathode rays were shown to be electrons

Thermionic emission

Definition

Thermionic emission is the emission of electrons from the surface of a hot metal (cathode). Heating gives the free electrons in the metal enough energy to escape from its surface.

A cathode is heated by a low-voltage filament. Coating it with metal oxides lowers the work function, so electrons escape more easily.

Cathode rays

When a high p.d. is applied between a heated cathode and an anode in an evacuated glass tube, the emitted electrons are accelerated towards the anode. Some pass through a hole in it as a beam, historically called cathode rays.

Properties of cathode rays

They travel in straight lines (they cast sharp shadows of an obstacle).
They carry negative charge.
They are deflected by electric fields (towards the positive plate).
They are deflected by magnetic fields (in the direction given by Fleming's left-hand rule for negative charge).
They cause fluorescence when they strike certain materials (zinc sulphide glows).
They have momentum and energy: they can turn a small paddle wheel and heat the target.
They produce X-rays when they strike a hard metal target.
They can ionise gases.

All these properties show that cathode rays are streams of fast-moving electrons. J. J. Thomson (1897) measured their charge-to-mass ratio, $e/m$ , by balancing electric and magnetic deflections — the discovery of the electron.

Energy of the electrons

An electron (charge $e$ ) accelerated through a p.d. $V$ gains kinetic energy

\tfrac{1}{2}mv^2 = eV

Worked example

Electrons are accelerated through 2000 V. Find (a) their kinetic energy, (b) their speed. (e = 1.6 × 10⁻¹⁹ C, m = 9.1 × 10⁻³¹ kg)

(a) $E_k = eV = 1.6 \times 10^{-19} \times 2000 = 3.2 \times 10^{-16}$ J.

(b) $v = \sqrt{\dfrac{2eV}{m}} = \sqrt{\dfrac{2 \times 3.2 \times 10^{-16}}{9.1 \times 10^{-31}}} = \sqrt{7.03 \times 10^{14}} = 2.65 \times 10^{7}$ m s⁻¹ (about 9% of the speed of light).

The cathode-ray oscilloscope (CRO)

The CRO is an instrument that draws a graph of a changing voltage on a screen.

Part	Function
Electron gun (heated cathode, grid, anodes)	produces a fine beam of electrons; the grid controls brightness, the anodes accelerate and focus the beam
Y-plates (horizontal)	deflect the beam up and down in proportion to the input voltage
X-plates (vertical)	deflect the beam sideways; a time base sweeps it steadily from left to right and flies back
Fluorescent screen	glows where the beam strikes
Evacuated tube	stops electrons colliding with air molecules

Uses of the CRO:

measuring d.c. and a.c. voltages (peak value from the trace height and the Y-gain setting)
displaying waveforms (sound from a microphone, heartbeats in an ECG)
measuring frequency and time intervals (from the time base)
comparing phases of signals

Worked example

A CRO's Y-gain is 2 V per division. An a.c. trace has a peak-to-peak height of 6 divisions. Find the peak voltage.

Peak-to-peak $= 6 \times 2 = 12$ V, so the peak voltage $= 6$ V.

Key points

Thermionic emission: electrons leave a hot metal surface.
Cathode rays are streams of electrons: they travel in straight lines, are negatively charged, are deflected by electric and magnetic fields, cause fluorescence and produce X-rays.
Electron energy gained through p.d. $V$ : $\tfrac{1}{2}mv^2 = eV$ .
CRO: electron gun, deflecting plates, time base, fluorescent screen; measures voltages and displays waveforms.

WASSCE examination focus

WAEC tests this

Define thermionic emission, list the properties of cathode rays, describe the main parts of a CRO and their functions, and calculate electron energies and speeds with $eV = \tfrac{1}{2}mv^2$ . Reading voltages and periods from a CRO trace may appear.

Quick check

Why is the tube of a CRO evacuated?
Which part of a CRO controls the brightness of the spot?
Give two properties of cathode rays.

Answers: 1. So that the electrons do not collide with air molecules and lose energy or scatter. 2. The grid. 3. For example: they travel in straight lines; they are deflected by electric and magnetic fields; they cause fluorescence.

Thermionic emission, cathode rays and the oscilloscope