Cathode Ray Production Revision Notes | A Level AQA Physics

1. Why Cathode Rays Matter

Before 1897, no one knew the electron existed. The experiments that revealed it began with a surprisingly simple device: a glass tube with most of the air pumped out. What happened inside that tube changed physics permanently.

Cathode ray production sits at the heart of the AQA 'Turning Points in Physics' topic because it represents the moment physicists first glimpsed sub-atomic structure. Understanding how cathode rays are produced — and what they actually are — is the foundation for everything that follows in this topic, including J.J. Thomson's measurement of the charge-to-mass ratio of the electron.

2. The Discharge Tube: Structure and Setup

Picture a sealed glass tube, roughly 30–40 cm long. At each end, a metal electrode is sealed into the glass: the cathode (negative electrode) at one end and the anode (positive electrode) at the other. A vacuum pump removes most of the gas inside, reducing the pressure to around $10^{-3}$ Pa or lower — far below atmospheric pressure of $10^5$ Pa.

A high-voltage supply — typically several kilovolts — is then connected across the two electrodes. This combination of very low pressure and high voltage is what makes cathode ray production possible. Neither condition alone is sufficient; both are required together.

The tube also typically contains a small hole or aperture in the anode. This allows a narrow beam of cathode rays to pass through and continue travelling in a straight line beyond the anode, where it can be detected on a fluorescent screen coated with a material such as zinc sulphide. The screen glows where the beam strikes it — giving a visible spot that can be deflected by electric or magnetic fields.

3. How Cathode Rays Are Produced

Here is where the physics becomes interesting. At atmospheric pressure, gas molecules are so densely packed that any electrons released from the cathode would immediately collide with them and be absorbed. Reducing the pressure removes most of these molecules, giving electrons a clear path from cathode to anode.

The high voltage creates a strong electric field between the electrodes. This field accelerates any free electrons in the tube towards the anode. In early discharge tubes, electrons were released from the cathode surface by a process called field emission — the intense electric field at the cathode surface pulls electrons out of the metal. In later, more controlled tubes, the cathode was heated so that electrons were released by thermionic emission.

Once released, electrons are accelerated through the potential difference between cathode and anode. The work done by the electric field converts electrical potential energy into kinetic energy of the electrons. This is described by the energy equation $eV = \frac{1}{2}mv^2$, where $e$ is the electron charge, $V$ is the accelerating voltage, $m$ is the electron mass, and $v$ is the speed gained. The electrons then travel in straight lines (in the absence of any deflecting field) from cathode to anode, forming the cathode ray beam.

Worked example: Calculating the Speed of Electrons Accelerated Through a Potential Difference

Electrons are accelerated from rest through a potential difference of 2000 V in a discharge tube. Calculate the speed of the electrons as they reach the anode. (Electron charge $e = 1.6 \times 10^{-19}$ C, electron mass $m = 9.11 \times 10^{-31}$ kg)

1. $eV = \frac{1}{2}mv^2$ — The work done by the electric field on the electron equals the kinetic energy gained. This assumes the electron starts from rest. [M1]
2. $v^2 = \frac{2eV}{m}$ — Multiply both sides by 2 and divide by $m$. [M1]
3. $v^2 = \frac{2 \times 1.6 \times 10^{-19} \times 2000}{9.11 \times 10^{-31}}$ — Substituting $e = 1.6 \times 10^{-19}$ C, $V = 2000$ V, $m = 9.11 \times 10^{-31}$ kg. [M1]
4. $2 \times 1.6 \times 10^{-19} \times 2000 = 6.4 \times 10^{-16}$ J — $2 \times 1.6 = 3.2$; $3.2 \times 2000 = 6400$; so $6400 \times 10^{-19} = 6.4 \times 10^{-16}$ J.
5. $v^2 = \frac{6.4 \times 10^{-16}}{9.11 \times 10^{-31}} = 7.03 \times 10^{14}$ m$^2$ s$^{-2}$ — $6.4 \div 9.11 \approx 0.703$; exponent: $10^{-16} \div 10^{-31} = 10^{15}$; so $v^2 \approx 7.03 \times 10^{14}$ m$^2$ s$^{-2}$.
6. $v = \sqrt{7.03 \times 10^{14}} \approx 2.65 \times 10^7$ m s$^{-1}$ — $\sqrt{7.03} \approx 2.65$; $\sqrt{10^{14}} = 10^7$. This is about 9% of the speed of light — fast, but non-relativistic at this voltage. [A1]

Answer: $v \approx 2.65 \times 10^7$ m s$^{-1}$

Notice that doubling the accelerating voltage does not double the speed — because $v \propto \sqrt{V}$. This is a common point of confusion in exam questions that ask you to compare speeds at different voltages.

4. Evidence That Cathode Rays Are Negatively Charged Particles

When cathode rays were first observed in the 19th century, their nature was genuinely unknown. Were they waves, like light? Were they charged particles? A series of experimental observations settled the question — and each piece of evidence is worth knowing for your exam.

First, cathode rays travel in straight lines. When a solid object is placed between the cathode and the fluorescent screen, a sharp shadow forms on the screen with no diffraction effects. This is consistent with particle behaviour; waves would be expected to show diffraction effects around obstacles, whereas cathode rays produce sharp shadows with no diffraction, consistent with particle behaviour.

Second, cathode rays are deflected by electric fields. When metal plates are placed above and below the beam inside the tube and a voltage is applied across them, the beam curves towards the positive plate. This confirms the rays carry negative charge, since negative charges are attracted towards positive potentials.

Third, cathode rays are deflected by magnetic fields. A magnetic field applied perpendicular to the beam causes it to curve in a direction consistent with the force on a moving negative charge (given by $\vec{F} = q\vec{v} \times \vec{B}$, or using Fleming's left-hand rule with the current direction reversed for negative charge). This deflection is the basis of Thomson's famous $e/m$ experiment.

Fourth, the properties of cathode rays — their speed and the ratio of charge to mass — are independent of the cathode material and the residual gas in the tube. Whatever metal the cathode is made from, and whatever trace gas remains, the same type of particle is produced. This was a crucial clue that cathode rays are a universal constituent of matter, not an artefact of a particular material.

5. The Role of Pressure: A Closer Look

It is worth examining the pressure dependence more carefully, because exam questions sometimes ask you to describe or explain what happens as pressure changes. At atmospheric pressure, the gas inside the tube is a good insulator — the mean free path of any electron is extremely short, so no sustained current flows and no beam forms.

As pressure is reduced, the mean free path increases. At intermediate pressures (around $10^2$ Pa), something visually striking occurs: the gas begins to glow in coloured bands and regions. This is because electrons gain enough energy between collisions to excite gas molecules, which then emit light as they return to their ground state. These glowing discharge patterns were the original 'discharge' that gave the tube its name.

At very low pressures (below about $10^{-1}$ Pa), almost all the gas has been removed. There are too few molecules to produce visible glowing, but the cathode ray beam itself becomes well-defined and travels cleanly from cathode to anode. The fluorescent screen at the end of the tube glows where the beam strikes it. This is the operating regime for cathode ray production in the context of Thomson's experiments.

6. Historical Context and Significance

The discharge tube was developed and refined by several physicists across the 19th century, most notably William Crookes, whose evacuated tubes (often called Crookes tubes) produced particularly clear cathode ray beams. Crookes believed cathode rays were a fourth state of matter — radiant matter — a state distinct from solid, liquid, and gas, consisting of molecules in a highly attenuated form. The debate between the 'particle' and 'wave' camps was not resolved until J.J. Thomson's 1897 experiments.

Thomson used a refined discharge tube to measure the deflection of cathode rays in both electric and magnetic fields simultaneously. By balancing these deflections, he could determine the speed of the particles and then calculate the ratio of charge to mass, $e/m$. The value he obtained was the same regardless of cathode material or residual gas — confirming that cathode rays consist of a single type of sub-atomic particle, which he called a 'corpuscle' and which we now call the electron.

This was a genuine turning point: for the first time, there was direct experimental evidence that atoms are not indivisible, and that a particle far lighter than any atom exists as a universal component of matter. The discharge tube was the instrument that made this discovery possible.

7. Exam Technique and Common Mistakes

Questions on cathode ray production tend to fall into two categories: 'describe' questions asking you to explain the setup and process, and 'explain' questions asking you to justify why specific conditions are needed or what the evidence tells us. The distinction matters for marks.

For a 'describe' question, give a clear sequence: low pressure in the tube, high voltage applied, electrons emitted from cathode (by field emission or thermionic emission), electrons accelerated towards anode, beam passes through aperture in anode, beam detected on fluorescent screen. Do not include 'because' — just state what happens.

For an 'explain' question, you must link each condition to its physical reason. Low pressure is needed because it increases the mean free path of electrons, allowing them to reach the anode without being absorbed by gas molecule collisions. High voltage is needed because it provides the electric field that accelerates electrons from the cathode and gives them sufficient kinetic energy to reach the anode and produce a detectable beam.

Taken together, the production of cathode rays in a discharge tube — requiring low pressure and high voltage, producing a beam of negatively charged particles whose properties are universal — provided the experimental foundation for Thomson's identification of the electron as the first known sub-atomic particle.