Cathode Rays

Properties of cathode rays

These are some properties of cathode rays: they are negatively charged, they travel in a straight route, and they ionise the gas inside the tube. The properties of cathode rays don’t change due to the gas used in the vacuum tube.

The cathode ray tube

Nowadays, we use the name cathode ray tubes. They were previously called gas discharge tubes or Crookes tubes (named after William Crookes whose experiments showed the earliest direct signs of electrons and their charge).

Cathode ray tubes consist of an evacuated glass with two metal electrodes and rarefied gas within the glass. When a potential difference (voltage) is applied across the electrodes, electrons begin to emit from the cathode towards the anode and accelerate inside the gas due to this high potential difference.

These electrons excite the gas atoms, leading to the emission of electromagnetic radiation. As a result, the route of the electrons becomes visible. The name cathode ray tube originally comes from the fact that the electrons are emitted from the cathode.

In his experiments, Crookes observed that these particles, which we now know are electrons, carry momentum. A magnet can bend their straight path in the expected route for moving negative charges.

How were cathode rays used in the discovery of the electron?

J.J. Thomson improved on Crookes’s experiments with gas discharge tubes. Through his experiments, Thomson

• verified that cathode rays have a magnetic field and an electric field,
• gathered the rays in a metal cup and discovered an overabundance of a negative charge, and
• measured the ratio between an electron’s charge and its mass (q_e/m_e), which was one of the most important steps in finding the exact charge of a single electron.

Take a look at the image below. In Thomson’s experiment, which was used to determine the velocity of electrons, an electric field E is applied between two metal plates, and there is a magnetic field B perpendicular to the electric field because the tube is placed between the opposite poles of a magnet. Since the two forces E and B are perpendicular, they apply opposite forces to the electrons. When these forces cancel each other, the net force on the electrons disappears, which means that the velocity of the charged particle (electron) is v = E/B.

Determining q_e/m_e

To understand how Thomson determined q_e/m_e, we look at the relation between the forces. Recall that

\[F = q_e \cdot E\]

where F is the net force on the electron measured in newtons (N), q_e is the charge of the electron measured in coulombs (C), and E is the electric field affecting the electron measured in newton per coulomb (N/C).

The vertical deflection is related to the electron’s mass, and we know from the acceleration equation that

\[a = \frac{F}{m_e}\]

where F is the net force on the electron (N), m_e is the mass of the electron measured in kilograms (kg), and a is the acceleration of the electron measured in metres per second squared (m/s²).

At that time, q_e was not yet known, and since F is not known, we can apply the first equation to the previous one to see things more logically:

\[a = \frac{F}{m_e} = \frac{q_e \cdot E}{m_e}\]

Now, if we solve the equation for q_e/m_e, we have

\[\frac{q_e}{m_e} = \frac{a}{E}\]

We can calculate the deflection to obtain a and calculate E using the applied voltage and the distance between the plates.

The importance of q_e/m_e

The importance of the electron’s charge-to-mass ratio was crucial because Thomson calculated it as -1.76 ⋅ 10^-11 C/kg. This was a considerable number, and Thomson stated that this implies the electron has such a small mass. Today we know it is so small that the mass of a proton is 1836 times the mass of an electron.

Later on, Thomson conducted different experiments using different methods (such as the photoelectric effect) to emit electrons from atoms. Still, he always came up with the same properties for the electron, determining it as an independent particle.

Cathode ray tube. https://www.flickr.com/search/?text=cathode%20ray%20tube&license=4%2C5%2C6%2C9%2C10