Cathode rays, also known as e-beam or electron beams, are electron streams detected in vacuum tubes. Cathode rays were part of very important experiments used to discover the electron. Let’s find out how!
What is a cathode ray?
Cathode rays, also known as electron beams, are streams of electrons detected in discharge tubes (vacuum tubes). These discharge tubes are devices that control the electric current between a potential difference applied to the electrodes in a high vacuum.
A glow behind the positive electrode (cathode) can be observed when a potential difference (voltage) is applied to the electrodes. The electrons emitting from the cathode is what causes this glow.
To find which electrode is the cathode and the anode, we need to look at the connections between the electrodes and the voltage supply:
- The electrode connected to the negative pole of the voltage source is the cathode.
- The electrode connected to the positive pole of the voltage source is the anode.
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.
Julius Plücker and Johann Wilhelm Hittorf first observed cathode rays in 1869, and Eugen Goldstein named them in 1876. The most important use of cathode rays was discovered by J.J. Thomson in 1897 when he concluded that cathode rays were made up of a previously unidentified negatively charged particle, the electron.
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 (qe/me), which was one of the most important steps in finding the exact charge of a single electron.
A diagram of J.J. Thompson’s cathode ray tube, Oğulcan Tezcan – StudySmarter Originals
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.
A schematic diagram showing the electron beam passing between the plates, Oğulcan Tezcan – StudySmarter Originals
Determining qe/me
To understand how Thomson determined qe/me, 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), qe 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), me 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, qe 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 qe/me, 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 qe/me
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.
It was only when I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms. - J.J. Thomson
Cathode Rays - Key takeaways
- Cathode rays, also known as electron beams, are streams of electrons in discharge tubes (vacuum tubes).
- Discharge tubes are devices that control electric current between a potential difference (voltage) applied across electrodes in a high vacuum.
- When a potential difference is applied to the electrodes in a cathode ray tube, a glow behind the positive electrode (cathode) can be observed. The reason for the glow is the electrons emitting from the cathode.
- Cathode ray tubes are also called gas discharge tubes or Crookes tubes.
- The charge-to-mass ratio of the electron was significant because Thomson calculated it to be -1.76 ⋅ 10-11C/kg, which was a considerable number. Thomson stated that this implies that the electron has such a small mass.
- After various experiments, Thomson concluded that the electron is an independent particle, making it the first subatomic particle discovered.
ImagesCathode ray tube. https://www.flickr.com/search/?text=cathode%20ray%20tube&license=4%2C5%2C6%2C9%2C10
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