Cathode Rays

Cathode Rays

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Cathode rays are basically electron streams found in vacuum tubes. They are also known as e-beam or electron beams. These rays played a significant role in discovering the electron. If you want to know more about this discovery, then keep reading!

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, and the electrode connected to the positive pole of the voltage source is the anode.

Properties of cathode rays

Cathode rays are negatively charged particles that travel in a straight route and ionise the gas inside the vacuum tube. The properties of cathode rays do not change regardless of the gas used in the 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

Cathode ray tubes, previously known as gas discharge tubes or Crookes tubes, are vacuum glass tubes with two metal electrodes and rarefied gas inside. When a voltage is applied across the electrodes, electrons are emitted from the cathode and accelerate towards the anode. These electrons excite the gas atoms, causing the emission of electromagnetic radiation, which makes the path of the electrons visible. The name cathode ray tube comes from the fact that the electrons are emitted from the cathode. William Crookes, after whom the tube was initially named, observed that these particles carry momentum, which can be altered by a magnetic field, as shown in the image below.

Check out our explanation on Momentum.

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

J.J. Thomson built on William Crookes's experiments with gas discharge tubes, and through his experiments, he made several significant discoveries. Thomson verified that cath had both magnetic and electric fields, collected the rays in a metal cup, and discovered an overabundance of negative charge. He also measured the ratio between an electron's charge and its mass (qe/me), which was a crucial step in determining the exact charge of a single electron.

Thomson's experiments with cathode rays, along with his discovery of the electron, revolutionized our understanding of atomic structure and led to the development of modern electronics. His work earned him the Nobel Prize in Physics in 1906.

A diagram of J.J. Thompson's cathode ray tube
A schematic diagram showing the electron beam passing between the plates
A schematic diagram showing the electron beam passing between the plates

Determining qe/me

J.J. Thomson was able to determine the ratio of the charge of the electron (qe) to its mass (me) by studying the forces acting on the electron. He used the equation F = qeE, 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). He then used the acceleration equation F = mea, 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). By combining these two equations, he was able to solve for qe/me, which is equal to E/a.

By performing experiments with charged particles in gases, Thomson was able to estimate the magnitude of qe/me. He then used a cathode ray tube to measure the deflection of the particles, which allowed him to calculate the acceleration (a) and electric field (E).

The importance of qe/me

J.J. Thomson's discovery of the electron and its charge-to-mass ratio was a crucial breakthrough in our understanding of atomic structure. By calculating the charge-to-mass ratio, Thomson was able to determine that the electron had an incredibly small mass compared to the proton. This realization led to the development of new theories about the structure of atoms, including the idea that atoms were made up of smalleromson's experiments cathode rays and the discovery of the electron also paved the way for further research into subatomic particles. It was the first time that a subatomic particle had been identified, and it opened up a new field of research in physics that continues to this day.

Thomson's work also had practical applications, leading to the development of new technologies such as the cathode ray tube, which was used in early television sets and computer monitors. Overall, Thomson's contributions to the field of physics were groundbreaking and continue to be studied and celebrated to this day.

Cathode Rays

What is the use of a cathode ray oscilloscope?

A cathode ray oscilloscope is used to examine the signal characteristics, oscillation distortion, and signal frequency response.

How are cathode rays produced?

Cathode rays are produced when there is a potential difference (voltage) applied across the electrodes. The 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. Thus, the route of the electrons becomes visible.

What is a cathode ray and what are its properties?

Cathode rays are electron streams detected in vacuum tubes. 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.

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