Cathode Ray Experiment

The cathode ray experiment revealed that electrons are negatively charged particles present in all atoms, discovered through the behavior of cathode rays in electric and magnetic fields.

1.0What is Cathode Ray Experiment

The Cathode Ray Experiment is a fundamental scientific procedure that demonstrates the existence of electrons. In this experiment, a cathode ray tube, a vacuum-sealed tube with two electrodes, is used. When an electric current passes through the tube, a beam visible in the presence of a low-pressure gas, termed "cathode rays," is emitted from the cathode (negative electrode) towards the anode (positive electrode). These experiments revealed that cathode rays are streams of electrons, providing key insights into the structure of matter at an atomic level. Here's a detailed explanation of the process and findings:

The Cathode Ray Experiment Setup

• Cathode Ray Tube: This glass tube contains two metal electrodes under a high vacuum. When a high voltage is applied across these electrodes, electricity flows through the tube, even at very low gas pressures.
• Electrodes: The tube includes a cathode (negative electrode) and an anode (positive electrode). A high voltage across them causes a stream of particles to move from the cathode to the anode, forming cathode rays.

Observations and Conclusions

• Origin and Direction: Cathode rays are emitted from the cathode and move toward the anode. They are invisible but can be detected by their impact on fluorescent materials.
• Detection: The rays are made visible by a fluorescent screen placed at the end of the tube, where they create a visible spot or image. This principle is similar to how old television screens or oscilloscopes work.
• Straight-Line Path: In the absence of external forces, cathode rays travel in straight lines.
• Charge Nature: The deflection of cathode rays in electric and magnetic fields indicates they are negatively charged particles, later named electrons.
• Independence of Material: The properties of cathode rays do not depend on the type of gas in the tube or the material of the electrodes, suggesting electrons are a universal component of atoms.

2.0Charge to Mass Ratio of Electron

Through these experiments, scientists like J.J. Thomson concluded that the electron is a fundamental, negatively charged particle present in all atoms, providing profound insights into atomic structure. This experiment marked a pivotal moment in the development of atomic physics and our understanding of the material world.

In 1897, British physicist J.J. Thomson quantified the charge-to-mass ratio (e/m_e) of the electron using a cathode ray tube with perpendicular electric and magnetic fields. The deflection of electrons from their path is influenced by three factors:

Charge Magnitude: Greater charge results in more significant interaction with the fields, thus more deflection.

Particle Mass: Lighter particles are deflected more easily than heavier ones.

Field Strength: Increasing the electric or magnetic field strength enhances electron deflection.

By applying only an electric field, electrons hit the tube at point A. With only a magnetic field, they strike at point C. Thomson adjusted the fields' strengths to make electrons follow their original path, hitting point B, as if no fields were present. Through precise measurements of these deflections against the field strengths, Thomson was able to determine the value of e/m_e as:

Where m_e is the mass of the electron in kg and e is the magnitude of the charge on the electron in coulomb (C). Since electrons are negatively charged, the charge on electron is –e.

3.0Charge on the Electron

R.A. Millikan (1868-1953) devised a method known as oil drop experiment (1906-14), to determine the charge on the electrons. He found that the charge on the electron to be –1.6 × 10^–19 C. The present accepted value of electrical charge is –1.6022 × 10^–19 C. The mass of the electron (m_e) was determined by combining these results with Thomson's value of e/m ratio.