Question

Summarize the evidence used by J. J. Thomson to argue that cathode rays consist of negatively charged particles.

Short answer

J.J. Thomson's experiments with cathode rays involved passing them through electric and magnetic fields. He observed the following: 1. The rays were deflected towards the positive plate in an electric field, suggesting the particles were negatively charged. 2. In a magnetic field, the rays bent in a circular path consistent with the Lorentz force acting on negatively charged particles. 3. The calculated charge-to-mass ratio (e/m) of the particles was much larger than any known ion, indicating lighter particles with a higher charge-to-mass ratio. From these observations, Thomson concluded that cathode rays consist of negatively charged particles, now known as electrons.

Step-by-step solution

Background information on cathode rays

Cathode rays are streams of particles observed in vacuum tubes when a voltage is applied between two electrodes. These rays were the subject of much study in the late 19th century, and various theories were put forth to explain their nature and properties.

J.J. Thomson's experimental setup

Thomson's experiments involved passing cathode rays through electric and magnetic fields. The setup consisted of a vacuum tube with an electric field created between two parallel plates (anode and cathode) and a perpendicular magnetic field created by a pair of Helmholtz coils.

Observation 1: Deflection in electric field

When the electric field was applied, Thomson observed that the cathode rays (which were initially traveling in a straight line) would bend towards the positively charged plate. This suggested that the particles in the cathode rays were negatively charged, as they were attracted to the positive electric field.

Observation 2: Deflection in magnetic field

When the magnetic field was applied instead of the electric field, Thomson observed that the cathode rays also bent in a circular path, with the direction of the force acting on the particles perpendicular to both the velocity of the particles and the magnetic field. This again suggested that the particles were negatively charged, as their behavior was consistent with the Lorentz force acting on a negatively charged particle in a magnetic field.

Observation 3: Properties of the deflection

By carefully measuring the deflection of the cathode rays in both the electric and magnetic fields, and knowing the strength of the fields, Thomson was able to calculate the charge-to-mass ratio (e/m) of the particles in the cathode rays. This value was found to be much larger than any known ion, indicating that the cathode ray particles were much lighter and had a higher charge-to-mass ratio.

Conclusion: Evidence for negatively charged particles

From these observations, Thomson was able to argue that cathode rays consist of negatively charged particles, which he called "corpuscles" but are now known as electrons. His experiments showed that these particles were deflected by both electric and magnetic fields in a manner consistent with having a negative charge, and their properties (such as the charge-to-mass ratio) differed from those of any known charged particles at the time. This evidence was crucial in establishing the existence of the electron as a fundamental particle.