Chapter 8, Problem 1AC

To determine

The evidence that convinced Thomson that he had discovered a subatomic particle, the electron is to be determined from the given options:

1. The charge-to-mass ratio was same for all materials.

2. Cathode rays could move through a vacuum.

3. Electrons were attracted toward a negatively charged plate.

4. The charge was always $1.60\times {10}^{-19}coulomb$.

Answer to Problem 1AC

Solution:

Option (a) is the correct answer.

Explanation of Solution

Introduction:

J.J Thomson performed an experiment of cathode rays and observed the green phosphorescence when these rays are passed under high magnetic and electric field in a high vacuum tube. This observation leads very interesting fact that the particles which comes out from the cathode rays were attracted towards the anode terminal or the positive terminal.

High vacuum tube filled with the inert gas provides the inert free medium for a particle to move inside the tube.

On the introduction of the magnetic field, the charge gets deflected. This is due to the magnetic force applied on the moving charge. This confirmed that the particle has some charge.

Explanation:

Reason for the correct option:

As the charge to the mass ratio of the particle is calculated. It is initially presumed it might depend upon the velocity of the particle or the intensity of the magnetic field. Thus, the velocity and magnetic field intensity also changed but the charge to mass ratio of the particle remains the same. This leads for the search of the source of the particle whether it is observed due to the gas filled inside the chamber or the cathode material. When both the material of the cathode and the gas filled was changed, then also the charge to the mass ratio remains unchanged.

This confirms the possibility subatomic particle which does not depend upon the material of the metal or cathode or the magnetic field. The charge to mass ratio of the particle will always remain constant for all materials.

Thus, the particle named as electron and its charge to mass ratio $\frac{e}{m}$ is approximately equal to:

$\frac{e}{m}=1.7\times {10}^7$

Hence option (a), is correct.

Reason for the incorrect options:

Cathode ray can travel through the vacuum but it does not infer that it is subatomic particle because energized particle or wave can also travel through vacuum. Hence, the option (b) is incorrect.

The attraction of the particle is towards the positively charged plate because electrons are negatively charged particles. Hence, the option (c) is incorrect.

Thomson found the charge-to-mass ratio of particle but not charge from the oil drop experiment. Hence, the option (d) is incorrect.

Conclusion:

The main evidence that convinced Thomson is that he had discovered a subatomic particle; the electron is the charge-to-mass ratio was the same for all materials.