Determination of Specific Charge of Electron J J Thomson Experiment

The entire subject of physics can be broadly divided into two categories like classical physics and the modern physics. Classical physics deals with the macroscopic objects which moves with the velocity that is very much smaller than the velocity of light. Modern physics deals with objects which are microscopic in nature and moves with the velocity comparable to the velocity of light.

As per the concepts of modern physics, both radiation and matter has dual nature. The travel like a wave and it interacts like a particle. Discovery of the cathode rays is one of the important starting points of modern physics.

In a discharge tube if a very low pressure of 0.01 mm of Hg and a high voltage of 10,000 V is applied, invisible cathode rays are generated. These rays are invisible and they start from the cathode. They are nothing but stream of electrons which travel in straight lines with high velocity which is equal to 1 by 10^th of velocity of light.

As they are having some velocity, they possess kinetic energy. They effect photographic plates and they can ionize the gases. As they are having a negative charge, they are affected by both electric and magnetic fields. The cathode rays are independent of the medium that is present in the discharge tube.

We can determine the specific charge of the cathode rays using JJ Thomson experiment. The apparatus consists of a discharge tube where a low pressure and high voltage is applied. There is a provision to apply electric and magnetic fields perpendicular to each other within the discharge tube. The cathode rays are allowed to strike a surface made up of zinc surface. When the invisible cathode Ray strikes the surface, there will be scintillation formed on the screen. Basing on the location of the scintillation we can identify the path of the cathode ray .

When no electric and magnetic fields are applied, cathode Ray travels along a straight line and strikes the screen exactly on the middle of the screen. When electric field is applied, there is a force acting on the cathode is therefore they were attracted towards the positive plate of the battery and hence modify their path.

When only magnetic field is applied perpendicular to previous electric field, the cathode Ray takes circular path. If both electric and magnetic fields are applied simultaneously in such a way that they apply equal force on the cathode ray, the cathode Ray goes in a undeviated manner.

We can derive the equations for specific charge basing on the equations of force experienced by the charged particle when electric field alone is applied and one magnetic field alone is applied.

When both the fields are applied perpendicular to each other, and if they are magnitudes are equal in size, the cathode Ray goes in a undeviated manner. It means that the cathode ray electron is in equilibrium position and hence we can equate both the forces.

When the cathode Ray is released, it is because of the high-voltage that is applied. Hence it will acquire some potential energy and by the time the cathode ray reaches the screen, all this potential energy is converted into kinetic energy. By applying the law of conservation of energy we can equate both these energies and derive an equation further for specific charge as shown below.