# Effect of Electric and Magnetic Fields

Question: What is the Effect of Electric and Magnetic Fields on cathode rays? Give its conclusion.

In the absence of any electric or magnetic field, cathode rays move in straight line perpendicular to the surface of cathode.

In 1895, J Pernn showed that when cathode rays are passed between the poles of the magnet the cathode rays was deflected which showed that they have charge.

Effect of Magnetic Fields

In 1897, Sir J.J.- Thomson established their electric charge by the application of electric field. The cathode rays deflected towards positive plate showing their negative nature.

Effect of Electric Fields

Measurement of e/m and Charge of an Electron (J. J. Thomson’s cathode rays experiment 1897)

JJ Thomson subjected a cathode ray (electron particles) to see the effects of electric and magnetic fields.

In the beginning, in the absence of any electric or magnetic field, the electron from cathode rays struck the fluorescent.

Effect of Electric and Magnetic Fields

Then under the effect of electric field, they strike at point P2.
Similarly they strike at point P3 under the influence of magnetic field only. Now adjusted in such a way that the electron again strike at point P1.

Conclusion
In this way by comparing the strength of the two fields, he determined the e/m value of an electron which is 1.7588×1011 coulombs kg-1. This means that 1kg of electrons have 1.7588×1011 coulombs of charge.