Question

Explain what would happen if we began Charge-to-Mass Ratio of Electrons experiment with a dot on the CRT (cathode-ray tube) screen instead of a line?

Answer 1

Electrons emitted from cathode are accelerated by the anode potential to give a line on the screen because this anode potential will generate transverse force. The length of the line shall depends on the strength of the applied potential. If we apply small anode potential then the length of the line will be small.

Now for finding specific charge of the electrons (e/m) we have to apply magnetic field through solenoid in such a way that the line focusses into a point. Now by balancing the electric and magnetic forces acting on the electrons, one can calculate the (e/m) ratio very easily.

But now if we apply very small anode voltage to the electron beam then we will get a point on the screen instead of the line. Now we can't find how much magnetic field has to apply to focus a point to another point. So, now we are unable to find the specific charge of the electron. Hence in Order to specific charge ratio we must get line on the screen instead of the point.