Thermionic emission

As you may know, metals contain many free electrons moving at high speed and at random between the atoms of the material. If these free electrons are given enough energy they may actually escape from the metal surface.




We can compare the motion of the free electrons (Figure 2) with that of a pond full of fish (Figure 1). The fish are crowded together in the pond and swim around vigorously - leaping in and out of the water to give a dynamic equilibrium (as many fish are leaving the water at any time as are returning to it).

If the energy of the fish is increased more of them will be leaping out and falling back - the process will be come more violent. The faster they swim the more will leap out and the further from the water will they get before they fall back in again. If some incentive is given for them to leave the water for good - for example a worm - there will be a flow of fish away from the pond!

One way to give the free electrons sufficient energy to allow them to escape is to heat the metal. If you put a wire carrying a fairly large current (6A) close (within a couple of mm) to the plate of a positively charged electroscope the emission of electrons from the wire will discharge the electroscope.

If we put a negative plate in front of our hot metal any electrons that are emitted will be pushed back towards the metal by the negative charge on the plate. None of them will get very far from the metal. This happens because the electrons are also negatively charged.







If we make the plate positive the electrons will get attracted towards it, but if the space between the metal and the plate contains air the electrons will not get far. They will collide with the molecules of the air, and so very few will reach the positive plate. Hence the reason for putting the hot wire close to the electroscope plate in the previous experiment.






However if we put the metal and the plate in an evacuated glass container the electrons will be pulled across. The hotter we make the metal the more electrons will escape and travel to the positive plate. Increasing the voltage on the plate will also increase the flow and make them move faster.