Question

Why don't we get maximum current (saturation current) at 0 potential

• A. If we increase the potential of the plate the number of electron coming out also increases as the field from the collector plate is also aiding in the ejection of electrons.
• B. The space near the emitter plate is full of freshly ejected slow moving electrons which restrict new low energy electrons from coming out.
• C. At 0 potential the energy of the electrons is very less so they take long time to reach the other plate hence the low current.
• D. Scientist are still trying to figure that out.

Answer 1

The correct option is B

The space near the emitter plate is full of freshly ejected slow moving electrons which restrict new low energy electrons from coming out.

We know that work function is the minimum work done (energy required) to remove an electron from the metal surface.

So the free electrons on the surface, when they get energy which is greater than the work function then they come out

Where as there are other free electrons that might not be on surface and after getting the same energy loose some in collisions with other electrons. If after collisions the remaining energy is not sufficient enough or not greater than the work function then it won’t come out else it will come out with a lesser kinetic energy.

These electrons which have low K.E hence clutter near the emitter plate as their velocity is low.

This cluttering creates a negative space charge.

Now this space charge repels new electrons that have low K.E. from coming out.

But once you increase the potential all electrons gets attracted or accelerated towards the positive plate thereby removing the earlier repulsive space charge

Option B is correct

Option A also sounds correct but technically you will need a very large potential to pull electrons from the emitter plate where as in photo electric effect we deal with potentials in the range of 2 – 5 v