The discovery of the electron

Question 1

What is thermionic emission?

Answer 1

When heat gives the free electrons in a metal enough energy to break free from the metal’s surface.

Question 2

Why does the glass tube in an electron gun have to be evacuated?

Answer 2

Electrons are easily stopped or deflected by atoms. So the glass tube in an electron gun has to be evacuated of air so that the electron can travel freely in the electric field.

Question 3

Where does the formula 1/2mv^2=eV come from?

Answer 3

• The formula for work done in moving a charge through a potential difference is ∆W=Q∆V.
• Charge ,Q , is the charge of the electron and ∆V is the potential difference between an anode and cathode, V, then ∆W=eV.
• The kinetic energy that an electron will have when it has been accelerated (from rest) is equal to the work done in accelerating it through the potential difference. ∴ 1/2mv^2=∆W= eV