DISCOVERY OF THE ELECTRON

Question 1

What is a cathode ray? And what is the setup?

Answer 1

• A cathode ray is a beam of electrons, called a cathode ray as it appears to be a glowing ray that originates from the cathode
• The glow is caused by excitation: the free electrons excite the gas particles and when they de-excite they release photons, this is also called “recombination”

Question 2

Thermionic emission and their use in electron beams

Answer 2

• This is when a metal is heated, and it’s free electrons gain enough kinetic energy to break free of the surface of the metal
• This is used to create electron beams as when free electrons are emitted they can be accelerated through a P.D

Question 3

Defining the electron volt

Answer 3

• Work done on a particle of charge Q, accelerated through a PD of V, is equal to QV joules
• replace Q with the charge of an electron, and we get eV
• work done = 1/2 mv^2 = eV
• new unit of energy

Question 4

Measuring specific charge of an electron

Answer 4

• fire an electron beam in a glass bulb containing hydrogen at low pressure
• the beam will excite the hydrogen atoms, and allow us to see its path
• put magnetic field coils either side of the bulb to create a uniform magnetic field that causes the electron beam to have a circular path
• this means magnetic force = centripetal force

Question 5

Millikan’s oil drop experiment

Answer 5

• an atomiser creates a thin mist of oil drops that are charged by friction as they leave the atomiser
• some drops fall through a hole in the top plate and reach terminal velocity where mg = 6π x viscosity x rv

mg = 4/3 πr³ x density x g

r² = 9 x visc x v / 2 x density x g

• when the electric field is turned on it is adjusted so that the drop is stationary, as F = QV / d

QV/d = mg = 4/3 πr³ x density x g

• values for Q were all multiples of 1.6 x 10^-19 so it was assumed this was the fundamental unit of charge and the charge of the electron