Discovery of the Electron

How do cathode rays work

Gases at a low pressure in a sealed discharge rube conduct electricity when a p.d. is applied across the terminals

Where does a cathode ray glow the brightest?

At the cathode (negative terminal) and has a characteristic colour

Why does each charge side glow in a cathode ray?

The positive column glows due to deexcitation of excited gas atoms (this glow is easily distorted by a magnet)
The negative glow us due to photons emitted in recombination

What did JJ Thompson demonstrate with cathode rays?

Cathode rays have:

• energy, momentum and mass

• negative charge

• the same properties, no matter what gas is used

• a very large charge to mass ratio (much bigger than that of hydrogen ions)

What was JJ Thompson’s conclusion from cathode ray experiments?

all atoms have the particles found in cathode rays and called them electrons

How do discharge tubes work?

The high p.d. pulls electron off the gas atoms, forming ions

The positive gas ions are accelerated towards the cathode, colliding with atoms and releasing more electrons

The electrons are accelerated towards the anode, colliding with gas atoms, so they become excited. The gas atoms de-excite, releasing visible photons

Why is the glow in a cathode tube brightest at the cathode (negative electrode)?

positive ion and electrons recombine, emitting photons

Thermionic emission

A metal is heated until free electrons on it’s surface gain enough kinetic energy to leave the surface

The electrons are attracted to the anode through an evacuated tube so the electrons form a small, accelerated beam when passed through a hole (an electron beam)

work done on an electron

ΔW = eV

How to calculate the speed of an electron in thermionic emission

as the electron moves towards the anode, the electric potential energy is converted to kinetic energy to at anode:
½ mv² = eV
v = (2ev/m)^1/2 where e/m is the specific charge of an electron

How to determine the specific charge of an electron

fire an electric gun perpendicular to a magentic field
work done, ΔW = Fd = eV so F=eV/d

the magnetic force on an electron is equal to the centripetal force to Bev = mv²/r
so r = mv/Be and v = Bre/m

as ½mv² = eV
mB²r²e²/2m² = eV

e/m = 2V/B²r²

the path of the electon beam is visible as the electrons collide with gas atoms, which excite and de-excite

2V/B²r² can be measure so e/m for an electron could be determined

What does the electron’s specific charge tell use?

The specific charge was ~1800 times greater than the specific charge of a proton so the mass of an electron must be 1800x less than the mass of a proton

MIlikan’s oil drop experiment set up

An atomiser creates a fine mist of oil drops that are charged by friction when they leave the atomiser
Milikan applies a p.d. between 2 plates so an electric field is produce, which exerts a force on the charged drop

Force on oil drop in Milikan’s experiment if drop is stationary

Force upwards, due to electric field = force downwards due to weight
QV/d = mg

Q = mgd/V

Force on oil drop in Milikan’s experiment when oil drop is moving

A force of drag opposes the motion of the drop
F_drag = 6πηrv where η is viscosity of the liquid

At terminal velocity:
6πηrv = mg
volume = 4/3 πr³
mass = ρ x volume

6πηrv = 4/3 πr³ρg
r² = 9vη/2ρg and as ρ, g, η are constant r² ∝v

What did Milikan’s results show?

the charge of a droplet was always nx-1.6×10^-19 where n is a whole number

this showed that charge is quantised