The Discovery of the Electron

Question 1

What is a cathode

Answer 1

A negatively charged electrode

Question 2

What is an anode

Answer 2

A positively charged electrode

Question 3

How are cathode rays produced in a discharge tube

Answer 3

A pd is applied between the anode and the cathode which caused the emission of cathode rays

Question 4

How does the discharge tube conduct

Answer 4

The electric field between the electrodes ionises the gas particles in the tube.
This separates the atoms into positive ions and electrons
Negative electrons are attracted to positive anode.
Positive ions attracted to the negative cathode.
This only happens because the pressure of the gas is low enough to allow particles to travel.
Electrons are emitted from cathode and travel to anode.
Conduction is a result of the electrons and positive ions being accelerated across tube by electric field.

Question 5

Why does the gas glow in a discharge tube

Answer 5

Electrons and positive ions are travelling in opposite directions in the tube.
Due to low pressure, they have space to gains lots of KE.
When they collide, they recombine in an excited state.
Electrons in the atoms de-excite to ground state, emitting visible photons

Question 6

What is thermionic emission

Answer 6

When the electrons in the heated cathode/filament have more energy in their kinetic energy stores. This is enough to leave the surface of the metal and move towards the anode.

Question 7

Explain why a passing current through the filament causes the emission of electrons

Answer 7

The current heats the wire.
The electrons in the filament gain enough KE to escape the filament and accelerate towards the anode

Question 8

Work done in an electron accelerated through a pd V =

Answer 8

1/2 mv^2 = eV

Question 9

What happens to a charge when moving in a magnetic field

Answer 9

It experiences a force perpendicular to its motion

Question 10

How do you find the specific charge of an electron using just a magnetic field

Answer 10

F = mv^2 / r

F = Bev

v = Ber / m

eV = 1/2 mv^2

e/m = 2V / B^2r^2

V, r and B can all be measured

Question 11

How do you find the specific charge using both magnetic and electric field - JJ Thompson’s method

Answer 11

The helmoltz coil generates a magnetic field.
Oppositely charged plates generate an electric field.
Magnetic field points into plane of page and exerts downwards force, whereas electric field generates upwards force.

Fe = Fb
Fe = eE = eV / d
Fb = Bev

Bev = eV /d
v = V/Bd
Electric field is then turned off and beam forms circular path so we can use magnetic field equation

v = Ber/ m
e/m = V/ rB^2d

Question 12

How do you find the specific charge using just an electric field

Question 13

What does the particles deflecting in magnetic and electric fields suggest

Answer 13

They are negatively charged

Question 14

Difference between specific charge of electron and hydrogen ion

Answer 14

S.C of electron is x1800 larger than hydrogen ion

This meant either :
Electron had much smaller mass or much larger magnitude of charge

Question 15

What did Millikan’s Oil Drop experiment determine

Answer 15

The value of fundamental/elementary charge

Question 16

Describe the method for Millikan’s oil drop experiment

Answer 16

A fine mist of atomised oil drops is sprayed into a chamber.
As the drops pass out of the spray nozzle, they are ionised by X-rays
The drops pass into a region between 2 metal plates and are viewed by a microscope

Question 17

Why is oil used instead of water in Millikan’s experiment

Answer 17

Oil does not evaporate quickly and so mass of drops remains constant.

Question 18

What happens to the drops when they are ionised by X-rays in Millikan’s experiment

Answer 18

It changes their charge from neutral to positive or negative depending on whether they gain or lose electrons

Question 19

What is the condition under which the oil drops are held stationary between oppositely charged plates

Answer 19

QV/d = mg

Negative oil drops with charge Q experience an upward force from the uniform electric field.
There is a downwards force of the weight of the oil drop.
Both forces must be equal so there is no resultant vertical force on each drop

Question 20

What happened to the oil droplets when the electric field was turned off

Answer 20

They fell at terminal velocity as their weight was balanced by drag forces as they fell.

Question 21

Stokes Law

Answer 21

For an object with radius r moving at v through fluid viscosity n, the viscous drag force is :

F = 6 x pi x n x r x v

Question 22

How was mass calculated from Stokes’ Law

Answer 22

6 x pi x n x r x v = mg
m = density x volume = 4/3 x p x pi x r^3
combine equations to get mass

Question 23

What was significant about Millikan’s results

Answer 23

Each oil droplet had different charges, but each value was an integer multiple of 1.60x10^-19.

This suggested that this was the charge of each electron since each negative oil droplet had been ionised and hence carried an integer number of electrons

Question 24

Explain why the temperature of the wire increases when the current is increased

Answer 24

When current is increased, the electrons have more KE and when they collide with lattice ions they transfer more KE to lattices’ energy therefore increasing wire’s temperature. The electrons are also moving faster therefore more collisions per second

Question 25

Explain why thermionic emission if negligible when the filament current is too low

Answer 25

Temperature depends on current.
KE of electrons depends on the temperature of the filament.
Electrons must do work/ overcome work function to leave the metal

Question 26

Why must the container be evacuated in thermionic emission

Answer 26

So the electrons do not collide with gas atoms and lose KE.
The electrons from the wire would be absorbed by the gas atoms