The Discovery of the Electron Flashcards
What is a cathode
A negatively charged electrode
What is an anode
A positively charged electrode
How are cathode rays produced in a discharge tube
A pd is applied between the anode and the cathode which caused the emission of cathode rays
How does the discharge tube conduct
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.
Why does the gas glow in a discharge tube
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
What is thermionic emission
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.
Explain why a passing current through the filament causes the emission of electrons
The current heats the wire.
The electrons in the filament gain enough KE to escape the filament and accelerate towards the anode
Work done in an electron accelerated through a pd V =
1/2 mv^2 = eV
What happens to a charge when moving in a magnetic field
It experiences a force perpendicular to its motion
How do you find the specific charge of an electron using just a magnetic field
F = mv^2 / r
F = Bev
v = Ber / m (1)
eV = 1/2 mv^2 (2)
substitute equation for v above (1) into equation (2)
e/m = 2V / B^2r^2
V, r and B can all be measured
How do you find the specific charge using both magnetic and electric field - JJ Thompson’s method
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
How do you find the specific charge using just an electric field
t = w/v
a = Fe / me
a = eV / dme
s = ut + 1/2 at^2
y = 1/2 x a x w^2 / v^2
a = eV / dme
e / me = ad / V
What does the particles deflecting in magnetic and electric fields suggest
They are negatively charged
Difference between specific charge of electron and hydrogen ion
S.C of electron is x1800 larger than hydrogen ion
This meant either :
Electron had much smaller mass or much larger magnitude of charge
What did Millikan’s Oil Drop experiment determine
The value of fundamental/elementary charge
Describe the method for Millikan’s oil drop experiment
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
Why is oil used instead of water in Millikan’s experiment
Oil does not evaporate quickly and so mass of drops remains constant.
What happens to the drops when they are ionised by X-rays in Millikan’s experiment
It changes their charge from neutral to positive or negative depending on whether they gain or lose electrons
What is the condition under which the oil drops are held stationary between oppositely charged plates
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
What happened to the oil droplets when the electric field was turned off
They fell at terminal velocity as their weight was balanced by drag forces as they fell.
Stokes Law
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
How was mass calculated from Stokes’ Law
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
What was significant about Millikan’s results
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
Explain why the temperature of the wire increases when the current is increased
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
Explain why thermionic emission if negligible when the filament current is too low
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
Why must the container be evacuated in thermionic emission
So the electrons do not collide with gas atoms and lose KE.
The electrons from the wire would be absorbed by the gas atoms
