AQA Physics Turning Points Flashcards
What did Thompson assume when researching cathode rays and how was this assumption used?
Electrons had the same charge magnitude as hydrogen ions
So as their charge to mass ratio was much higher than hydrogen ions, then they must have a high charge or tiny mass
How are cathode rays produced in a discharge tube?
The gas must be at sufficiently low pressure.
If voltage applied to the tube is high enough, some of the gas atoms are ionised.
Positive ions created near cathode surface are attracted to cathode. Photons are emitted when some of the ions and electrons produced by ionisation recombine.
Some electrons do not recombine and move towards anode. This causes excitation by collision of gas atoms. Photons are emitted when the atoms de-excite.
Thermionic emission and their use in electron beams
- This is when a metal is heated, and its 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
Defining the electron volt
- 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
a new unit of energy
Measuring the specific charge of an electron
- 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 on 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
Millikan’s oil drop experiment
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
What were J.J. Thomson’s conclusions about cathode rays?
- They have energy, momentum and mass.
- They are negatively charged.
- They have the same properties, regardless of what gas is used.
What affects the speed of the electrons produced in thermionic emission?
- Increasing the p.d. increases the force attracting the electrons towards the anode.
- This results in the electrons gaining higher speed.
How can you calculate the specific charge of an electron?
Using a magnetic field
r = mv/Be
e/m = v/Br
What was the significance of the specific charge of an electron?
Thomson showed that the specific charge of an electron is 1.76 x 10^11 C/kg.
This was 1860 times larger than a hydrogen ion (the largest specific charge known at the time).
Describe Millikan’s oil drop experiment.
Controlled the motion of charged oil droplets using the electric field between oppositely charged parallel plates.
Made any charged droplet stay stationary by adjusting the p.d. between the plates until the electric force was equal and opposite to the weight of the droplet.
How did Millikan measure the mass of an oil droplet?
Define strokes law
To hold an object stationary in a fluid, the force of a field must equal the viscous drag force F= 6pi × nrv
What is an atomiser
A device which changes a liquid into a mist(small drops) by forcing it through a very small hole
How does an atomiser charge droplets
Friction, postive is electrons lost. Negaticlve if electrons gained
