Discovery of the Electron Flashcards
How does a discharge tube produce cathode rays?
A discharge tube contains a low pressure gas and an anode and cathode.A voltage is applied across the cathode and anode.This creates a strong electric field which ionises the gas atoms by pulling electrons off them.This creates positive ions which are accelerated by the voltage towards the cathode.As a result they move a high speed and when they collide with the cathode electrons are released as a result.These electrons are attracted towards the anode so as they move towards the anode they collide with the gas and glass atoms exciting the electrons within them.When these electrons deexcite visible photons are released which causes the light.These electrons also recombine with the positve ions creating excited atoms and then dexcite producing light.
Why is the gas at a low pressure?
To allow the ions and electrons to travel to their destinations without losing a lot of energy through collisions.
What is an electron volt?
It is the kinetic energy carried by an electron after being accelerated from rest through a pd of 1v.
What is the ke gained by an electron equal to as it passes an anode?
1/2mv^2=eV.
What is thermionic emission?
This is when electrons are emitted from a metal by heating using a filament.Heating a metal provides the free electrons with sufficient energy to overcome the work function of the metal.This leads to them being emitted and accelerated through the pd to the anode
How does an electron gun work?
An electron gun contains a filament, cylindrical anode, cathode and and evacuated tube.The cathode is heated using the filament,this provides the free electrons with sufficient ke to overcome the work function of the cathode metal and leave the cathode.These electrons are accelerated by the electric field towards the anode.Only some electrons are able to pass through the narrow gap of the anode creating a beam.These electrons are travelling at a constant speed as there is no electric field past the anode.
What effect does increasing the current and voltage have?
Increasing the current supplied to the filament causes more electrons to be emitted per second.Increasing the voltage increases the speed of the electrons towards the anode.
Why does an electron gun have to be evacuated?
The electrons are much smaller than the air atoms in the gun so they may cause the electrons to stop or get deflected.Therefore, the gun must be evacuated to allow the electrons to reach the anode.
What was the term cathode rays used to describe in the 1800s?
They were believed to cause a glow on the end of the discharge tube.
How does a fine beam tube work?
An electron gun fires a beam of electrons into a bulb containing low pressure hydrogen gas which is surrounded by 2 circular magnetic coils.The electrons collide with the gas atoms causing them to excite and the dexcite which emits light.The beam is fired at right angles to the magnetic field so the force exerted on the electrons is at right angles and causes them to take a circular path.By using the circular motion formula and the ke formula we can find the specific charge of the electron.The two coils create a uniform magnetic field either side of the bulb.
How do you calc the specific charge of an electron?
Use F=BeV and F=mv^2/r and 1/2mv^2=eV and you will get e/m=2V/B^2R^2
What was the significance of Thomson’s discovery?
He calculated that the specific charge of electrons was 1800 times larger than the specific charge of H+ ions which was the largest at the time.This enabled him to conclude that protons and electrons had the same charge.But the mass of electrons is 1800 times smaller than protons.He was also able to confirm that they were charged particles with mass energy and momentum.
