Turning points - Discovery of the electron Flashcards
To produce cathode rays, why is the gas inside a discharge tube at low pressure?
The gas is at low pressure because at low pressure, there is a significant amount of space for the positive ions to gain a sufficient amount of kinetic energy store, and be accelerated towards the cathode.
What are cathode rays?
The positive ions created from the ionisation of gas atoms by the electric field collide with the metal surface of the cathode, causing electrons to be released. These electrons, due to attraction, travel towards the anode, and they travel in a straight line. A beam of these electrons are called cathode rays.
We can get discharge tube with a paddle which s seen to rotate. What is the significance of this?
The electrons from the cathode are accelerated towards the anode due to the electric field. In this process the electrons collide with the paddle and transferred momentum to the paddle, causing it to move. The transfer of momentum is a particle property, and this showed that cathode rays are particle.
How does the discharge tube glow?
The electrons released from the cathode due to colWelisions of the positive ions with the cathodes metal surface, and these electrons collide with the gas atoms in the discharge tube causing excitation or ionisation of these atoms. When the atoms deexcite they emit visible light photons causing the gas to glow, causing the discharge tube to glow.
What is thermionic emission?
Thermionic emission is a way of producing cathode rays (electrons) without the need for a gas, like in a discharge tube.
What is the effects of increasing the voltage during thermionic emission?
Increasing the voltage strengthens the electric field, so there is an increase in the force attracting the electrons to the anode, causing an increased acceleration of the electrons towards the anode, meaning the electrons have a greater speed.
What is the effect of increasing the current of the filament during thermionic emission?
Increasing the current means the temperature of the filament is greater, so there is an increase in the rate of electrons ejected each second, and this means an increase in the intensity.
What is the derivation of finding the specific charge of an electron using a cathode ray?
qE=Bqv (electric field and magnetic field must be equal for no deflection)
v=E/B
1/2mv^2 = QV (W=QV)
Q/m = v^2/2V
We can sub E/B into v
Q/m (specific charge) = E^2/2VB^2
What was the significance of Thompsons method to find the specific charge of an electron (method using both magnetic and electric field)
From the deflection of the particle in the field, it was shown how the particle was negatively charged.
The value Thompson got for his specific charge was much larger than the value for the specific charge for helium which was already knows during his time. Specific charge = charge/mass and this showed that the particle must have either a smaller mass than helium, or a greater charge.
In the fine beam tube experiment to find the specific charge and why did the tube glow?
There is an electron gun firing beams of electrons into a bulb which contains low pressure gas. There is also a magnetic field in the bulb. As the electrons are fired, they bombard with the gas atoms in the bulb and this causes the atoms to be excited. The atoms immediately de-excite, and during this process they release visible light photons.
In the fine tube beam experiment, why was the path of the electrons seen to be circular?
This is due to the magnetic field that the electron passes through. The electron feels a magnetic force Bqv and this force is equal to the centripetal force, which is why the electrons path is circular.
In Milikans oil drop experiment, the oil droplets were neutral, so how did they become positively or negatively charged?
The oil droplets were sprayed using an atomiser, and these droplets were initially neutral. Milikan exposed the chamber to X-rays which ionised the air. The oil droplets collided with free electrons to become charged by gaining or losing electrons.
Another way they became charged was due to friction.
What was Milikans oil drop experiment?
He wanted to find the elementary charge.
When an electric field was turned on, the electric field upwards was equal to the weight downwards when the oil droplet was stationary.
qE=mg
When the electric field was turned off, the resistive force , the drag force upwards was equal to the weight downwards. The drag force was found using stokes law and then equated to mg.
The droplets were assumed to be spheres, so we can use the equation density x volume to find the mass, remember that the equation for the vol of a sphere is 4/3 pi r ^3.
What did Milikan find in his experiment?
He found that the charge on the oil droplets was always a multiple of the elementary charge 1.6x10^-19. Milikan concluded that this was the elementary charge and that charge does not exist in a quantity smaller than the elementary charge. He concluded that this was the charge f n electron.
