Turning Points Flashcards
1
Q
Who discovered cathode rays?
A
Thomson
2
Q
How are cathode rays formed?
A
- a potential difference is applied across a discharge tube with a low pressure gas
- the tube would start to glow brightly (brightest at the cathode)
3
Q
Properties of cathode rays?
A
- They have a mass
- They have a negative charge
- They have the same properties no matter what gas is used in the discharge tube
- very large charge to mass ratio
4
Q
Cathode rays modern day name?
A
- electrons
5
Q
How does the discharge tube begin to glow?
A
- high potential difference across the discharge tube will pull electrons off the gas atoms forming ions and electron pairs
- the positive gas ions are accelerated towards the cathode and when they collide with it they release even more electrons
- the electrons are accelerated along the tube (low pressure gas means they accelerate quickly)
- they collide with gas atoms casing them to become excited
- the atoms quickly de-excite and release visible photons
6
Q
Why is the glow brightest at the cathode?
A
- here the gas ions and electrons can recombine and emit photons of light
7
Q
Cathode Rays Experiment
A
8
Q
What is thermionic emission?
A
- it is where a metal is heated up until free electrons on its surface gain enough energy and are emitted
9
Q
How do electron guns work?
A
- they use a potential difference to accelerate electrons, which are released from the cathode through thermionic emission
- the electrons are accelerated towards the anode which has a narrow gap
- this forms a narrow electron beam
- work done on electron = eV
