9. Turning Points Flashcards
Why does a discharge tube apply a high voltage?
To ionise the gas atoms in the tube
What happens to gas atoms after they have been ionised in a discharge tube?
They are postive ions and are attracted to the cathode
What effect does positive ions being attracted onto the cathode surface have?
Free electrons from the cathode surface are emitted
Where do the electrons come from in a discharge tube?
They are both emitted by the cathode and produced in the production of positive ions
Why were electrons called cathode rays?
Because they were observed to originate at the cathode
How do electrons cause a glow in the discharge tube?
By collisions with gas atoms, causing them to be excited, emitting visible light on de-excitation
At what point is visible light emitted in cathode ray tubes?
When a gas atom de-excites, after an electron has collided with it and excited it
When are photons emitted in cathode ray tubes?
When positive ions and electrons recombine
Which way do electrons travel in cathode ray tubes?
Towards the anode
What type of pressures are the gases in discharge tubes?
Low
What does the colour of light emitted by a discharge tube depend on?
The type of gas inside the tube
What is created when a high voltage is applied between the anode and cathode in a discharge tube?
A strong electric field
Why is the gas in a cathode ray tube kept at low pressure?
So there is space between gas atoms to allow positive ions to be accelerated towards the cathode
What is thermionic emission?
When a metal filament is heated by passing a current through it, and some of the free electrons gain enough Ek to leave the metal surface
During thermionic emission, what is the anode and cathode?
Cathode - the wire filament
Anode - Metal plate with a small hole in it
In thermionic emission, how are electrons accelerated through the hole in the anode?
A high pd between the cathode and anode
In thermionic emission, what is work done on the electrons, eV, equal to?
Kinetic energy gained (1/2mv²)
What does thermionic emission essentially produce?
An electron beam
How is the metal heated in thermionic emission?
By passing an electric current through it
Does the glass tube used in thermionic emission have gas molecules in it?
No - it is evacuated
In thermionic emission, what is the equation that gives the speed of each electron in the beam from the anode?
eV = 1/2mv²
Why can the speed of electrons in thermionic emission be calculated by eV = 1/2mv²?
Their Ek doesn’t change after they pass the anode, as the electric field is no longer acting
Which is a more effective way of freeing electrons from a metal to create electron beams; thermionic emission or cathode ray tubes?
Thermionic emission
What is the work function of a metal?
The minimum energy needed to remove one of the free electrons to just beyond the surface of a metal