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
What are the assumptions when using eV = 1/2mv²?
- each electron starts from the cathode with negligible Ek compared to the work done on it by the accelerating pd
- the speed of the electrons in the beam is much less than the speed of light in free space, so the non-relativistic formula for Ek applies
How is the specific charge on an electron found in a deflection tube using a magnetic field?
- Bev = mv²/r so r=mv/Be
- combine this with 1/2mv²=eV (rearrange r for v)
- e/m = 2V/B²r²
Why are electron tubes usually evacuated?
So that the electrons are not slowed down by collisions
How is the specific charge on an electron found in a deflection tube using a magnetic field to deflect the beam and an electric field to balance the deflection?
- radius of curvature of electron beam in magnetic field is measured
- electron field applied to beam path is straight and forces due to fields are balanced
- eE = BeV and E = V/d so V = E/B
- r = mv/Be
- so e/m = v/E
How is the specific charge on an electron found in a deflection tube using an electric field to deflect the beam and a magnetic field to balance the deflection?
- horizontal speed of electron beam found from eE = BeV and E = V/d so V = E/B
- vertical deflection y of beam at end of plates is measured
- s = ut + 1/2at²
- hence e/m = ad/V where d is separation of plates
Before the specific charge of the electron was measured, what was known to have the largest specific charge of any charged particle?
Hydrogen at 9.6 x 10⁷ Ckg-1
Why could Thomson not conclude that the electron has a much smaller mass than a the hydrogen atom from e/m?
As neither the mass nor the charge of the electron was known at that time
How does the specific charge of an electron compare to that of a hydrogen atom?
The e/m of an electron is 1860 times larger
How did Millikan measure the charge on the electron?
- used oil spray and oppositely charged parallel metal plates
- plates horizontal so field vertical
- could make charged droplet stay stationary by adjusting pd between plates until electric force was equal and opposite to weight of droplet
In Millikan’s oil droplet experiment, what equations are used?
QV/d = mg
so charge on droplet, Q = mgd/V
In Millikan’s oil droplet experiment, how do the oil droplets become charged?
Frictional effects between the sides of the nozzle and the oil (or using X-rays)
In Millikan’s oil droplet experiment, why do some droplets move up and some move down?
- some have +ve/-ve charges
* some are heavy/light compared to their charge - move in different directions
In the equation Q = mgd/V, which quantity is hard to measure? How can this be solved?
- mass - use density
- so m =4/3πr³
- of which radius is hard to measure
- so use stokes law F (mg) = 6πηrv to find r
What significance did the results from Millikan’s oil droplet experiment have?
- could measure charge of each droplet
- found that the charge was always a whole number multiplied by 1.6 x 10⁻¹⁹
- so charge is quantised in whole number multiples
