Section 13 - Turning points (Discovery of the electron) Flashcards
(16 cards)
What is a discharge tube?
a glass tube with an anode and cathode with a low pressure gas within it which has a large pd across tube, marking parts of it glow.
what did JJ thompson demonstrate about cathode ray tubes?
1) they have energy/ momentum
2) they have a negative charge
3) they have the same properties, regardless of type of gas
4) specific charge greater than hydrogen ions, so they either had a smaller mass or greater charge
so what were electrons known as?
cathode ray particles
Explain how the glowing areas form in 3 steps
1) naturally occuring free electrons in the gas acclerate towards positive anode
2) electrons collide with gas atoms and ionise them, creating more positive opns and more free electrons
3) free electrons create three glowing areas
Explain the negative glow
1) positive ions accelerate towards negative cathode
2) ions collide with cathode, so more free electrons released from the cathode
3) electrons begin to accelerate towards the anode
4) some of these electrons collide with positive ions near cathode to form photons
Explain the positive column glow
1) free electrons accelerate towards anode, they collide with gas atoms, exciting them, so excited gas atoms de-excite and emit photons
Explain the glowing glass
1) some electrons accelerating towards anode go so fast that they miss it
2) electrons collide with end of glass tube
3) exciting molecules in the glass, as it glows it de-excites and emits photons
Explain process of thermionic emission
1) A metal is heated by the current, and it’s free electrons will gain kinetic energy
2) if they have enough energy, the electrons break free from the surface
3) once emitted, these electrons can be accelerated by an electric field in an electron gun
Explain how the electrons are acclerated by an electric field
1) heating coil will emitt electrons which are accelerated towards cylindrical anode
2) some electrons pass through a little hole in the anode, which makes a narrow beam of electrons
3) electrons in the beam move at a constant v due to no field beyond anode, so no force
Define 1 eV
the kinetic energy carried by one electron after it has been accelerated from rest through a potential difference of one volt.
Explain the processes of how Thomson measured the specific charge of the electron
1) beam of electrons pass through low pressure gas, and H atoms along path absorb energy
2) excited H atoms emit light as they dexcite
3) Two circular magnetic field coils generate a uniform electric field in the tube
4) beam is initially fired at right angles to the magnetic field, beam curves around a circle so m(of e)xv^2/r = BeV
5) rearrange with eV = 1/2mv^2 to make e/me = 2V/B^2r^2
What is stokes law, as a formula?
F = 6π𝜂 rv
Draw set up for milikans experiment, explain each use on the diagram
Atomiser - made a fine mist of oil drops which are charged by friction
Microscope - to view the fallen drop in the hole with a scale to measure d and therefore its velocity
Variable pd - application of a pd between two plates to produce a field that exerted a force on the drop
What were the forces acting on the oil drops when the electric field was turned off?
1) weight of the drop acting downwards
2) viscous force from the air acting upwards
what would the formula be when the oil drops meet terminal velocity ?
mg = 6π𝜂 rv
What are the forces on the oil drop when it is stationary ?
1) weight of drop acting downwards
2) force due to uniform electric field acting upwards
QV/d =mg
