Section 13: Option D - Turning Points in Physics Flashcards
What is thermionic emission?
The process of releasing charged particles (electrons or ions), from a heated metal surface
How are electrons emitted at the filament?
- Filament is heated due to the circuit
- Electrons gain sufficient energy (work function) to leave the metal surface
- This process is called thermionic emission
How is a cathode ray formed in a gas discharge tube?
- Electric field ionises gas atoms
- Positive ions in tube are accelerated to cathode and strike electrons in the surface
- Electrons in cathode emitted and accelerated towards the anode
Why should the gas in thermionic emission be low pressure?
Less molecules of gas therefore the electrons collide less, meaning they slow them down less and don’t reduce their energy as much
How high should the pd across the anode and cathode be in thermionic emission?
Discharge tube - high voltage supply
Electron gun - higher voltage supply
What were the conclusions from thermionic emission?
Electrons must be negatively charged
How do you calculate the speed of an electron from an electron gun?
Use the ½ 𝑚v^2 = 𝑒V equation where V is the accelerating pd, m is the mass of the electron and e is the charge of the electron
What was the method for how electrons moving through cross fields used to calculate specific charge?
- Electron beam from electron gun fired at 2 plates (positively and negatively charged)
- With just a pd across the plates, the electrons would be attracted to the positive plate
- Adding a strong enough perpendicular magnetic field where magnetic force = electric force, the electron will carry on in a straight line
Who and how was it decided that electrons were particles?
William Crooke’s
1. Electrons go from cathode to anode
2. Waves would reflect/diffract
3. They must be particles because they collide and transfer kinetic energy to the paddle
What are the equated equations for using electrons moving through cross fields?
- Electric force F=Eq F=Ee (F=Ve/d)
- Magnetic field F=Bev set Fs equal from electric field Ee=Bev E=Bv v=E/B
- Specific charge from eV=1/2mv^2 e/m=v^2/2V
- All together e/m = E^2/2VB^2
What was the method for how electrons moving through a fine beam tube used to calculate specific charge?
- Electron gun fires electron at a magnetic field in a fine beam tube
- Causes a centripetal force as the resultant force is always towards the centre by Flemming’s LH rule
- Measure the radius of the path
What are the equated equations for using electrons moving through fine beam tube?
- Centripetal force F=mv^2/r
- Magnetic field F=Bev
- Set forces equal Bev=mv^2/r Be=mv/r e/m=v/Br
- Using eV=1/2mv^2, sub in v from forces equal equation to get e/m=2V/B^2r^2
What did J.J. Thompson do?
- Discovered that electrons are negatively charged subatomic particles that exist in all atoms - plum pudding
- Established the specific charge of electrons - 1800 times greater than a hydrogen ion, this meant that cathode ray particles have a smaller mass than a hydrogen ion and so must be smaller than an atom
What was the method for Millikan’s oil drop experiment at terminal velocity?
- Atomiser (oil spray)
- Oil droplets - falling down due to weight mg
- Drag acted in the opposite direction (Stoke’s law)
- When at terminal velocity these are equal
What are the equated equations for Millikan’s oil drop experiment at terminal velocity?
- m=ρV m=ρ4/3πr^3
- Equated forces so ρπr^3g=stoke’s law
- So r = √ 9ηv/2ρg
What is the other method for Millikan’s oil droplet experiment and what other equation is used?
- Use a varying voltage until the oil droplet is stationary
- Set mg equal to VQ/d where Q=mgd/V
How did we quantize electron charge?
All the answers in Millikan’s oil droplet experiment had a common factor of 1.60 × 10^–19
What was Newton’s corpuscular theory of light?
All light is composed of corpuscules; tiny, weightless particles which travel in straight lines
* Light travels in straight lines split into horizontal and vertical components
- Reflection ✓ force pushes them away
- Refraction ✓ when they enter a denser material speeds up
- Interference X diffraction X polarisation X
What was Huygen’s wave theory?
Longitudinal wave in an aether
* Refraction ✓ wavelets slowed down in a denser material - wavefornts bend
* Reflection ✓
* Interfernece ✓ diffraction ✓ (Young)
* Polarised X
What were the problems with Huygen’s wave theory? How were they solved?
- Needed coherent sources - used a point source
- Didn’t polarise - Young deduced light is a transverse wave
What did James Clerk Maxwell theorise?
- An electric field and magnetic field oscillate perpendicular to each other in an EM wave
- EM waves were a spectrum of light with the same speed/different frequencies
From Maxwell’s equation for the speed of light what do ε0 and μ0 relate to?
- ε0 relates to the electric field strength due to a charged object in free space
- μ0 relates to the magnetic flux density due to a current-carrying wire in free space
What was Fizeau’s experiment for the determination of the speed of light?
- Light source
- Reflection at a mirror
- Passes through a rotating cog
- Reflects straight back at a mirror the other end,passes back through cog sometimes and reaches the observer
What were Fizeau’s calculations?
- When light passes through a gap between teeth on the cog, c=2d/t t=2d/c t=T (period)
- When returning light is blocked by a tooth t=T/2n t=1/2nf
- Equated together 2d/c=1/2nf c=4dnf