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
How did Hertz disover radio waves?
- Maxwell’s model - oscillating magnetic and electric fields are perpendicular
- Oscillating current in a radio wave transmitter indicates presence of electric field
- This then produces an oscillating magnetic field
- The varying magnetic field induces an emf
What is the ultraviolet catastrophe?
- Disagreement between the practical and theoretical predicitons of the intensity of energy at different wavelengths
- The theoretical prediction was that there was an infinte amount of energy/intensity at the very low wavelengths and at long wavelengths the intensity/energy was very similar
How did Planck solve the ultraviolet catastrophe?
He suggested that EM radiation is released in photons (which are related to a frequency and not continuos , therefore it is not infinite)
What is the de Broglie wavelength equation?
- 1/2mv^2=eV so v=√2eV/m
- v=h/mλ
- √2eV/m = h/mλ
- λ=h/√2eVm
What pattern was observed in electron diffraction and why is the pattern seen?
- A pattern of concentric rings/diffraction rings are seen
- Due to the constructive and destructive interference that occurs from when the electron beam is diffracted
How does increasing the electron speed change the diffraction pattern?
- The wavelength of the wave will decrease
- The diffraction rings would appear closer together
What is an estimate of anode voltage needed to produce wavelengths of the order of the size of the atom?
150V
What is resolving power?
An object’s ability to acknowledge two seperate objects close together are separate objects, not just one
How are images formed in the TEM?
- Electron beam produced by thermionic emission is accelerated using an anode
- Condenser lense - magnetic field which focuses the electrons by deflecting them so they all fall straight
- Thin sample - thin bits lots of electrons go through, thick bits less electrons go through
- Objective lens - focuses the image
- Projector lens - flips and projects the image onto a screen
- Fluorescent screen - lights up when hit by electrons
What are the factors that affect the quality, and level of detail in the image produced by a TEM?
- Wavelength depends on the speed of electrons
- Lower wavelength = greater detail
- Emitted electrons come from the heated cathode so have a speed that’s dependant on temperature
- Speed of the electrons isn’t always the same therefore different pathways through the lens so aberration
- Sample thickness reduces speed of electrons, increasing wavelength, and decreasing detail
How does decreasing the anode potential affect the detail of the image?
- A lower anode potential would mean the electrons gain less kinetic energy
- Their wavelength will increase
- The resolving power would decrease making it less detailed
How does a STM work?
- Creates images using quantum tunneling
- A fine probe/needle is placed close to the surface of a material to make a tunneling current between the surface and the probe
- A small pd goes between the probe and the surface so electrons cross the gap from negative to positive
What are the 2 methods to using a STM and why do these work?
- Fix the distance, keep the probe height the same and measure the changes in current
- Fix the current by adjusting the probe height and measure the changes in probe height
The closer the gap > probability of tunnelling > current
What was the aim of the Michelson-Morley experiment?
To prove that there was ether by showing that the speed of light changed with different rotations in respect to the ether
What is absolute motion?
Motion is independent of where it is being observed from - Galileo and Newton
What is the ether theory of light?
There should be a measureable difference in the time taken for light to travel parallel and perpendicular to the motion of the Earth
What are the components of an interferometer?
- Monochromatic light source
- Semi silvered mirror
- Glass compensator block - 2 beams travel through the same amount of air and glass
- 2 mirrors at either end of the light paths
- Interference pattern is observed
What was the expectations of the Michelson-Morley experiment?
It was expected that when rotating the blocks and the mirrors by 90 degrees, the interference pattern seen should change as the speed of light should be changing in respect to the ether
What were the conclusions of the Michelson-Morley experiment?
- No shift in interference pattern
- Can’t measure absolute motion - ether didn’t exist
- Speed of light is invariant
- All rays of light were travelling at the same speed no matter which direction
- Nul result
What were the 2 postulates of Einstein’s theory of special relativity?
- Physical laws have the same form in all inertial frames
- The speed of light in free space is invariant
What are the consequences of special relativity?
- Time dilation
- Length contraction
- Relativistic mass
What is time dilation?
Time runs slower for an observer when observing a moving object than it does for the object itself the effect increases the closer you approach to the speed of light - a moving clock runs slow
What is proper time?
The time an observer in an inertial reference frame measures the time interval to be that it takes for something to happen
What is the equation for muon decay?
μ − -> e- + νμ + anti ve
How does muon decay provide evidence for time dilation?
- The intensity of muons with distance decreased less than expected
- The half life to human observers is much larger than the actual half life
How do you calculate the expected count rate in muon decay?
(0.5)^half life time x number of muons
How does special relativity effect lengths?
Length contraction means an observer parallel to a moving object will observe a shorter length than an observer at rest relative to the rod - a moving rod looks short
LENGTHS AT RIGHT ANGLES DON’T CHANGE
What is a relativistic object?
One that is moving so fast that the effects of relativity become significant
How does special relativity affect mass?
A moving mass weighs more - linked to E=mc^2, using relativistic mass E=m0c^2
How does Newton’s corpuscular theory explain refraction?
- Light is made up of particles/corpuscles
- Force acts attracting them to the water.
- Attraction only affects motion at the
interface/boundary. - Only one component of velocity / momentum
(vertical) changes at the interface. - The (vertical component of) velocity /
momentum increases which causes the
change in direction
How does Huygen’s wave theory explain refraction?
i)Wave front is incident on interface
ii)Huygens secondary wavelets at wave fronts.
iii)Wavelets travel more slowly in the water.
iv)The slowing down of the waves / wavelets
causes the change in direction.
What was the experimental proof that Huygen’s theory was correct?
- When the speed of light was measured in
water, the value found supported Huygens’
prediction that light would travel slower in water - Interference effects in Young’s double slit
experiment can be explained by Huygens’
wave theory but not by Newton’s corpuscular
theory.
What is an EM wave?
(vibrations of) the electric wave/field and magnetic wave/field are
* perpendicular to each other
* perpendicular to the labelled direction of motion
* in phase with each other
What is de Broglie’s hypothesis?
- All particles have a wave-like nature
- An associated wavelength λ=h/ρ
How are diffraction rings changed so that the maxima are closer together?
- Use target with greater spacing between atoms - larger separation gives rise to narrower fringes
- Increase the p.d. - Reduces wavelength (which gives rise to narrower fringes)
- Decrease distance between foil and film - for same angles, maxima closer together
How does special relativity affect EK?
- v has upper limit as Ek increases
- As v increase mass increases
- As v approaches c, mass approaches infinity
- Near c, Ek tends to infinity
Calculated with E-E0
What is black body radiation?
- Electromagnetic radiation
- In a spectrum with a peak that is dependent on temperature
What is quantum tunnelling?
- Electrons have to overcome the potential barrier
- So a fraction will move from the barrier to the tip
What is the size of an atom?
0.1nm
How can you use electron diffraction to support the de Broglie hypothesis?
- De Broglie hypothesis suggests that λ will
decrease/increase if the momentum
increases/decreases - Measure pd across anode and cathode to determine (KE of electrons and
therefore) momentum/speed of electrons - Measure ring diameter as
increased/decreased diameter indicates
increased/decreased wavelength - Increasing/decreasing pd resulting in
decreased/increased ring diameter
What is an inertial reference frame?
One which moves at a constant velocity and in which Newtons laws are obeyed
