1
Q
What is an electrode? What is a cathode and anode?
A
Electrode: conductor through which electricity passes
Cathode: negatively charged electrode
Anode: positively charged electrode
2
Q
What are discharge tubes made of?
A
- Glass chambers containing low pressure gas, with an anode at one end and a cathode at the other, connected to high voltage supply
3
Q
What was the observation in the discharge tube?
A
- When p.d. was applied, the gas glowed
- Hypothesised that glow was caused by emissions from cathode called cathode rays
4
Q
How was charge of cathode rays found?
A
When magnetic field applied to tube, path of cathode rays were deflected in a direction consistent with negatively charged objects, showing that cathode rays are made from negatively charged particles (electrons).
5
Q
How does the discharge tube conduct electricity?
A
- Electric field between electrodes ionises the gas particles in the tube
- Separates atoms into positive ions and electrons
- Postive ions attracted to cathode
- Can only happen because pressure is low enough to allow charged particles to travel
- Electrons are emitted from the cathode and travel towards the anode
- Conduction is a result of electrons and ions moving across the tube
6
Q
Why does the gas in a discharge tube glow when p.d. is applied?
A
- Electrons and positive ions travelling in opposite directions
- Due to low pressure, space to gain a large amount of kinetic energy
- When the ions and electrons collide, they recombine in an excited state
- Electrons in atoms de-excite to ground state, emitting visible photons (and other frequencies)
7
Q
How do you create a cathode ray?
A
- Cathode rays are made of electrons
- Use a strong electric field to pull electrons across the tube
- Can also be made easier by heating the cathode (called thermionic emission)
8
Q
What is thermionic emission?
A
The elctrons in the heated cathode have more energy in their kinetic stores. This is enough to leave the surface of the metal and move towards the anode.
9
Q
What are electron guns?
A
Electrons are accelerated towards the anode but pass through a hole in it and continue towards the target. This creates a concentrated beam of electrons.
10
Q
How to work out work done to an electron and what assumptions do we make?
A
W = qV = eV
Therefore:
eV = 1/2 mev2
Where
me = mass of an electron
v = speed of the electron
We assume all work done on an electron is transferred to its kinetic store.
11
Q
How is the specific charge of an electron determined with a magnetic field only?
A
- When moving in a magnetic field, a charge experiences force perpendicular to its motion
- This causes a circular path for an electron
So by setting centripetal force equal to the magnetic force:
F = mev2/r = Bev
Which we rearrange to get:
v = Ber / me
We can substitute back into the work done = kinetic energy equation and use the potential difference (VA) of the anode to rearrange for the specific charge:
eVA = 1/2 me(Ber / me)2
So:
e/me = 2VA / B2r2
12
Q
How to find the specific charge of an electron using both a magnetic and electric field?
A
- J.J. Thomson balanced magnetic and electrostatic forces on a beam of electrons
- Oppositely charged plates generate an electric field
- Helmholtz coils generates a magnetic field
- Electric field strength is varied by changing p.d. across plates until the electrostatic force FE = FB
- This is seen visually when the electron beam is not deflected
FE = eE = eV / d
FB = Bev
By equating both:
Bev = eV / d
v = V/Bd
We then use the same equation from when we switch off the electric field (just magnetic) and find:
e/me = V/rB2d
V = p.d. with anode
r = radius of circular motion in magnetic field
B = magnetic flux density of the field
d = distance between plates in electric field
13
Q
How to find specific charge of an electron using an electric field only?
A
- Use constant acceleration equation
t = w / v
Where v is horizontal speed and width of plates w
a = FE / me = eV / dme
Use s = ut + 1/2 at2
y = 1/2 a w2/v2
Where y is horizontal displacement
e/me = ad/V
14
Q
What did the specific charge of an electron mean?
A
Since specific charge was far greater for an electron, electron had a smaller mass or a much larger magnitude of charge.
15
Q
What was the method for Millikan’s oil drop experiment?
A
- A fine mist of atomised oil drops is sprayed into a chamber
- Oil is used instead of water because it does not evaporate quickly
- Mass of drops remain constant
- Drops are ionised as they are sprayed out of the nozzle by X-rays
- This changes their charge from neutral
- They will become positively charged if they lose electrons
- They will become negaticely charged if they gain electrons
- Drops pass into a region between two metal plates and are viewed using a microscope
16
Q
What are the conditions for oil drops to be stationary in Millikan’s oil drop experiment?
A
- Charged drops fall into uniform electric field between plates by distance d with potential V
- Negative oil drops with magnitude of charge Q experience upward force in uniform electric field
- If stationary, force is equal to weight
QV/d = mg
Use Stokes’ Law to determnine mass of each oil drop:
mg = 6πηrv
But we don’t know mass and radius of oil drop so just use density:
m = 4/3 ρ r3
Combine equations to find radius and mass for an oil drop with known velocity
17
Q
What was Newton’s corpuscular theory of light?
A
Newton said light was made of small particle-like bodies called corpuscles.
- Predicted light-emitters were losing mass slowly
- Able to explain reflection, refraction and dispersion but not diffraction
18
Q
How did Newton’s corpuscular theory explain relfection and refraction?
A
Reflection: corpuscles simply hit reflective surface and experienced and equal and opposite repulsive (Newton’s third law)
Refraction:
- force of attraction between light and matter
- when in a single medium, no resultant forces
- but moving more less dense to more dense medium caused light to move faster (was the theory) in denser in new medium as there is more matter in denser medium so resultant force perpendicular to boundary
19
Q
What was Huygens’ wave theory of light?
A
- Huygens proposed light was a wave
- Since all waves (at the time) travelled through a medium, suggests Universe filled with a massless medium known as luminiferous aether
- Suggests light travels in wavefronts emitted from a point source and any point of wavefront acts as a secondary point source from which wavelets propogate
- These wavelets join to form new wavefronts and so on
20
Q
What are the similarites between Newton’s corpuscular theory of light and Huygens’ wave theory of light?
A
- Both explain reflection
- Both explain refraction
- Both explain dispersion
21
Q
What are the differences between Newton’s corpuscular theory of light and Huygens’ wave theory of light?
A
- Corpuscular theory said light made of particles with mass, while wave theory said it was a wave travelling through massless medium
- Corpuscular theory said light is faster in denser media, whilst wave theory said light was slower in denser media
- Corpuscular couldn’t explain diffraction or interference, wave theory could
22
Q
Why was Newton’s corpuscular theory more accepted than Huygens’ wave theory?
A
- Both explained phenomena of light, both had flaws
- Newton was already widely respected due to work on motion and gravity
- No way of measuring speed of light or observing diffraction at the time
23
Q
What results did Newton’s corpuscular theory predict for Young’s double slit experiment?
A
Two bright regions, not an interference pattern
24
Q
What is the significance of Young’s double slit experiment for the development in the theory of light?
A
- Interference pattern was definite evidence for the wave theory of light as opposed to corpuscular theory
- Interference is strictly a wave property but corspuscular theory was no immediately rejected
25
What did Maxwell discover about electromagnetic waves?
- A charged particle has an electric field
- An accelerating charge produces an electric field which **alternates** perpendicular to the particle's motion
- Alternating electric field produces **alternating magnetic field**
- Alternating magnetic field produces electric field and so on (self-propogation)
- This mean light does not need a medium to travel
26
What was Maxwell's formula for electromagnetic waves speed?
*c = 1 / √(μ0ε0)*
where
μ0 is the permeability of free space (constant that relates magnetic flux density witht the current in free space that produces the field)
ε0 is the permittivity of free space (constant that relates electric field strength with the charged object in free space producing the electric field)
27
What was Hertz's setup for the discovery of radio waves?
- short air gap between wires and put a large potential difference across the gap, so high voltage sparks bridged the gap
- sparks generated radio waves, so this was a radio wave **transmitter**
- circular wire with a small break in the circuit sparks when **held near source of radio waves**
- concave metal sheet with two parallel metal rods at centre which had oscillating potential difference induced across them by the radio waves' alternating magnetic field
28
How did Hertz show EM waves could be reflected?
- He placed a metal screen behind the source and measured a stronger signal with the detector
- Showed some radio waves were reflected off the screen and back towards the detector
29
How did Hertz show that the waves were able to penetrate insulators?
When an insulator was placed between the transmitter and detector, no difference in signal detected
30
How did Hertz show waves were polarised?
- When detector rotated 90 degrees perpendicular to path of radio waves, sparks stop being produced in the detector
- Showed that magnetic fields of radio wave only oscillated in a single plane
- Caused by electrons only accelerating in one direction
31
Why was Fizeau's speed of light experiment needed?
- Previously thought that light covered distance **instantaneously** and travelled at **infinte speed**
- Some astronomical observations contradicted this
- Fizeau measured a **finite speed of light**
32
What was Fizeau's method for measuring the speed of light?
- Shone a beam of light at a **mirror** several kilometers away
- Placed a **toothed wheel** in the path of light that was spinning at a very high speed
- Toothed wheel was position so teeth of wheel and gaps between them periodically passed over the beam of light
- Created regular pulses of light travelling towards the distant mirror
- Wheel rotated in a way so that the reflected beam was blocked by the next tooth in the cog (by adjusting frequency of rotation)
- Use **frequency** of rotation, **time light took to travel to mirror and back** could be calculated
- Use c = 2d / t with d being large
- Found same value of c as Maxwell
- Used *c = 4dnf* for calculation
33
What did Fizeau's experiment in **water** find?
- Slower speed of light by same method
- Evidence contradicted Newton's corpuscular theory
34
What is a perfect black body?
A theoretical object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation.
- Spectrum of EM radiation that is emitted changes **dependent on temperature of the black-body**
35
Graph of radiation emitted by a black-body at different temperature
36
What was the ultra-violet catastrophe?
- Disagreement with experimentally measured black-body spectra and the spectra predicted by classical physics
- Treating EM radiation as a **wave** suggested that the object would release an **infinite** amount of UV as temperature of object increased, which isn't the case
37
How did Planck address UV-catastrophe?
- Came up with mathematical descriptions for the emission and absortption occurring within a black-body
- Assumed the energy emitted was quantised
- Could only be emitted in intefer multiples of these packets of energy
- E = nhf
- Allowed Planck to develop a theory that explained the observed spectra of almost-perfect black bodies
38
How did the photoelectric effect contradict wave theory?
- Photoelectrons only emitted for radiation above a certain frequency
- Radiation below threshold frequency, no matter how great intensity, photoelectrons will not be emitted from the metal's surface
- Direct contradiction to wave theory which predicted EM wave transferred energy continuously
39
What was Einstein's explanation for photoelectric effect?
- Einstein proposed EM radiation was made of discrete quanta
- Explained photoelectric effect
- Explained why radiation below the threshold frequency doesn't cause photolectric emssion because only one photon was able to transfer its energy to only one electron which couldn't energise electron if hf isn't high enough
40
What was De Broglie's hypothesis?
- Hypothesised that all particles can behave as **both particles and waves**
- Equated E = mc2 = hf to get p = h / λ
- Used idea to find wavelength of accelerated electrons
- used eV = 1/2 mv2
- rearranged for λ = h / √(2meV)
- hence hypothesised, if electron **accelerated to higher speed**, then De Broglie **wavelength decreases**
41
What is the set up for the electron diffraction experiment?
- Electron gun and directed into a thin film of **graphite**
- Diffraction produces circular pattern on **fluorescent screen** made of **phosphor**
42
What was the effect of varying potential difference in the electron diffraction experiment and what did it conclude?
- As accelerating voltage increased, the concentric rings got closer to the centre of the phosphor screen
- Proved that De Broglie's hypothesis was correct
43
What is the use of varying anode voltage to control electron wavelength?
- Change voltage across anode to manipulate wavelength
- Microscope's **resolving power** depends on wavelength of radiation being used
- Electrons had wavelengths much shorter than X-rays
- Size of atom is 10-10 m so could find accelerating voltage needed to make wavelength same size as an atom
44
What is the setup of a transmission electron microscope and how does it work?
- Focused beams of electrons using **magnetic lenses**
- Electrons passed through a sample and formed an image on a fluorescent screen
45
What is the use of the electron gun in a TEM?
- Accelerated to high speeds by a large potential difference
- Emits electrons through thermionic emission
46
What is the function of the condenser lens?
- Condenser lens' magnetic field deflects the electrons into a wide beam travelling parallel to the axis of the microscope
- Parallel beam is **uniformly** incident on the sample
47
What is the function of the objective lens?
- Forms an **image** of the sample
- Deflects the outer electrons in the beam towards the central axis like convex optical lens does for light
- Electrons along microscope's axis are not deflected
48
What is the function of the projector lens?
- Lens causes beams from the objective lens to spread out, magnifying image created by the objective lens
- Magnified image is directed onto a fluorescent screen, emitting light where electrons are incident
49
What are the drawbacks of the TEM?
- Electrons must **pass** through the sample
- This reduces speed and resolving power since wavelength reduces
- Not all electrons emitted by thermionic emission have same speeds so **range of speeds**, leading electrons passing through same part of sample to be projected onto a range of points, forming a **blurrier** image
50
Transmission electron microscope mark scheme (with how to control resolution)
A typical TEM can accelerate electrons to very high speeds and form high
resolution images.
Explain:
- the process of image formation
- the factors that affect the quality of, and the level of detail in, the image
51
What is a scanning tunneling microscope and how does it work?
- Probe with fine tip is held few nanometres above surface of object
- Probe is moved in any direction by increments of 0.001 nm
- Probe held at constant p.d. with surface of sample
- Some electrons are able to **tunnel** to the tip
- Creates a **tunneling current** which is measured
- Used to map the surface of sample (higher tunnelling current means raised atom)
52
What are the two modes of operation of a STM?
- Constant height : tip remains fixed verically and changes in tunnelling current used to produce image of surface
- Constant current : change in height of tip to keep tunnelling current constant and vertical motion of probe is used to map image of surface
53
What was luminiferous aether?
- All know waves travelled through a medium
- Wave theorists (Huygens) said that this medium for light was luminiferous aether
54
What was the aim of the Michelson-Morley experiment?
- Light travelling in direction of Earth’s motion is travelling against aether
- Like with water, hypothesis was that light travelling against aether would be travelling more slowly than light travelling perpendicular to aether wind
- Aimed to use the difference in these speeds of lights to determine the absolute motion of the Earth relative to the aether
- Phase difference in two beams of light were meant to be used for this
- Apparatus was rotated 90º and there was expected to be a change in the interference pattern due to change in phase difference between beams
55
What were the components of the Michelson-Morley interferometer and what were the results?
Set-up:
- Partially reflective surface used to split into two beams
- A compensating plate used to ensure both beams pass through same amount of glass so no other factors affect results
- After being reflected on mirrors, the beams return to a detector where the interference pattern between them is recorded
Results:
- No change in interference pattern when apparatus was rotated
56
What is meant by the invariance of the speed of light?
The speed of light can be lower than c (3 x 108 m/s) but can never **exceed** it.
57
What is a frame of reference?
A set of co-ordinates to record the position and time of events
58
What is an inertial frame of reference?
A frame of reference that is not accelerating.
- move at constant velocity with respect to each other
59
What is Gallilean relativity?
If a person travelling at 4 m/s throws a ball which they see travelling at 2 m/s, a stationary observer would notice the ball moving at 6 m/s.
- Gallilean relativity assumes Newton's laws of motion work in **all** frames of reference
60
What was the problem with Gallilean relativity and why was Einstein's postulates for special relativity required?
- Vector addition of speeds works at speeds much lower than the speed of light
- If vector addition leads to a value greater than the speed of light, this disobeys the invariance of the speed of light
61
What was Einstein's first postulate for special relativity?
The laws of physics are the same in all inertial frames of reference.
- i.e. we don't know if we are moving or not so the laws of physics must work regardless
62
What was Einstein's second postulate for special relativity?
The speed of light in a vacuum is the same for **all frames** of reference.
- i.e. a moving object would measure a beam of light as moving at c and a non-moving object would measure the **same** beam of light as moving at c as well, not c + v (where v is the speed of the moving object)
63
What is the equation for time dilation?
t = t0 / √(1 - (v/c)2)
- where v is the velocity at which the stationary observer is travelling
- t0 is proper time (time experienced passed by stationary observer)
- moving clocks tick slower
64
What is the stationary observer and external observer in time dilation?
- The stationary observer is stationary relative tot the frame of reference where an event is ocurring
- An external observer sees the frame of reference as in motion
65
What is the importance of the muon lifetime experiment?
Provides **experimental evidence** for time dilation and length contraction
66
What are the observations in the muon lifetime experiment?
- Muons are unstable and produce electrons when they decay
- Travel at 0.98c and have a half-life of 1.6μs
- Created 10km above surface
- Expect very few muons to reach surface
- In practice, larger number of muons detected at Earth's surface due to time dilation or length contraction
67
What is proper time?
Time measured in the reference frame which is stationary relative to the event being time.
68
What is proper length?
Length measured in the reference frame stationary relative to the distance being measured.
69
What is length contraction?
The length of objects moving at high speeds appear shorter to external observer.
- **Width** will remain constant
70
What is the equation for length contraction?
L = L0√(1-(v/c)2)
- L is length measured by observer moving relative to length being measured
- L0 is proper length
71
What was the mass-energy equivalence equation?
*E = mc2*
where
E = energy (J)
m = mass (kg)
c = speed of light (m/s)
72
Where is conversion between mass and energy used?
- Fusion of hydrogen into helium
- Fission of uranium
- Nuclear weapons
- High-energy particle collisions
73
What is relativistic mass and what is its equation?
m = m0 / √(1 - (v/c)2)
- transferring energy to object causes mass to increase
- relativistic speeds doesn't support 1/10th of speed of light
74
What is the total energy of a relativistic object?
E = m0c2 / √(1-(v/c)2)
- take away rest energy to leave kinetic energy
75
Graph of relativistic kinetic energy vs. Newtonian energy
76
What was the aim of Bertozzi's experiment?
- Accelerate electrons close to speed of light and measured kinetic energy
- Plotted squared speeds against kinetic energies and compared results to Newtonian and relativisitic predictions
77
What was the set up of Bertozzi's electron experiment?
- Strong electric field used to transfer to the kinetic store of the electrons
- Speed of electrons calculated by distance over time
- Signal after electric field and after aluminium target used to measure time using an oscilliscope
- Distance between two signals measured and speed calculated
78
What were the results of Bertozzi's experiment?
- Data agreed more with relativistic predictions
79
How was the kinetic energy of the electrons verified in Bertozzi's experiment?
- Measured temperature change of the aluminium target and using ΔE = mcΔθ
- Energy change is equal to total kinetic energy lost by incoming electrons
- Charge transferred to target used to work out number of electrons which hit the target
- Can work out kinetic energy per electron and these agreed with energy transferred to electron by electric field
- Shows results are valid
80
Explain why an electric current through a filament causes the wire to emit electrons.
- Current heats wire
- Electrons gain sufficient kinetic energy to leave filament
81
How can the electric nature of the emitted radio waves in Hertz’s experiment be detected?
Use a dipole that is aligned with the transmitter to detect a changing electric field.
82
Explain how the detection of the wave by the loop demonstrates the magnetic nature of the
radio waves in Hertz’s experiment.
- emf is induced in the loop
- can only occur because of an alternating magnetic field which is caused by the radio wave
83
Explain how the nature of light is implied by Maxwell's theory of electromagnetic waves and Fizeau's result.
- Maxwell came up with a prediction for the speed of an EM wave
- Fizeau's measured value was very close to this prediction
- Hence, this indicated that light was an EM wave
84
What is meant by Einstein's postulate that the speed of light is invariant?
- The speed of light is independent of the motion of the source and the observer
85
Michelson-Morley interferometer experiment MS
Physics
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