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CGP qu Flashcards

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1
Q

What do we mean by cathode rays?

A

invisible rays thought to cause glow that appears on wall of discharge tube when pd applied across terminals

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2
Q

What is meant by thermionic emission?

A

when heat gives electrons in metal enough energy to break free from metal’s SURFACE

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3
Q

Why does glass tube have to be evacuated in electron gun?

A

electrons tiny compared to smallest atom - so easily stopped/deflected by atoms
so electrons can travel freely in electric field

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4
Q

Where are electrons emitted from in electron gun?

A

from cathode, attracted towards anode

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5
Q

Prove eV=KE

A

Wd in moving charge through e field = QV = eV
eV=KE
work d to accelerate e- through pd = KE that it has when accelerated from rest

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6
Q

Why e- spiral in fine beam tube?

A

collide w He atoms in tube
e- slow down
slower e- have less curvature - r directly prop to v

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7
Q

What allows us to measure radius of e- beam in fine beam tube?

A

sufficient mg field
e- beam fired at right angles to to mg field generated by 2 circular mf coils on either side of tube

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8
Q

magnetic f in fine-beam tube: Why is there He in the tube?

A

cgp: H
passes through this low pressure gas -collide w H atoms + transfer energy to atoms
e- in atoms move to higher energy levels - excitation - will de-excite + emit light
So see e- beam as glowing trace through gas

remember small amt gas so most e- unaffected by it!

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9
Q

What are 3 methods to find sc of electron?

A
  • Thomson: electron deflection: e- gun firing e- between 2 deflecting parallel plates w pd applied across them - mg field perpend to el field (applied w electromagnet) - vacuum - equate forces & eV=KE
  • mg field in fine beam tube: e- beam fired at right angles to to mg field generated by 2 circular mf coils on either side of tube -> beam curves in circle - mg f= Fc & eV=KE - always have h/he?

[- parallel plates but w no mg field: e- go straight through (efs =0) - then increase pd so parabolic motion w constant a = use suvat (won’t need pd as given speed since this has sc in it), a=F/m=Ee/m]

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10
Q

What do you expect to happen to beam of e- when electromagnet is off (electron gun firing bet parallel plates w e+mg f)

A

beam will deflect towards positive plate

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11
Q

Derive eqn for radius beam of e- travelling in fine beam tube?

A

magnetic force on e- = centripetal force
r = mv/Be

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12
Q

Describe experiment to calc specific Q of e- ?

A

Fine beam tube - glass bulb w mf field coils either side filled w low pressure H & containing an e- gun
Direct e- beam at right angles to mg field so beam travels in circle, it will excite H atoms which de-excite + emit photons
Path of e- beam visible
Measure radius of circle, pd of gun, mfs of m field

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13
Q

Significance of discovery that specific Q of e- greater than H ion?

A

sc on e- nearly 1800 times more than H ion
either e- has bigger charge or v light mass
so might have particles smaller than atom

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14
Q

What forces are acting on oil droplet moving at steady speed when no pd is applied across plates in Millikan’s experiment? What can we use this for?

A

steady speed => terminal v
weight = viscous drag
find radius of oil drop!

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15
Q

What forces are acting on oil droplet when applied across plates in Millikan’s experiment? (hint it is stationary)
CHECK QU!

A

stationary - not moving - no viscous drag!!
electric force = weight
find charge of oil drop!

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16
Q

What is Stokes Law?

A

F=6rvpiviscosity
viscosity measured in Pa s
viscous drag f acts in opp direction to velocity
“how thick fluid is”

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17
Q

What equipment did Millikan use?

A

2 parallel plates
atomiser
oil drops
variable pd
microscope!

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18
Q

Name + formula for force that causes oil-drop’s velocity to change when electric field applied bet plates in Millikan’s oil drop experiment?

A

electric force

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19
Q

Forces acting on charged oil drop in Millikan’s experiment when e field turned off?

A

weight, viscous drag f

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20
Q

Forces acting on charged oil drop in Millikan’s experiment when e field turned on and drop in motion?

A

weight, viscous drag f, electric f

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21
Q

Forces acting on charged oil drop in Millikan’s experiment when e field turned on and drop is stationary?

A

weight, electric f

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22
Q

What result led Millikan to find the charge on e-?

A

found charge on oil droplets always and integer value of e
concluded charge cannot exist in ‘smaller packets’ than this and is quantised packets of e
this must be size of Q on e-

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23
Q

Significance of Millikan’s discovery on quantisation of charge in determining properties of electrons?

A

mass of e- calc and Q deduced
(at time this was lightest particle to be discovered)

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24
Q

Consider the experiment where light shone through pinhole onto screen - What was predicted to happen to the width of beam using corpuscular theory as pinhole width was decreased?

A

light beam reaching screen predicted to get thinner
corpuscles of light travel in straight lines
so if pinhole narrower -> beam of light of corpuscles will be narrowed too

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25
Q

Consider the experiment where light shone through pinhole onto screen - What actually happened to the width of beam using corpuscular theory as pinhole width was decreased?

A

Beam got wider
light is wave and can diffract
if pinhole smaller, gap will be closer to wavelength of light so it will diffract more

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26
Q

Explain Newton’s corpuscular theory of light?

A

light was made up of stream of tiny particles called corpuscles
reflection - force pushed particles away from surface
refraction - worked if corpuscles travelled faster in denser medium

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27
Q

Explain Huygens’ principle?

A

every point along wavefront can be considered a point source of secondary wavelets that spread out in forward direction at the speed of wave
the new wavefront is the surface that’s tangential to all of these secondary wavelets

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28
Q

What are point sources of secondary wavelets?

A

hemispherical wavelets that come from each point

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29
Q

Why was Newton’s theory preferred to Huygens’?

A

N was v successful + respected
H’s theory couldn’t explain light polarisation/double refraction and why sharp shadows were formed by light

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30
Q

What difficulty did scientists face when trying to observe interference + diffraction of light?

A

struggled to get 2 coherent light sources as light is usually emitted in random bursts

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31
Q

Explain YDSE and how he got round the problems scientists previously faced?

A

used 1 light source shining through slit to create a point light source
put slide w 2 thin slits in front of this light source -> created 2 coherent light sources & projected these onto screen
observed bright + dark fringes being formed

32
Q

Significance of YDSE?

A

gave evidence of light diffracting + showing interference = wave properties

33
Q

What convinced scientists of Huygens’ wave theory of light in the end?

A

discovery that light behaved as transverse wave

34
Q

What is an EM wave?

A

transverse waves made up of oscillating electric + magnetic fields that are perpendicular to each other & direction of travel

35
Q

What did Maxwell predict? Equation?

A

predicted there’s a spectrum of EM waves travelling at same v w different f
model showed theoretically that all EM waves travel at same speed c in vacuum
c=1/square root permeability * permittivity of free space -> speed of em wave in vacuum

36
Q

What is Maxwell’s Equation?

A

c=1/square root permeability * permittivity of free space -> speed of em wave in vacuum
permittivity = relates to efs due to charged object in free space
permeability = relates to mg flux density due to current carrying wire in free space

37
Q

Describe Fizeau’s experiment?

A

measured c by passing a beam of light through a gap bet cog teeth to a reflector abt 9 km away
cog was rotated at exactly the right speed so that reflected beam passed through next gap in cog teeth
knew distance and calc t from frequency of rotations + # of gaps (for light to travel from+back reflector)

38
Q

How did Fizeau’s experiment provide evidence that light is an EM wave?

A

Maxwell calc his speed of light which was v close to estimate made by Fizeau yrs earlier
-> suggested that light (+ UV + infrared radiation) is EM wave

39
Q

Describe how Hertz discovered radio waves?

A

used an induction coil & capacitor to produce a high voltage, and showed that radio waves were produced when high voltage sparks jumped across a gap of air

40
Q

What properties of radio waves did Heinrich Hertz discover?

A

showed that radio waves had all the properties of EM waves, including reflection, refraction, diffraction, interference, polarisation & same velocity

41
Q

Why do emitted photoelectrons have range of KE?

A

e- emitted from deeper down in metal lose more energy as they escape than those at surface
so photoe- have range of energies

42
Q

3 main conc from photoelectric effect experiments?

A

of photoelectrons emitted per s is directly proportional to intensity of radiation.

For a given metal, no photoelectrons are emitted if the radiation has f below threshold f.

The photoelectrons are emitted with a variety of KE ranging from 0 -> some max value.
(value of max KE increases with f of radiation, but is unaffected by intensity of radiation.)

43
Q

Einstein’s photon theory of light?

A

EM waves (& energy that they carry) can only exist in discrete packets called photons

photons have 1-1particle-like interaction w electron in a metal surface. A photon will transfer all its energy to that 1 e-

44
Q

What did Planck suggest about light?

A

suggested that EM waves can only be released in discrete packets, or quanta.

45
Q

When low f light is shone on metal we get no photoe-
How does wave theory fail to explain this, and how photon theory can?

A

Wave theory predicts that electrons should gradually gain energy from incident waves until they have enough energy to escape - but no explanation for why this never happens.

photon theory of light says that photons have a one-on-one, particle-like interaction with an electron

each photon transfers all its energy to 1 specific electron.

Electrons cannot build up energy - they either get enough from one photon of the correct frequency to escape, or don’t get any. hence a threshold frequency.

46
Q

What is work func of metal?

A

The work function of a metal is the minimum amount of energy that an electron needs to break the bonds holding it in the metal.

47
Q

What does wave theory predict happens to KE of photoe- when intensity increases? What happens?

A

Wave theory predicts that the kinetic energy of a photoelectron should increase with light intensity. In reality, the kinetic energy of a photoelectron is only affected by the frequency of the light, and the intensity has no effect at all.

48
Q

Explain why photoe- have max KE according to Einstein’s photon theory?

A

Photoelectrons have a kinetic energy equal to the energy of the photon that collided with them, hf, minus the energy used to break the bonds of the metal, the work function, ф, and any energy they lose whilst leaving the metal. The maximum kinetic energy a photoelectron can have is just the energy supplied by the photon, minus the work function.

49
Q

What is the ‘ultraviolet catastrophe’?

A

Classical wave theory could explain the slope of black body radiation curves at long wavelengths (low frequencies) - it suggested that the power radiated was proportional to
X-4, but this meant that the power output was predicted to head towards infinity in the ultraviolet region. This was the ultraviolet catastrophe - wave theory, then widely accepted, had predicted something that was impossible.

50
Q

Describe the effect increasing velocity has on diffraction pattern of electrons?

A

Increasing the electron velocity will cause the diffraction pattern circles to squash together towards the middle, i.e.
the amount of diffraction decreases.

51
Q

What is meant by wave-particle duality?

A

All particles have both particle and wave properties. Waves can also show particle properties.

52
Q

Name 2 effects that show EM waves have both wave and particle properties?

A

E.g. Diffraction shows light has wave properties, and the photoelectric effect shows light has particle properties.

53
Q

Name the phenomenon that shows electrons have wave-like properties?

A

Electron diffraction

54
Q

What anode potential produces electrons with de Broglie wavelengths similar to size of atom?

A

around 150V

Size of atom = 0.1 nm??

55
Q

Describe how transmission electron microscope produces image of v thin sample?

A

An electron gun produces a beam of electrons with a certain de Broglie wavelength (dependent on the anode potential).

A set of EM coils known as a condenser lens focuses the electrons into a thin, straight beam onto the sample, the structure of which may cause some of the electrons to diffract.

2 more sets of electromagnetic coils use magnetic fields to deflect the electrons so that they form a magnified image, which is projected onto a fluorescent screen.

56
Q

Describe how scanning tunnelling microscope produces a 3D image of surface of sample?

A

A very fine probe is positioned very close (around 1 m) to a sample’s surface.

A potential is applied so that the probe is negatively charged in relation to the sample. Electrons ‘tunnel’ from the probe to the surface, which produces a small electric current.

A bigger distance results in a smaller current.

The probe scans the surface of the sample and produces a 3D image of it, by measuring either the current at a set probe height, or the height of the probe from the sample at a set current.

57
Q

Describe how position of STM probe is made to move by tiny distances

A

Piezoelectric transducers are used. When a p.d. is applied to them, they experience a tiny change in length, which moves the probe.

58
Q

Describe idea of absolute motion

A

Everything, including light, moves relative to a fixed background known as the ether.

59
Q

How was it hoped that Michelson-Morley experiment would provide evidence for absolute motion?

A

hoped to compare speed of light parallel & perpendicular to motion of the Earth, & find a difference between them.

With this difference -> measure absolute speed of Earth.

60
Q

In the Michelson-Morley experiment, how was semi-silvered glass block used?

A

a) To split the light source into two beams, by reflecting some light and transmitting the rest.

61
Q

In the Michelson-Morley experiment, how were plane mirrors used?

A

b) To reflect the light back to the semi-silvered glass block, where the two beams will once again converge, forming an interference pattern.

62
Q

In the Michelson-Morley experiment, how was unsilvered glass block used?

A

c) To make sure that both beams of light travel through the same amount of glass and air.

63
Q

Results from Michelson-Morley experiment?

A

Expected that rotating interferometer would result in a shift in the interference pattern seen, due to the change in the angle at which each beam was moving relative to the absolute motion of the Earth.

64
Q

What was the significance of the results from the Michelson-Morley experiment?

A

Failed to detect absolute motion
showed that c has same value for all observers (it is invariant) + that it’s impossible to detect absolute motion
-> ether doesn’t exist

Light is invariant - cannot measure something relatively?

65
Q

What is a frame of reference?

A

a) A space or system of coordinates that we use to describe the position of an object.

66
Q

What is an inertial frame of reference?

A

b) A frame of reference in which Newton’s first law is obeyed.

67
Q

2 postulates of special relativity?

A

Physical laws have the same form in all inertial frames.
speed of light in free space is invariant.

68
Q

What is meant by time dilation?

A

An observer of 2 events that is moving at a constant velocity, v, in relation to the events will measure a longer time interval, t, between the 2 events than the time measured by an observer who is stationary relative to events, t0.

69
Q

What provides evidence for time dilation?

A

muon decay
Muons travel from the upper atmosphere towards the ground at speeds close to c. They have a short half-life, and we expect the intensity of muons to decrease a certain amount as they travel between two points in the atmosphere. But the time taken in their reference frame is shorter than the time taken in our reference frame, so the intensity of the muons drops less than we would expect (if we ignore relativity). This is evidence for time dilation.

70
Q

What is meant by relativistic mass?

A

faster an object moves, the more massive it gets. The relativistic mass of an object is the mass of the object when it is moving at a certain velocity.

71
Q

Describe Bertozzi’s experiment and explain how it provides evidence for special relativity?

A

Bertozzi used particle accelerators to accelerate pulses of electrons to a range of energies from 0.5 MeV to 15 MeV.
The particles were smashed into an aluminium disc a set distance away. The time taken by electrons of each energy to reach the aluminium disc was measured so that their speeds could be calculated. As the energy of the electrons was increased, the speed of the electrons didn’t increase as you would expect from E = ½mv?, but instead tailed off towards a maximum value approaching 3 × 108 ms’ (c). This showed that as the energy increased, the mass increased. To check that the electrons had the energy he thought they had, Bertozzi used the heat generated by the collisions at each energy to calculate the kinetic energy of the particles immediately before impact. Bertozzi found that plotting a graph of kinetic energy against speed gave a curve that closely matched that predicted by Einstein’s formula. This was the first direct evidence for special relativity.

72
Q

Suggest a suitable anode potential to view silicon atoms in a sample?

A

The de Broglie wavelength should be similar in size to the diameter of the atom, 2.20 × 10-^10 m.

73
Q

Why do electrons have to behave as waves in order for a transmission electron microscope to work?

A

The electrons diffract around the atoms and molecules of a sample’s structure to produce an image of it - this will only happen with waves
The electrons would not be able to do this as particles, as they would either pass straight through the sample or be absorbed/blocked by it

74
Q

Explain pattern expected to form on screen according to Newton’s corpuscular theory of light for YDSE?

A

Two bright fringes formed on the screen behind the two slits
Newton’s corpuscular theory said that light behaved as a particle and naturally travelled in straight lines, so only two fringes would be formed

75
Q

Explain pattern actually forms on screen according to Newton’s corpuscular theory of light for YDSE?

A

A pattern of many fringes will be seen, with a bright central fringe and decreasingly bright fringes further out. In this experiment light shows the wave properties interference and diffraction, which supports Huygens’ wave theory of light

76
Q

Describe how cathode rays are produced in discharge tube & key discoveries of their properties that led scientists to predict existence of sub-atomic particles?

A
  • A discharge tube is an evacuated glass tube with a cathode and an anode and a potential difference applied between them.
  • When the potential difference is applied, a glow appears on the wall of the discharge tube.
  • Cathode rays were shown to have energy, momentum and mass, showing that they were made up of particles.
  • Cathode rays were shown to have a negative charge, also showing that they were made up of particles.
  • They were shown to have the same properties no matter what gas was in the tube and what the cathode was made of.
  • They were shown to have a huge charge-to-mass ratio, showing they were either highly charged or subatomic.

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