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Physics > Modern Physics > Flashcards

Modern Physics Flashcards

1
Q

Thermionic emission

A
  • the emission of electrons from the surface of a hot metal
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2
Q

uses of thermionic emission

A
  • xray tube
  • cathode ray tube (in old tv as screen glows)
  • increase in temperature = faster electrons
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3
Q

Cathode ray

A
  • in a vacuum (avoids collisions of e- with gas particles)
  • low voltage passed through a metal filament
  • causes thermionic emission of cathode plate
  • high voltage anode accelerator with a hole (2kv potential difference between cathode and anode)
  • pass through due to high velocity (e- gun)
  • pass through 2 sets of parallel plates (y/x)
  • allows electrons to be focused on a point
  • electrons transfer kinetic energy into a bright flash on phosphorescent screen
  • fast enough to look like a single image to human eye

use = oscilloscopes, electro cardiograms, xrays

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4
Q

x-ray

A
  • cathode ray tube
  • higher potential difference between cathode and anode means faster electrons (more frequency) (50-90 kv)
  • anode is tungsten (high melting point)
  • e- hit tungsten and kinetic energy is converted - produce 1% x ray 99% heat
  • causes e- in tungsten to get excited emitting an x ray
  • higher energy electrons = more penetrating
  • oil coolant backs tungsten; prevents over heating
  • mica window - allows xrays out
  • lead wall for protection
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5
Q

who found xray

A
  • Rontgen
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6
Q

properties of xrays

A
  • high frequency E.M radiation
  • penetrates certain materials - bone absorbs
  • can ionise certain materials; knock off electrons; semi conductors
  • photographic film
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7
Q

photoelectric effect

A

the emission of electrons from the surface of a metal when E.M radiation of a suitable frequency is incident on it

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8
Q

Photon energy

A

E=hf

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9
Q

Work function

A 
ϕ = h fo
fo = thresh hold frequency

the minimum energy required to cause the photoelectric effect (release an electron from the surface of a metal)

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10
Q

Einstein’s photoelectric law

A

the kinetic energy of the fastest moving electron emitted is calculated as the difference in energy between the energy of the incident photon and the work function of the metal

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11
Q

photoelectric effect demonstration

A
  • positive/negatively charged zinc plate
  • GL electroscope
  • red laser - nothing on either
  • ultraviolet - nothing on = + charged
  • ultraviolet - causes leaf to fall on - charged because there is an excess of electrons which will be emitted > Fo for - charged
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12
Q

x-ray definition

A

electromagnetic radiation of a high frequency that is produced when high speed electrons strike a metal target

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13
Q

threshold frequency

A

the minimum frequency of light needed to equal the work function

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14
Q

Einsteins Explanation of the photoelectric effect

A
  • hf = Φ + 1/2 mv²
  • light travels in photons of energy
  • each photon gives all of its energy to one electron
  • if the energy of the photon is greater than or equal to the work function an electron is released
  • any energy exceeding the work function is given to the electron as kinetic energy
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15
Q

Rutherford’s gold experiment

A
  • alpha particle, gold foil, zinc sulfide screen
  • fired at foil
  • most went straight through - atom mostly empty space
  • some slightly deflected - repelled by nucleus
  • some bounced straight back - hit nucleus straight on

= mostly empty space
small dense positive core
electrons orbit nucleus (not embedded)

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16
Q

isotopes

A

different atoms of the same element with the same atomic number but different mass numbers (numbers of neutrons)

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17
Q

defining a substance

A 
protons = element 
neutrons = isotope (of same element)
electrons = ionisation (dif e = ionised)
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18
Q

Henry Becquerel

A

discovered radioactivity

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19
Q

Rutherford

A

structure of the atom

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20
Q

einstein

A

photoelectric effect

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21
Q

hertz

A

uv light has energy

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22
Q

rontgen

A

x-ray

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23
Q

stooney

A

names electron

24
Q

radioactivity

A

the spontaneous decay of unstable nuclei with the emission of one or more types of radiation

25
Q

alpha particle

A

penetrating power = lowest (paper)
ionising ability = highest (larger)
charge = +2

26
Q

beta particle

A 
penetrating = middle (aluminium foil/plastic)
ionising = medium 
charge = - 1
27
Q

gamma ray

A 
penetrating = greatest 
ionising = least 
charge = 0 (no deflection in EM field)
28
Q

radioisotope

A

any isotope that decays and emits radiation

29
Q

Uses of radiation

A

1) food irradiation - kills bacteria
2) medicine - γ - sterilises instruments
- β - tracers (α would stay)
- γ - cancer
3) non - destructive stress testing - thickness
4) Carbon dating - ratio different after death
- C14 - C12
- compare to atmospheric
5) smoke detectors - two electrodes
- air ionised by α
- smoke too dense

30
Q

ionisation

A

radiation collides on an atomic level with the nucleus of cells and can knock an electron out of their shells

  • can cause rapid uncontrollable growth of cells (cancer)
  • can kill a cell
  • can cause genetic abnormalities
31
Q

cloud chamber

A
  • based on penetrating ability
  • low pressure gas
  • mica window
  • contrails
  • α - common, short
  • β - less common, longer
  • γ - rare - long
32
Q

Gieger Muller tube

A
  • based on ionising ability
  • doesn’t identify type
  • only count rate
  • low pressure, saturated gas
  • mica window (low density)
  • counter

1) - radiation ionises 1 molecule
- creates ion pair
2) - e attracted to cathode shell
- cation attracted to anode
3) causes avalanche effect
- further ionisation
- increases count rate
4) - causes current to flow
- converted to counts

33
Q

Activity/Rate of decay

A

number of nuclei of a radioactive source that decay every second

34
Q

Law of radioactive decay

A

the activity of a radioactive substance is always ∝ the number of nuclei that remain undecayed

35
Q

half-life

A

the time taken in seconds for half of the nuclei of a radioactive source to decay

36
Q

Nuclear energy

A

energy that originates from the nucleus of atoms

37
Q

radiation

A

the spontaneous release of α, β, γ particles from an unstable nucleus

38
Q

nuclear fission

A

the splitting of a large nucleus into two approximately similar smaller nuclei with the release of neutrons and energy

39
Q

nuclear fusion

A

the joining of two smaller nuclei into one larger single nucleus with the release of energy

40
Q

Uranium

A

largest nucleus that can be held together with strong nuclear force naturally

41
Q

critical mass

A

the minimum mass of fissile material required in order to sustain a chain reaction

42
Q

chain reaction

A

a self-sustaining fission reaction where at least one neutron is emitted in order to carry on the reaction

43
Q

Nuclear Fission of Uranium

A

1) uranium enriched with U235
(0.7 % = natural)
(3 % = plant)
(9% = bomb)
(U238 would absorb neutrons produced so no chain reaction)

2) critical mass brought together
3) neutron strikes U235
4) briefly turns into U236 - unstable
5) U236 undergoes fission - Kr
- Ba
- 3 neutrons
6) neutrons carry Ek liberated (e = mc²)

7) as at least 1 neutron released a chain reaction occurs (further fission)
(slow = plant)
(fast + uncontrollable = bomb)

8) reaction carries on until it is slowed or stopped by human intervention

44
Q

fate of the products of nuclear fission

A

Barium - used for tracing in hospitals
- detected by xrays
Krypton - energy saving fluorescent lights

reprocessed (enriched again) - Sellafield

45
Q

Critical Mass Properties

A
  • varies for each radioisotope
  • depends on - temp, shape, density, purity
  • too little = no chain reaction
  • too much = uncontrollable = can lead to a melt down
46
Q

Speed of neutrons

A
  • slow neutrons needed to cause fission
  • too slow - won’t enter nucleus
  • too fast - radiative capture
    - absorbed by nucleus
    - increases mass
47
Q

Nuclear reactor

A
  • generator
  • steam created by nuclear energy turns a turbine in a magnetic field to produce a.c electricity
  • Fuel rods - enriched uranium fissile material
  • control rods - absorbs neutrons being
    emitted from fuel rods
    - lowered/raised proportionally
  • moderator - slows down fast neutrons
    - allows for sustainable fission
    - prevents radiative capture
  • coolant - liquid/gas
    - absorbs heat and carries it to the
    heat exchanger
  • concrete shield - protects from leaks
48
Q

Advantages of Nuclear Fission

A
  • produces massive amounts of energy economically
  • does nos emit greenhouse gasses
  • low accident rate
  • will be no shortage of nuclear fuel
  • products can be reused
49
Q

disadvantages

A
  • radioactive waste - long half life
    - expensive to store
  • reprocessing expensive
  • major accident = large scale health hazard
50
Q

factors affecting Nuclear fusion

A
  • two nuclei forced together
  • need enough energy to overcome repulsive coulomb force
  • extreme temp + pressure
  • high velocity - particle accelerator
51
Q

isotopes of helium

A

deuterium/triterium

52
Q

advantages of fusion

A
  • no greenhouse gases
  • no radioactive waste
  • large amounts of energy produced
  • fuel required is cheap + plentiful (heavy water = deuterium)
53
Q

disadvantages of fusion

A

unable to create a sustainable fusion reaction due to the energy required to overcome the repulsive coulomb force

54
Q

difference between nuclear and chemical reactions

A
  • in the nucleus
  • spontaneous
  • new elements created
  • doesn’t involve electrons
  • no forming/breaking of chemical bonds
55
Q

solid state detector

A
  • PN junction
  • radiation hits depletion layer
  • electron - hole pairs produced
  • current flows
56
Q

neutrons and chain reaction

A

at least 1 neutron must be released to cause further fission for a chain reaction to occur

  • > 1 = uncontrollable + unsafe
  • < 1 = no chain reaction occurs

Physics (16 decks)

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