Modern Physics Flashcards
Thermionic emission
- the emission of electrons from the surface of a hot metal
uses of thermionic emission
- xray tube
- cathode ray tube (in old tv as screen glows)
- increase in temperature = faster electrons
Cathode ray
- 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
x-ray
- 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
who found xray
- Rontgen
properties of xrays
- high frequency E.M radiation
- penetrates certain materials - bone absorbs
- can ionise certain materials; knock off electrons; semi conductors
- photographic film
photoelectric effect
the emission of electrons from the surface of a metal when E.M radiation of a suitable frequency is incident on it
Photon energy
E=hf
Work function
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)
Einstein’s photoelectric law
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
photoelectric effect demonstration
- 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
x-ray definition
electromagnetic radiation of a high frequency that is produced when high speed electrons strike a metal target
threshold frequency
the minimum frequency of light needed to equal the work function
Einsteins Explanation of the photoelectric effect
- 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
Rutherford’s gold experiment
- 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)
isotopes
different atoms of the same element with the same atomic number but different mass numbers (numbers of neutrons)
defining a substance
protons = element neutrons = isotope (of same element) electrons = ionisation (dif e = ionised)
Henry Becquerel
discovered radioactivity
Rutherford
structure of the atom
einstein
photoelectric effect
hertz
uv light has energy
rontgen
x-ray
stooney
names electron
radioactivity
the spontaneous decay of unstable nuclei with the emission of one or more types of radiation
alpha particle
penetrating power = lowest (paper)
ionising ability = highest (larger)
charge = +2
beta particle
penetrating = middle (aluminium foil/plastic) ionising = medium charge = - 1
gamma ray
penetrating = greatest ionising = least charge = 0 (no deflection in EM field)
radioisotope
any isotope that decays and emits radiation
Uses of radiation
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
ionisation
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
cloud chamber
- based on penetrating ability
- low pressure gas
- mica window
- contrails
- α - common, short
- β - less common, longer
- γ - rare - long
Gieger Muller tube
- 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
Activity/Rate of decay
number of nuclei of a radioactive source that decay every second
Law of radioactive decay
the activity of a radioactive substance is always ∝ the number of nuclei that remain undecayed
half-life
the time taken in seconds for half of the nuclei of a radioactive source to decay
Nuclear energy
energy that originates from the nucleus of atoms
radiation
the spontaneous release of α, β, γ particles from an unstable nucleus
nuclear fission
the splitting of a large nucleus into two approximately similar smaller nuclei with the release of neutrons and energy
nuclear fusion
the joining of two smaller nuclei into one larger single nucleus with the release of energy
Uranium
largest nucleus that can be held together with strong nuclear force naturally
critical mass
the minimum mass of fissile material required in order to sustain a chain reaction
chain reaction
a self-sustaining fission reaction where at least one neutron is emitted in order to carry on the reaction
Nuclear Fission of Uranium
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
fate of the products of nuclear fission
Barium - used for tracing in hospitals
- detected by xrays
Krypton - energy saving fluorescent lights
reprocessed (enriched again) - Sellafield
Critical Mass Properties
- varies for each radioisotope
- depends on - temp, shape, density, purity
- too little = no chain reaction
- too much = uncontrollable = can lead to a melt down
Speed of neutrons
- slow neutrons needed to cause fission
- too slow - won’t enter nucleus
- too fast - radiative capture
- absorbed by nucleus
- increases mass
Nuclear reactor
- 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
Advantages of Nuclear Fission
- 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
disadvantages
- radioactive waste - long half life
- expensive to store - reprocessing expensive
- major accident = large scale health hazard
factors affecting Nuclear fusion
- two nuclei forced together
- need enough energy to overcome repulsive coulomb force
- extreme temp + pressure
- high velocity - particle accelerator
isotopes of helium
deuterium/triterium
advantages of fusion
- no greenhouse gases
- no radioactive waste
- large amounts of energy produced
- fuel required is cheap + plentiful (heavy water = deuterium)
disadvantages of fusion
unable to create a sustainable fusion reaction due to the energy required to overcome the repulsive coulomb force
difference between nuclear and chemical reactions
- in the nucleus
- spontaneous
- new elements created
- doesn’t involve electrons
- no forming/breaking of chemical bonds
solid state detector
- PN junction
- radiation hits depletion layer
- electron - hole pairs produced
- current flows
neutrons and chain reaction
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
