Physics Unit 1 - Theory Stuff

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  • Created on: 15-05-15 11:39

Stabe and Unstable Nuclei

Graph showing the strong nuclear and electromagnetic force (

  • Strong nuclear force is repulsive for very small separations of nucleons (>0.5)
  • As nucleon separtion increases past about 0.5fm, the strong nuclear force becomes attractive. Itreaches a maximum attractive value and then falls rapidly towards zero after about 3 fm
  • The electrosmagnetic repulsive force extends over a much larger range
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Nuclear Decay

Alpha decay - occurs in very bug atoms ( more that 82 protons ). Nuclei of theses atoms are just too bug for strong nuclear force to keep them stable. To make themselves more stable, they emit an alpha particle from their nucleus

.alpha decay  (

Beta Decay - The emmision of an eletron from the nucleus along with an antineutrini particle. Beta Decay happens in isotopes that are "neutron rich". When a neutron ejects a beta particle, one of the neutrons in the nucleus turns into a proton. The anineutrino released carries aqay some enery and momentum.

beta minus decay (               

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Pair Production

  • When energy is converted into mass you get equal amounts of matter and antimatter.
  • Only occurs when there is enough enough energy to produce the masses of particles. It must always produce a particle and antiparticle because certain quantities must be conserved
  • Eg. two protons at high speed fired at each other, you will end will aalot of energy at the point of impact. This energy might be converted into more particles. if an extra proton is formed there will always be an antiproton to go with it.
  • If a photon has enough energy, it can produce an electron positron pair. It tends to happen whena photon passes near a nucleus. Particles produced in a detector curve away in opposite directions because of the oppsoite charges of an elecron and positron.
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When a particle meeets its antiparticle the result is annihilation. All the mass of the particle and anti particle gets converted back to energy in the form of two gamma ray photons. Antiparticles can only exist for a fraction of a second before this happens, so you don't get them in ordinary matter.

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Classification of Particles


  • Made up of protons and neutrons.
  • Since protons are positively charged they need a strong force to hold them together
  • Not all particles can feel strong force, the ones that canare called hadrons


  • Protons, neutrons, sigmas
  • short lived
  • Decay into other particles except protons
  • All baryons except protons decay to a proton

 Neutron decay

  • beta - minus decay
  • weak interaction
  • n --> p + e- +  ν
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  • Interact with baryons via the strong force
  • Unstable
  • B = 0


  • Swap protons with neutrons and neutrons with protons
  • leave total baryon number unchanged


  • heavier more unstable than pions
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  • Fundametal particles
  • interact via weak interaction and a bit of gravitational force
  • Also electromagnetic force ( if charged)

Electrons are stable leptons. But there are other leptons too.Muons are heavy electrons but they are unstable and decay eventually into ordinary electrons and positrons

Electrons and muon leptons each come with their own neurinos. Neutrions are take part in weak interaction have (almost) zero mass.Each of four leptons have their corresponding antiparticles

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quark composition


  • proton - uud
  • neutron - udd
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Quark Confinement

Energy supplied to remove a quark, is instead used to make more quarks and antiquarks . It


It is not posible to get a quark by inself

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Beta - and Beta + decay


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Electron Capture

Proton rich nucleues can 'capture' an electron from inside the item and turn into a neutron.


The proton is 'acting' on the electron as it captures the electron. The 'W' boson comes from the proton. Like with B- decay an elctron neutrino is emmited to conserve the lepton number.

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Electron Proton collisions

Where an electron collides with a proton at high speed. In an electron proton collision the electron is the particle that is acting because it is being fired at the proton so the W boson comes from th eelctron. It must be W- to conserve charge


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Photoelectric effect

If electrons absorb enough energy, the bonds holding it to the metal break and the electron is released. This is called the photoelectric effect and the electrons that are emmited are know as photo electrons.

  • For a given metal, no photelectrons are emmited if the radiation has a frequencey below a certain value - called threshold frequency
  • Photelectrons emmited with a variety of kinetic energies ranging from zero to some maximum value. This value of maximum kinetic energy increases with the frequency of the radiation.
  • the intensity of radiation is the amount of energy per second hitting an area of the metal. The maximum kinetic energy of the photoelectrons is unaffected by varying the intensity of the radiation
  • The number of photoelectrons emmited per second is proportional to the intensity of the radiation.
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The photoelectric effect and wave theory

Threshold frequency 

Wave theory states that for a particular frequency of EM wave, the energy carried should be proportional to the intensity of the beam. The energy carried by the EM wave should also be proportional to the intensity of the beam. The energy energy carried out by the EM wave would be spread evenly over the wavefront.  This would mean if an EM wave ewas shone on to a metal , the electrons would gain little energy from the incomming wave evenytually gaining enough energy to leave the metal. If EM has lower frequency it would take longer for the electrons to be liberated. However electrons are never emmited unless the wave is above a threshold frequency, so wave theroy cannot explain threshold frequency.

Kinetic energy of photoelectrons

the higher the intensity of the wave, the more energgy it should transfer to each electon - the kinetic energy should increase with intensity.

Wave theory cannot explain the fact that kinetic energy relies on the the frequency of the photoelectric effect.

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Explaining the photoelectric effect

  • photon - discrete wave packets of energy 
  • When EM radition hits a metal, the metal surface is bombarded with photons. If one of the photns collides with a free electron then the electron gains nergy equal to E =hf
  • Howver before an electron can be liberated, it needs enough energy to overcome the bonds in the metal. This is known as the work function energy and its value depends on the metal.
  • If enegy gained by photon is greater that work function then electron is emmited. 
  • If not sufficient energy is gained by the photon then electron will just shake about and release energy to another photon. The metal will heat up but no electrons will be released
  • fo = ϕ / h

Max kinetic energy

The kinetic energy it wil be carrying when it leaves the metal is hf minus any other energy loses i.e the work function. This is why the electrons emmited have a range of kietic energies. The mimum energy an electon can lose is workfuction hence : Ek = hf - ϕ.  Rearranging gives the photolectric equation: hf = ϕ + Ek , Ek = \begin{smallmatrix} \frac{1}{2}mv^2 \end{smallmatrix}  (      

hf = ϕ + \begin{smallmatrix} \frac{1}{2}mv^2 \end{smallmatrix}  (

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Energy levels in atoms

1 eV = 1.6 x 10-19J

Elctrons in atoms exist in energy levelsm, n =1 represents the lowest energy level an electron can be in in - the ground state

  • An electron is excited if it moves up an energy level
  • Electons can move down an energy levek by emmiting a photon.
  • Since energy levels are definite, the energy of each photon emitted can only take a certain allowed value. 

Ionisatoion energy - The amount of energy needed to for an electron to be completely removed from the ground state.

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Photon emission - fluorescent tube

  • Fluorescent tubes use excitation of eletrons and photon emmisions to produce visble light
  • Contains a mecury vapour, across which a high voltage is applied.
  • When fast moving free electrons ( emitted by elctrodes in the tube and are accelrated by a high voltage ) collide with the elctrons in the mecuryatoms, the atomic mecury electrons are excited to a higher energy level.
  • When these excited electrons return to their ground states, they lose energy by emitting high energy photons in the UV range. The photons emmitied have a range of energies and wavelengths that coreespond to the different transitiosn of electrons.
  • A phosphourous coationg on the inside of tube abosorb these photons , exciting its electrons to much higher energy levels. These electrons then cascade down the energy levels and lose energy by emmiting many lower photon of visible light.
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Line emission spectra

When light is splut from a flurescent tube with a prism or a difreacttion gratting you get a line spectru,. The light spectrum is seen as a series of bright lines gainst a black background. Each line coresspomns to a different wavelength of light emitted by the source. 

This provides evidence that the electrons in atoms exist in discrete energy levels. Atoms can only emit photon energies equal to the difference between the two energy levels, as seen the corresponding wavelength in the line specteum. 

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Line absorption spectrum

Continuous spectra

A spectrum of whitelight is continuous. If you split the light up with a prism, the colours all merge together

Hot things emits a continuous spectrum in the visible and infrared 


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What are line absorption spectra?

You get a line absorbtion spectrum when light with a contiuous spectrum of energy ( white light) passes through a cool gass. At low temperatures, most of the electrons in the gas atoms will be in their ground states. Photons  of the correct wavelength are absorbed bby the electrons to excite them to higher energy levels. These wavelengths are then missing from the contiuous spectrum when it comes out the other side of the gas. You see a continuous spectrum with black lines in it corresponding to the absorbed wavelengths.

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Wave Particle Dualitiy

  • When light passes through a narrow gap, it spreads out this is known as diffraction.
  • Photoelectric effect and diffraction show that light behaves as both a particle and a wave - wave partlicle duality 
  • The de Broglie equation relates a wave property to a moving particle property..

Electron diffraction - shows that electrons have wave like properties

Electron microscope - A shorter wavelength gives less diffraction effects. Diffraction effects blur details on an image. If you want to resolve tiny detail n image, you need a shorter wavelength. Light blurs out detail more than 'electron-waves' do, so an electron microscope can resolve finer detail than a light microscope. They can let you lookat things as tiny as a single stand of DNA.

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Finding the IV characteristics of a component

1. Use a variable resistior to decrease (or increase) the resistance of the circuit in small, equal steps. Changing the resistance changes the amount of current flowing through the circuit.

2. For each change in resistance, take a reading from the ammeter to find the current through the component, and a reading from the voltmeter to find the voltage across it. Take a sensible nmber of measurements - enough so you can plot a graph and be able to spot a pattern.

3. Reverse the direction of electricity flow by switching the wires connected to the power packs. Then repeat steps 1 and 2 to collect data for negative values of V and I.

Once you've collecte dall your data, plot it on a pair of axes and valid - you've got yourself the I-v characteristic for the component. 

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Filament lamps


When current flows through a metal conductor, some of the electrical energy is transferred into heat energy and causes the metal to heat up. The extra heat energy causes the particles in the metal to viberate more. These viberations make it more difficult for the charge carrying electrons to get through the resistor therefore resistance increases

For most resistors there is a limit to the amount of current thatr can flow through them. More current means an increase in temperature, which means an increase in resistance, which menas the current decreases again. This is why the I-V graph for a filament lamp levels off at high current. 

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Why filament lamps blow?

 When you first switch a bulb on, the filament has lower resistance because it is cold. It means that the initilal current flowing through the filament has a lower resistance because its' cold. This means that the initial current flowing through the filament will be larger than the normal curve, so the filament is more likely to burn out at time.

The filament also heats up very quickly from cold to its operating temperature whn it's switched on. The rapid temperature change could cause the filament wire to blow.

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Super Conductors

You are able to decrease the resistivity of many metals by cooling them down. If you cool osme materials down to below a 'trastion temperature', their resistivity dissappears entirely and they become a superconductor. Without any resistance, none of the lectrical energy is turned into heat so none is wasted.

However most metals have extremely low 'trasnsition temperatures' which can be very expensive.

Uses of Superconductors:

- Power cables that transmit electricity without any loss of power

- Really strong electromagenets that have lots of applications, e.g. in medicine and Maglev trains.

- Elctronic circuits that work really fast, because there;s no resistance to slow the current down. 

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Emf and Internal resistance

internal resistance - when chemical energy is used to make electrons move whihc collide with atoms inside the battery. Hence cause the battery to warm uip. 

EMF- the amount of elctrical energy the battery produces and tansfers to each coulomb of charge is called the electromotive force or e.m.f

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Measuring internal resistance and e.m.f


Vary the value on the variable resistor and take readings in the same way as in the experiment to find the resistance of a component. Once you have data for current and potential difference of a power supply for different resistances, plot a graph V against I - the grpah you get will be a straight line.

Emf of the power source could also be measured by connecting a high resistance voltmeter across its terminals. Note a small current flows through the voltmeter so there must be some lost volts- hence you measure a value slighltly less that the e.m.f 

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  • Voltmeter
  • Time base controls how fast the beam is moved across the screen. 
  • Vertical height of the trace at any point shows the input voltage at that point.
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