Unit 1

Unit 1

1. Particles

   • Photons
     • EM radiation
       • Radio
         • >0.1m
       • Microwave
         • 0.1m-1mm
       • Infrared
         • 1mm-700nm
       • Visible light
         • 700nm-400nm
       • Ultraviolet
         • 400nm-1nm
       • X-ray
         • <1nm
       • Gamma
         • <1nm
       • Electric and magnetic waves
         • Travel together and vibrate
         • At right angles to eachother and in phase
     • Photons
       • Electromagnetic waves released by a charged particle when it loses energy
       • This happens when
         • Fast moving electron is stopped, slows down or changes direction
         • An electron in a shell of an atom moves to a lower energy level
       • Emitted as short bursts of waves known as a photon
       • Photon energy E = hf
   • Particles and antiparticles
     • Antimatter was predicted by Paul Dirac
     • For every particle, there is a corresponding antiparticle which
       • Annihilates the particle if they meet, converting their total mass to photons
       • Has exactly the same rest mass
       • Has an exactly opposite charge
     • Pair production
       • A photon with sufficient energu could suddenly change into a particle-antiparticle pair
       • These woulld then separate from eachother
   • Sorting
     • Hadrons
       • Interact through the strong interaction and through EM interaction if charged
       • Apart from the stable proton, they decay through the weak interaction
       • Split into baryons and mesons
         • Baryons are protons and all other hadrons that decay into protons
         • Mesons dont decay into protons
     • Leptons
       • Interact through the weak interaction and through the EM interaction if charged
     • Energy
       • Rest energy of products = total energy before - kinetic energy of products
     • Quarks
       • Up, down and strange quarks
       • Quarks have a property known as strangeness
         • Always conserved in a strong interaction
       • Make up hadrons
         • Proton is made from up, up down
         • Neutron is up, down down
         • Baryons are made from 3 quarks
         • Mesons are made from a quark and an antiquark

2. Quantum

   • Photoelectric effect
     • Electrons are emitted from a metal surface when EM radiation above a certtain frequency is directed at the metal
     • Problems
       • Photoelectric emission doesnt occue below a threshold frequency
       • Rate of electron emission is proportiaonal to the intensity of radiation
       • Emission occurs without delay
       • If EM radiation was waves, emission should take place with waves of any frequency, and would take longer using low intensity waves
     • Einstein
       • When light is incident on a metal surface, a surface electron absorbs a single photon
       • This gives the electron an energy equal to hf
       • An electron can leave the surface if the energy of the photon xceeds the metals work function
       • Maximum kinetic energy of the emitted electron = hf - work function
       • Threshold frequency of the metal = work function/h
   • Energy levels
     • The lowest energyy state of an atom is its ground state
     • When an atom in this state absorbs energy, one of its electrons moves to a shell at higher energy
     • The atom is now in its excited state
     • De-excitation
       • Atoms absorb energy as a result of exitation, but they don't permanently retain it
       • The electron configuration in an excited atom is unstable because there is a vacancy
       • This vacancy is filled by an electron from an outer shell transferring to it
       • When this happens, the electron emits a photon
     • Fluorescence
       • Atoms in a substance absorb UV photons and become excited
       • When the atoms de-excite, they emit visible photons
       • When the source of UV radiation is removed, the substance stops glowing
   • Wave partical duality
     • Lights wave nature is observed when light is diffracted
     • Lights particle nature is observed in the photoelectric effect
     • Electrons also have a dual wave-particle nature
     • This is characterised by a wavelength - its de Broglie wavelength
     • de Broglie wavelength = h/mv

3. Electricity

   • Circuit rules
     • At any junction, the total current leaving the junction is equal to the total current entering the junction
     • Components in series
       • The current entering a component is the same as the current leaving it
       • The current passing through 2 or more components in series is the same through each component
     • Potential difference
       • If the charge carriers lose energy, potential difference is a potential drop
       • If the charge carriers gain energy, the potential difference is a potential rise
       • For 2 or more components in series, the total pd across all components = the sum of pds across each component
       • The pd across components in parallel is the same
       • For any complete loop of a circuit, the sum of the emfs round the loop is equal to the sum of the potential drops round the loop
   • Potential difference and power
     • Each electron in a battery has a potential to deliver energy even when the battery isnt part of a complete circuit
     • When the battery is in the circuit, each electron passing through a component does work and uses elecrical potential energy in doing so
     • Potential difference - the work done (or energy transfer) per unit charge
     • W = VQ - for work done or energy transfer
     • The emf of a source is defined as the electrical energy produced per unit charge passing through the source
     • P = IV
     • Q = It
   • Resistance
     • R = V/I
     • Ohms law - pd across a metallic conductor is proportional to the current, provided conditions don't change
     • Resistivity = RA/L
     • Superconductivity
       • Has 0 resistivity at and below critical temperature that depends on the material
       • When a current passes through there is no pd across it
       • This means the current has no heating effect
       • Make high power electromagnets and power cables that transfer energy without waste
   • Emf and internal resistance
     • Internal resistance of a source is the loss of potential difference per unit current in the source when current passes through the source
     • emf = IR + Ir, where r is internal resistance
     • Therefore power, Ie = I^2R + I^2r
   • Potential dividers
     • Consists of 2 or more resistors in series with eachother and with a source of fixed pd
     • Uses
       • To supply a pd which is fixed at any value between 0 and the source pd
       • To supply a variable pd
       • To supply a pd which varies with a physical condition
     • To supply fixed pd
       • The ratio of pds caross each resistor is equal to the resistance ratio of the 2 resistors
       • V1 = V0R1/R1 + R2 OR V2 = V0R2/R1 + R2
     • To supply variable pd
       • Using thermistors or LDR's in sensor circuits, allowing pd to vary with a physical condition
       • Can be used as volume control or to vary lightbulb brightness
   • Alternating current
     • Measurements
       • Peak value = max current of pd - same in either direction
       • Current that repeatedly reverses direction
       • Frequency = the number of cycles each second
     • Oscilloscopes
       • Use an oscilloscope to display the waveform
       • Increasing the output pd from the signal generator - taller trace
       • Increasing frequency - more cycles on the screen
       • X plates are connected to the time base circuit, and the pd is connected to the Y-plates so the spot moves up and down as it moves left to right across the screen
     • RMS
       • Direct current that would give the same power as the mean power
       • Mean power is hlaf the peak power
       • RMS is root mean square - the value of direct current that would give the same heating effect as the alternating current in the same resistor
       • Peak value/root2