Physics

• Energy level units


  Joules or electron volts (1 eV = 1.6x10^-19 J)
• Emission spectrum


  Diagram showing wavelengths of emitted photons
• Absorption spectrum


  Diagram showing wavelengths absorbed by a gas or plasma
• Fluorescent tube lights


  1. Electrons accelerated through mercury gas, emit UV

  2. UV absorbed by fluorescent coating, re-emitted as visible light
• Photoelectric effect


  1. Light shone on metal liberates electrons

  2. Kinetic energy of electrons depends on photon frequency, not intensity
• Electron diffraction


  Electrons exhibit wave-like behaviour, forming interference patterns
• Electricity


  Flow of electric charge, transferring energy from source to components
• Diffraction


  Less diffraction according to n Lambda equal D sin Theta
• There is a standard graph showing how intensity varies with distance from the center
• The intensity doesn't decrease to zero unlike diffraction patterns for light
• You might have to convert kinetic energy into momentum or vice versa

  The trick is: kinetic energy is equal to half mv^2, multiply both sides by m which gives me equal half P (momentum) squared, then rearrange for p
• Electricity is a flow of charge or charges like electrons
• Cells and batteries


  The same thing, they do the same job
• How a battery works

  1. Battery has a store of chemical potential energy

  2. When connected in a complete circuit, this energy is transferred to the electrons which move through the wires

  3. This movement of charge is called a current and always goes from the positive terminal of the battery to the negative

  4. As the electrons pass through the bulb, their energy is converted into light

  5. The electrons don't just disappear, they are pushed back round to the battery by the ones behind them where they're refilled with energy ready for another trip around the circuit
• Potential difference (PD) or voltage


  Tells us how much energy is transferred per Kum of electrons
• Voltmeter


  Measures potential difference, always connected in parallel to the components
• Potential difference equation


  PD in volts = energy in Joules / charge in Kums
• Current


  The rate of flow of charge, measured in amps
• Current equation


  Current in amps = charge in Kums / time in seconds
• Ammeter


  Measures current, must be connected in series with the component
• Resistance


  Components resist the flow of charge or current through them
• Increasing potential difference


  Results in a greater current flowing, PD and current are directly proportional
• Ohm's Law


  V = IR, resistance can be calculated from V and I
• Resistance of a metal filament in a bulb is non-ohmic, it changes with PD and current
• Reason for changing resistance in metals

  Metals consist of a lattice of ions surrounded by a sea of delocalized electrons, higher current causes more collisions with ions which increases resistance
• Diode


  Only allows current to flow in one direction, has very low resistance in one direction and very high resistance in the other
• LED (light emitting diode)


  Acts like a diode but also emits light
• Superconductor


  Material with zero resistance, needs to be cooled to a critical temperature
• Resistivity


  The resistance of a cube of unit length sides of a material, measured in ohm-meters
• Measuring resistivity


  Measure diameter of wire to calculate area, measure V and I for varying lengths L, plot R against L to get a proportional relationship, use equation row = RA/L to calculate resistivity
• Series circuits


  • Total PD is shared between components, current is the same for all, total resistance is the sum of all resistances
• Parallel circuits


  • PD is the same for every branch, current is shared between branches, total resistance is lower than individual resistances
• Potential divider circuit


  Series circuit where total PD is shared between components
• Thermistor


  Resistance decreases as temperature increases, opposite of metals
• LDR (light dependent resistor)

  Resistance decreases as light intensity increases
• Power


  The rate of energy transfer, calculated as P = IV
• AC (alternating current)


  Needed to transmit electricity over long distances, voltage varies between +325V and -325V
• RMS (root mean square) values

  Used to convert AC to an equivalent DC value, RMS voltage = Peak voltage / √2
• Battery internal resistance


  Causes a voltage drop, so terminal PD is less than the EMF (electromotive force)
• EMF = terminal PD + I * internal resistance