Physics

• Energy is not something you can hold in your hand, it's just an idea
• Total energy in any interaction is always conserved, energy cannot be created or destroyed
• Energy can be turned into matter (Mass) but it's still technically true that energy cannot be created or destroyed
• Energy stores


  Different types of energy
• Kinetic energy


  Calculated using e = 1/2 mv^2
• Gravitational potential energy (GPE)


  Calculated using e = mgh
• Elastic potential energy


  Calculated using e = 1/2 ke^2
• Thermal energy


  Calculated using e = mc∆T
• Chemical potential energy is found in food and fuels, but there is no equation for it
• For anything to happen in a system, energy must be transferred from one object to another or one store to another store
• In a closed system, no energy is lost to the surroundings and no energy comes in from the surroundings
• Gravitational potential energy (GPE) at the top of a roller coaster


  Converted to kinetic energy (KE) at the bottom
• The equation V = sqrt(2KE/m) can be used to find the speed of an object at the bottom of a hill
• If more energy is lost to the surroundings than is gained by the system, it cannot be a closed system
• Specific heat capacity practical

  Use an electric heater, voltmeter, ammeter, balance, timer, and thermometer to measure the specific heat capacity of a material
• Power


  The rate of energy transfer, calculated using P = E/t
• Efficiency


  The ratio of useful energy output to total energy input, calculated as useful energy out / total energy in
• Electricity


  • The flow of charge (electrons) that carries energy from a source to a component
  • Requires a complete circuit loop for the charges to flow
• Potential difference (PD) or voltage

  The amount of energy transferred per unit of charge, calculated using V = E/Q
• Current


  The rate of flow of charge, calculated using I = Q/t
• Resistance


  The property of a component that resists the flow of current, described by Ohm's law V = IR
• Resistors have constant resistance, shown by a straight line on an I-V graph
• Bulbs have variable resistance, shown by a curved line on an I-V graph
• PD and current or V and I

  Directly proportional
• Drawing a graph of PD and current makes a straight line
• Turning the battery round gives negative values for PD and current but still a straight line through the origin
• Straight line with constant gradient

  Shows that a resistor has constant resistance, it's ohmic
• Steeper gradient of the line

  The lower the resistance of the resistor
• Ohm's law


  V = I * R, PD in volts equals current in amps times resistance in ohms
• Finding resistance from an IV graph

  1. Pick a point on the line

  2. Rearrange Ohm's law to R = V/I
• For a resistor, you'll get the same resistance value no matter what point you pick on the line
• For a bulb instead of a resistor

  You get a curved graph, showing the resistance is changing
• Resistance of a metal

  Increases with higher PD and higher current, as the metal heats up and the ions vibrate more
• Resistors are specially made so their resistance stays constant even if temperature changes
• Diode


  Only lets current 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
• Measuring resistance of a metal wire

  1. Connect to circuit with crocodile clips

  2. Measure V and I

  3. Calculate R using Ohm's law

  4. Move one clip to change wire length and see effect on resistance
• Resistance and length of wire are directly proportional
• Series circuits


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


  • PD is the same for each branch
  • Current is shared between branches
  • Total resistance decreases as more resistors are added