Topic 8

• Law of energy conservation


  Energy cannot be created or destroyed (it can only be transferred into different forms)
• Energy changes when a ball is thrown upwards and returns to starting position (ignoring air resistance)


  1. Upwards: KE is converted to GPE

  2. Peak: Maximum GPE, zero KE

  3. Downwards: GPE is converted to KE
• The total energy of the system remains constant due to the conservation of energy when a ball is kicked (assuming no external forces)
• Energy changes in a filament light-bulb


  1. Electrical energy is transferred into light and heat energy

  2. Light is a useful energy form, heat is waste energy
• Energy transfers for a bungee jumper

  1. When falling, the GPE is converted to KE of jumper

  2. As the cord tightens, KE is converted and stored as Elastic Potential Energy (EPE)

  3. At lowest point, the jumper's initial GPE equals the EPE stored in the cord
• A bungee jumper slows down once the cord begins to stretch

  • Kinetic energy decreases since it is converted to elastic potential energy
  • Since KE is proportional to (velocity)², as KE decreases, so does velocity
• Energy changes in a power station

  1. Heat energy released in the reaction heats water to make steam

  2. Steam moves the turbine, converting kinetic energy

  3. Kinetic energy turns the generator

  4. Electrical energy as the generator generates electricity
• Ways the energy of a system can be changed

  • Through work done by forces
  • Through electrical input
  • Through heating
• How energy can be changed through work done by forces

  A force which moves an object (kinetic energy) causes work to be done over a distance
• Equation to measure work done on an object


  • Work done (J) = force (N) x distance (m)
  • W = Fd
• Wasteful mechanical processes cause a rise in the temperature of the surroundings because they cause energy to be transferred by heating the surroundings
• Equation for kinetic energy


  • E = ½ m v2
  • Energy = ½ x Mass x (Velocity)²
  • Energy (J), Mass(kg), Velocity(m/s)
• Equation for gravitational potential energy


  • E = mgh
  • Energy = Mass x Gravitational Field Strength x Height
  • Energy (J), Mass(kg), Gravitational Field Strength (N/kg), Height (m)
• Power


  The rate at which energy is transferred (or rate at which work is done)
• Equations for power


  1. power = energy transferred / time

  2. power = work done / time
• Quantities involved in power equations

  • Energy (J)
  • Work Done (J)
  • Time (s)
• Unit of power


  Watt (W)
• 1 Watt


  1 Joule per second (J/s)
• Power rating of 10W for an electrical device

  It converts 10 Joules of energy every second
• Two motors lift the same mass through the same height

  Motor A does this in half the time of Motor B, so Motor A dissipates the most power
• Efficiency


  The proportion of energy that is converted usefully, rather than wasted/dissipated
• How to calculate efficiency

  Efficiency = useful energy out / total energy in
• Units of efficiency


  Efficiency is a ratio so has no units, but it can be expressed as a percentage