♡ Topic 8_Energy: forces doing work ♡

• Law of energy conservation: Energy cannot be created or destroyed, only transferred into different forms
• Energy changes when a ball is thrown upwards and returns:

  • Upwards: Kinetic Energy (KE) converts to Gravitational Potential Energy (GPE)
  • Peak: Maximum GPE, zero KE
  • Downwards: GPE converts back to KE
• Total energy of a ball that is kicked remains constant due to the conservation of energy
• Energy changes in a filament light-bulb:

  • Electrical energy transforms into light and heat energy
  • Light is useful energy, heat is waste energy
• Energy transfers for a bungee jumper:

  • Falling: GPE to KE
  • Cord tightens: KE to Elastic Potential Energy (EPE)
  • Lowest point: Jumper's initial GPE equals EPE stored in the cord
• Bungee jumper slows down as cord stretches because:

  • Kinetic energy decreases, converting to elastic potential energy
  • Since KE is proportional to (velocity)², velocity decreases as KE decreases
• Energy changes in a power station:

  • Heat energy heats water to make steam
  • Steam moves turbine, converting to kinetic energy
  • Kinetic energy turns generator
  • Generator produces electrical energy
• Ways energy of a system can be changed:

  • Work done by forces
  • Electrical input
  • Heating
• Energy changed through work done by forces:

  • Force moving an object (kinetic energy) does work over a distance
• Equation for work done on an object:

  Work done (J) = force (N) x distance (m)

  W = Fd
• Wasteful mechanical processes cause a rise in temperature of surroundings by transferring energy through heating
• Equation for kinetic energy:

  E = ½ m v²

  Energy (J), Mass (kg), Velocity (m/s)
• Equation for gravitational potential energy:

  E = mgh

  Energy (J), Mass (kg), Gravitational Field Strength (N/kg), Height (m)
• Definition of 'Power':

  • The rate at which energy is transferred (or rate at which work is done)
• Two equations for power:

  • Power = energy transferred / time
  • Power = work done / time
  • SI units: Energy (J), Work Done (J), Time (s)
• Unit of power: Watt (W)
• One Watt is equivalent to 1 Joule per second (J/s)
• Meaning of a power rating of 10W for an electrical device:

  • It converts 10 Joules of energy every second
• In a scenario where two motors lift the same mass through the same height, and Motor A does this in half the time of Motor B, Motor A dissipates the most power because the energy transferred is the same but the time taken is less (P=E/t)
• Efficiency:

  • Efficiency is the proportion of energy that is converted usefully, rather than wasted/dissipated
• Calculation of 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