P1.7

• When you lift an object

  1. Energy stored in your muscles is transferred

  2. To the gravitational potential energy store of the object
• Amount of energy transferred


  Depends on the object's weight and how high you lift it
• Weight


  • Measured in newtons (N)
  • Weight of a 1 kilogram object on the Earth's surface is about 10N
• Energy


  • Measured in joules (J)
  • Energy needed to lift a weight of 1 newton by a height of 1 metre is equal to 1 joule
• Input energy


  Energy supplied to the device
• Useful output energy


  Useful energy transferred by the device
• Energy cannot be created or destroyed
• No device can be more than 100% efficient, because you can never get more energy from a machine than you put into it
• Efficiency


  The greater the percentage of energy that is usefully transferred in a device, the more efficient the device is
• Efficiency and percentage efficiency

  Numbers without units, the maximum efficiency is 1 or 100%, if calculation produces a number greater than this, it must be wrong
• Efficiency of a device

  Useful energy transferred by the device / total energy supplied to the device * 100
• No energy transfer can be more than 100% efficient
• Machines


  • Waste energy because of friction between their moving parts, air resistance, electrical resistance, and noise
• Making machines more efficient

  Reducing the energy they waste, for example using lubrication to reduce friction between moving parts
• How can the efficiency of a system be improved?

  Reducing Waste Output - Lubrication, Insulation

  Recycling Waste Output
• Devices waste energy


  • Friction between moving parts causes heating
  • Resistance of wire causes it to get hot when current passes through
  • Air resistance causes force on moving object that opposes motion, wasting energy
  • Sound created by machinery causes energy transfer to surroundings
• Improving efficiency


  1. Lubricate moving parts to reduce friction

  2. Use wires with low electrical resistance in circuits

  3. Streamline shapes of moving objects to reduce air resistance

  4. Cut out noise (e.g. tighten loose parts to reduce vibration)