P1: Energy

• Energy is never used up. Instead it's just transferred between different energy stores and different objects
• Energy is Transferred Between Stores

  When energy is transferred to an object, the energy is stored in one of the object's energy stores
• Energy stores


  • Thermal energy stores
  • Kinetic energy stores
  • Gravitational potential energy stores
  • Elastic potential energy stores
  • Chemical energy stores
  • Magnetic energy stores
  • Electrostatic energy stores
  • Nuclear energy stores
• System


  A single object or a group of objects that you're interested in
• Closed systems are systems where neither matter nor energy can enter or leave. The net change in the total energy of a closed system is always zero
• Energy can be Transferred by Heating


  Energy is transferred to the water's thermal energy store (causing the temperature of the water to rise)
• Energy can be Transferred by Doing Work


  Work done is just another way of saying energy transferred-they're the same thing
• Work done


  • Current flowing (work is done against resistance in a circuit)
  • A force moving an object
• Initial force exerted by a person to throw a ball upwards

  Causes an energy transfer from the chemical energy store of the person's arm to the kinetic energy store of the ball and arm
• Gravitational force on a ball dropped from a height

  Causes energy to be transferred from the ball's gravitational potential energy store to its kinetic energy store
• Friction between a car's brakes and its wheels

  Causes an energy transfer from the wheels' kinetic energy stores to the thermal energy store of the surroundings
• Normal contact force between a car and a stationary object in a collision

  Causes energy to be transferred from the car's kinetic energy store to other energy stores, e.g. the elastic potential and thermal energy stores of the object and the car body
• Kinetic energy


  Energy in an object's kinetic energy store
• Gravitational potential energy


  Energy in an object's gravitational potential energy store
• When something falls, energy from its gravitational potential energy store is transferred to its kinetic energy store
• Elastic potential energy


  Energy in an object's elastic potential energy store
• Specific heat capacity


  The amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C
• Different materials have different specific heat capacities
• Materials that need to gain lots of energy in their thermal energy stores to warm up also transfer loads of energy when they cool down again
• Energy is never destroyed, it is always conserved
• Dissipated energy


  Energy that is stored in a way that is not useful, usually thermal energy
• In a closed system, the net change in energy is zero
• Power


  The rate of energy transfer or the rate of doing work
• A powerful machine transfers a lot of energy in a short space of time
• Lubrication


  Reduces frictional forces
• Frictional forces cause energy to be dissipated, e.g. air resistance can transfer energy from a falling object's kinetic energy store to its thermal energy store
• Powerful machine


  One which transfers a lot of energy in a short space of time
• Two identical cars race the same distance

  The car with the more powerful engine will reach the finish line faster
• Ways to reduce unwanted energy transfers

  • Lubrication
  • Thermal insulation
• Lubrication


  Reduces frictional forces between objects that are being rubbed together
• Insulation


  Reduces the rate of energy transfer by heating
• Investigating the effectiveness of materials as thermal insulators

  1. Boil water in a kettle

  2. Pour some into a sealable container

  3. Measure the mass of water

  4. Measure the initial temperature

  5. Seal the container and leave for 5 minutes

  6. Measure the final temperature
• The lower the temperature difference, the better the material is as a thermal insulator
• Thicker insulating layers make better thermal insulators
• Efficiency


  The ratio of useful output energy transfer to total input energy transfer
• Calculating efficiency


  • Change efficiency from percentage to decimal
  • Rearrange equation to find useful power output
  • Plug in given values
• No device is 100% efficient, wasted energy usually ends up in thermal energy stores
• Electric heaters are 100% efficient as all energy input is transferred to useful thermal energy
• All energy ultimately ends up in thermal energy stores
• Non-renewable energy resources like fossil fuels and nuclear fuel will run out one day