Unit 4

• Conservation of Energy


  You can never make it or destroy it - you just transfer it from one energy store to another
• Energy stores


  • Kinetic energy
  • Gravitational potential energy
  • Chemical energy
  • Thermal energy
  • Elastic potential energy
  • Electrical potential energy
  • Nuclear energy
• Anything moving has energy in its kinetic energy store, and the faster it is moving the more energy it has in this store
• Anything that can release energy in a chemical reaction, e.g. food, has energy in its chemical energy store
• Anything raised in a gravitational field (i.e. anything which can fall) has energy in its gravitational potential energy store
• Anything stretched like springs and rubber bands has energy in its elastic potential energy store
• Anything with separated electric charges that attract or repel each other has energy in its electrical potential energy store
• Anything undergoing nuclear reactions can release energy from its nuclear energy store
• Anything moving due to a force acting on it (e.g. pushing, pulling, stretching, squashing) has energy transferred to its kinetic energy store
• Anything with an electric charge moving through a potential difference (e.g. charges moving round a circuit) has energy transferred to its electrical energy store
• Anything with a temperature difference (e.g. heating a pan of water on a hob) has energy transferred to its thermal energy store
• Anything emitting or absorbing electromagnetic waves like light (e.g. energy from the Sun reaching Earth as light) has energy transferred by radiation
• The total energy of a closed system has no net change - energy can be stored, transferred between stores, or dissipated, but it can never be created or destroyed
• Dissipated is a fancy way of saying that the energy is spread out and less useful
• Most energy transfers involve some losses, often by heating
• Useful energy


  Energy that is transferred from one store to a useful store
• Wasted energy


  Energy that is transferred to less useful stores, normally by heating, or by light or sound
• The less energy that's wasted, the more efficient the device is said to be
• Efficiency


  Useful energy output / Total energy output x 100%
• Describing energy transfers


  1. Identify the energy stores involved

  2. Describe how energy is transferred between the stores

  3. Identify any wasted energy transfers
• Thermal radiation


  Transfer of energy by infrared electromagnetic waves
• Conduction


  Energy transfer by particles in solids
• Convection


  Energy transfer by particles in liquids and gases
• The bigger the temperature difference, the faster energy is transferred between the thermal energy stores of a body and its surroundings
• All objects are continually emitting and absorbing infrared radiation
• An object that's hotter than its surroundings emits more radiation than it absorbs, and an object that's cooler than its surroundings absorbs more radiation than it emits
• Some colours and surfaces absorb and emit radiation better than others
• Conduction


  Occurs mainly in solids, where vibrating particles transfer energy from their kinetic energy store to the kinetic energy stores of neighbouring particles
• Convection


  Occurs in liquids and gases, where more energetic particles move from hotter regions to cooler regions, transferring energy as they do
• Convection cannot happen in solids because the particles cannot move apart from vibrating
• Immersion heater example


  1. Energy transferred from heater coils to thermal energy store of water by conduction

  2. Particles near coils get more energy, become less dense, and rise

  3. Colder water sinks towards heater coils

  4. Convection currents form, circulating energy through the water
• Convection currents are all about changes in density
• Demonstrating convection currents with potassium permanganate crystals

  1. Crystals placed in cold water, one side heated

  2. Crystals dissolve, forming purple solution

  3. Purple solution carried by convection currents
• Work done


  Energy transferred when a force moves an object through a distance
• Power


  Rate of doing work, or rate of energy transfer
• Power is measured in watts (W), where 1W = 1J/s
• Kinetic energy


  Energy an object has due to its motion, depends on mass and speed
• Gravitational potential energy


  Energy an object has due to its position in a gravitational field, depends on mass, height, and gravitational field strength
• When an object falls, energy is transferred from its gravitational potential energy store to its kinetic energy store
• In real life, air resistance acts against falling objects, causing some energy to be transferred to other stores like thermal energy