Energy

• Conservation of energy principle

  Energy can be transferred, usefully stored or dissipated, but can never be created or destroyed
• Some energy is dissipated as wasted energy, generally in the form of heat
• In a closed system, the overall energy doesn't ever change
• Heating an object

  1. Energy is transferred to its thermal energy store

  2. Temperature increases
• Heat transfer

  • Can take place in three different ways depending on the medium involved
  • Conduction in solids
  • Convection in fluids
  • Radiation through empty space
• Conduction

  Vibrating particles transfer energy to neighboring particles
• Heating one end of a metal

  • Energy is transferred to the kinetic energy store of the particles at that end
  • Particles vibrate faster and collide with neighbors more often and with more energy
  • Collisions transfer kinetic energy to neighbors
  • Process repeats until heat is spread out evenly
• Thermal conductivity

  How well objects transfer energy by conduction
• Thermal conductivity

  • Metals have high thermal conductivity and transfer heat rapidly
  • Plastics have low thermal conductivity and are used as insulators
  • Fluids have low thermal conductivity
• Convection


  1. Heated fluid particles gain kinetic energy and move around faster by random diffusion

  2. Warmer, less dense particles rise above cooler, denser particles

  3. Cooler particles sink down and take their place

  4. Cycle repeats as long as fluid is being heated, creating a convection current
• Conduction and convection both involve particles gaining kinetic energy, but in conduction only the energy is transferred between particles, while in convection the particles themselves move
• Radiation

  1. Energy is carried by infrared waves

  2. All objects constantly absorb and emit radiation

  3. Hotter objects emit more radiation
• Reducing unwanted energy transfers

  1. Using thermal insulation

  2. Using lubrication
• Reducing heat loss in a house

  1. Make the house sealed closed to prevent air passing out (reduce heat loss by convection)

  2. Reduce heat loss by conduction through walls and windows (use materials with low thermal conductivity, cavity walls with insulating foam)

  3. Reduce heat loss through windows (use double glazing)
• Conduction

  Heat is lost directly through a solid like a wall or a window
• Convection

  Heat is transferred through liquids and gases
• Friction

  Reduces the efficiency of energy transfer and can cause objects to heat up
• Reducing friction

  1. Adding oil (a lubricant) to reduce friction (e.g. between cogs and chain of a bicycle)

  2. Making cars and planes streamlined to reduce air resistance friction
• Reducing friction allows vehicles to use less fuel and be more efficient
• Power

  The rate at which energy is transferred
• Power

  The rate at which work is done
• Calculating power (energy transfer)

  Energy transferred / Time taken
• Calculating power (work done)

  Work done / Time taken
• Efficiency


  The proportion of the energy supplied (input energy) that is actually transferred into the useful energy output
• Calculating efficiency


  1. Useful energy output / Total energy input

  2. Useful power output / Total power input
• Efficiency of a device

  Proportion of input energy transferred into useful energy output
• Efficiency can be calculated as a decimal or percentage
• Calculating efficiency from power

  Useful power output / Total power input