Energy: Conservation of Energy and Power

Energy: Conservation of Energy and Power

Conservation of Energy

  • The principle of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed.
  • Energy transfers occur in all physical and chemical processes, for example when you cycle up a hill or light a match.
  • Not all energy transfers are useful. In most scenarios, some of the initial energy is ‘wasted’ as it is transferred into a form that is not useful for the intended purpose. For example, in a light bulb, only a small portion of electrical energy is transformed into light. The rest is wasted as heat energy.
  • The ‘wasted’ energy is not destroyed, as this would violate the principle of conservation of energy. Instead, it is usually dissipated into the surroundings, often as thermal energy.

Energy Efficiency

  • The efficiency of an energy transfer is calculated by comparing the useful energy output to the total energy input. It is always a value between 0 and 1, or a percentage between 0% and 100%.
  • The formula for calculating energy efficiency is: efficiency = (useful energy output / total energy input).
  • A machine that is 100% efficient would convert all input energy into useful output energy. However, in practice, no machine can be 100% efficient due to energy losses, usually in the form of heat due to friction.
  • Efforts to improve energy efficiency are key to reducing energy waste, saving resources and controlling environmental impacts.


  • Power in physics is the rate of doing work or transferring energy. It tells us how quickly energy is being transferred or transformed.
  • The unit of power is the watt (W). One watt equals one joule per second.
  • The formula to calculate power is P = E/t, where P is power, E is the amount of energy transferred or work done, and t is the time taken.
  • When dealing with larger units of power, it’s often useful to convert watts to kilowatts (1 kilowatt = 1000 watts). This is particularly common in the context of electricity usage.

Sankey Diagrams

  • Sankey diagrams are a visual tool used to represent energy transfers.
  • In a Sankey diagram, the width of the arrows is proportional to the amount of energy they represent.
  • The input energy is usually represented by a single vertical arrow on the left, while output energies are represented by arrows leading to the right.
  • Useful output energies are usually represented by arrows pointing upwards, while ‘wasted’ energies are shown by downward arrows. The larger the downward arrow, the less efficient the device.