  ## Section 4 Energy

Notes
• For example, 100J of chemical energy from a battery is transferred electrically into 100J of output energy in a flashlight. A flashlight is used to provide light.

If 15J comes out as light, then the other 85J is wasted as thermal energy, and the efficiency of the flashlight is 15%.

If 10J comes out as light, then 90J is wasted as thermal energy, and the efficiency of the flashlight in this case is 10%.

100J of input energy is always transferred into 100J of output energy.

### Top Tip

Remember:

total energy into system = total energy out of system

• A Sankey diagram is a useful visual representation showing energy input and output. The diagram above represents the energy of a light bulb with an efficiency of 10%, i.e. of the 100J input only 10J of the output is transferred by radiation (light). The rest is ‘wasted’ as the surroundings are heated.

When drawing a Sankey diagram, make sure that the thickness of the arrows represents the relative amounts of energy.