Section 4 Energy
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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
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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.
