Energy is stored in different ways. A moving object has a kinetic store. A raised object has a gravitational potential store. A stretched spring has an elastic store. Food, fuel and batteries have chemical stores. A hot object has a thermal store. Energy is transferred between stores by forces doing work, by electric current, by heating and by waves such as light and sound.
Energy is never created or destroyed; it is only transferred. This is the law of conservation of energy. When a ball falls, energy moves from its gravitational store to its kinetic store. Some energy is always transferred to the surroundings as wasted heat and sound, so no machine is 100% efficient.
Worked example
Describe the energy transfer when you cycle up a hill.
- Energy comes from the chemical store in your food.
- Your muscles do work on the pedals.
Answer:Energy goes to the kinetic store and the gravitational store as you rise, with some wasted as heat.
Key words
- energy store
- a way energy is held: kinetic, chemical, thermal…
- kinetic
- energy of a moving object
- gravitational potential
- energy of a raised object
- conservation of energy
- energy cannot be created or destroyed
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NGSS-referenced: MS-PS3-5