Conservation of energy — Science, 14–17 years
Energy can be transferred or transformed, but in a closed system the total amount stays the same.
Idea
Energy is not a material that gets used up. It is a way of accounting for changes, such as motion, height, temperature or electrical activity. When one store decreases, energy appears in other stores or is transferred between objects, while the total is conserved.
Why it matters
Early explanations often treated heat, motion and electricity as separate things. Energy conservation gives one reliable rule for following them through a process. It helps answer where energy went, reveals missing measurements, and prevents the false idea that a machine can produce energy from nothing.
Worked example
A 2 kg book rests on a shelf 3 m above the floor. Its gravitational energy is E = mgh = 2 × 10 × 3 = 60 J. If it falls, that store decreases by 60 J while its kinetic store increases by about 60 J, ignoring air resistance. The energy has changed form, not vanished.
Common trap
People often say that a moving object loses all its energy when it stops. That is reasonable because its motion has disappeared. In reality, friction transfers much of its kinetic energy to thermal energy in the surfaces and nearby air; the energy is harder to notice, not gone.
Outside school
Energy accounting is used to compare solar panels, cars, house insulation and power stations. It shows that a fuel may transfer energy to motion, heat and sound at the same time. Designers try to send more energy into the wanted output and less into unwanted heating or noise.
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