Energy is conserved — Physics, 11–13 years
Energy can move between objects and change form, but it is not created from nothing or destroyed. Counting all the forms before and after helps explain where energy has gone, even when some becomes thermal energy.
Energy changes clothes
Energy is a way of keeping track of what can cause change. It may be stored in a stretched spring, carried by moving water or released as light and sound. The form changes, but the total amount in a closed system stays the same.
Why keep an energy account?
A machine can seem to waste energy when it slows down or gets warm. Conservation gives us a better question: which forms did the energy become? This idea grew from experiments showing that motion, heating, electricity and chemical changes are connected rather than separate supplies.
A falling ball
A 1 kg ball is held 5 m above the ground. Its gravitational store is about 50 J. Step 1: before release, energy is 50 J gravitational and 0 J kinetic. Step 2: just before impact, it is about 0 J gravitational and 50 J kinetic. Step 3: air resistance turns some of that into thermal energy.
Energy has not vanished
The word “lost” makes it sound as if energy has disappeared. That is a reasonable shortcut when a device becomes less useful, but physics asks us to include heating, sound and movement in the surroundings. Energy may spread out and become difficult to use, yet it has not been destroyed.
Making devices efficient
Engineers use energy accounts to improve bikes, cars, lamps and heating systems. They cannot make energy disappear, but they can reduce unwanted transfers, such as friction warming a machine. A more efficient device sends more of its input energy towards the job people want.
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