Conservation of energy — Physics, 14–17
Energy changes form and moves between places, but it is not created or destroyed. Physics, 14–17 years.
The idea
Energy is a way to keep track of what can cause change: movement, heating, light, sound, or lifting. It can be stored in different ways and transferred between objects or systems. In a closed system, the total amount stays the same, even when the useful form becomes less obvious.
Why we need it
Machines often look as if they create energy: a motor moves a lift, or a lamp produces light. The conservation idea asks where the input went and what else was produced. It grew from careful studies of heat, work, and engines, giving scientists a reliable account even when the process is complicated.
Worked example
A 2 kg book is lifted 3 m. Taking gravitational field strength as 10 N/kg, its gain in gravitational potential energy is mgh = 2 × 10 × 3 = 60 J. If the lifter transfers 75 J, the remaining 15 J becomes mainly thermal energy in muscles and surroundings. The total is still 75 J.
The common trap
People often say that energy is “lost” when a machine becomes warm or noisy. This is understandable because heat and sound may not help the machine do its intended job. The energy has not vanished; it has spread into the surroundings, where it is harder to collect and use again.
Where it appears
Energy conservation guides the design of power stations, batteries, insulation, and renewable systems. A home energy audit traces electricity or fuel into heating, light, motion, and waste heat. It also helps compare claims: a device cannot deliver more energy than it receives unless energy was already stored inside it.
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