Conservation and dissipation of energy: the principle of conservation of energy in a closed system, how energy is dissipated to less useful stores, and why mechanical processes waste energy by heating.

What this dot point is asking

Edexcel statements 3.4 and 3.6 to 3.8 want you to explain what conservation of energy means, that the total energy in a closed system does not change, that energy is dissipated so that it is stored in less useful ways, and that mechanical processes become wasteful when they raise the temperature of the surroundings.

The principle of conservation of energy

This principle is one of the most important in all of physics. When something appears to "lose" energy, the energy has actually been transferred somewhere, usually to the thermal store of the surroundings, where it is harder to use. Energy is never lost in the sense of vanishing; it is only ever moved or spread out.

Dissipation to less useful stores

In every real change, some energy is dissipated. A light bulb transfers most of its energy usefully as light but some is wasted heating the surroundings; a phone battery powers the device but the circuits warm up. The energy is still there, but once it is spread thinly through the surroundings it cannot be gathered back, so we call it wasted.

Why mechanical processes waste energy

Whenever surfaces rub or an object moves through air, some kinetic energy is transferred to thermal energy and the temperature rises. This is unavoidable, which is why no real machine is perfectly efficient. Reducing friction (with lubrication) or streamlining a shape reduces this wasteful heating, a point developed in the next dot point.

How Edexcel examines this

These statements are examined on both tiers, with the conservation principle frequently asked as a two-mark recall question whose mark scheme requires both that energy is never created or destroyed and that the total in a closed system is constant. The more demanding application questions describe a real situation where the outcome falls short of an ideal calculation, a slide, a bouncing ball, a swinging pendulum that loses height, and ask you to explain it; the full-mark answer names the useful transfer, identifies friction or air resistance dissipating energy to the thermal store of the surroundings, and stresses that energy is still conserved overall even though some is now in a less useful form. Examiners reward precise language: write "dissipated to the thermal store of the surroundings" rather than "lost as heat", and avoid implying that energy disappears. Linking dissipation to a rise in temperature, and noting that this is why real machines are never 100% efficient, sets up the efficiency dot point that follows.

Try this

Q1. State what happens to the total energy of a closed system during an energy transfer. [1 mark]

• Cue. It stays the same (energy is conserved).

Q2. Name the store that dissipated energy usually ends up in. [1 mark]

• Cue. The thermal store of the surroundings.