Energy, work & power
Energy stores and transfers
Energy stores
Energy is the ability to do work. It cannot be created or destroyed — only transferred from one store to another. This is the principle of conservation of energy.
SI unit of energy: the joule (J). 1 J = 1 N·m = 1 kg·m²/s².
SI unit of energy: the joule (J). 1 J = 1 N·m = 1 kg·m²/s².
| Energy store | Description | Example |
|---|---|---|
| Gravitational potential energy (GPE) | Stored due to an object's height above a reference level | Book on a shelf, water in a reservoir |
| Kinetic energy (KE) | Stored due to an object's motion | Moving car, rolling ball |
| Chemical energy | Stored in chemical bonds, released by reactions | Fuel, food, batteries |
| Thermal energy (internal energy) | Stored in the random motion of particles | Hot water, a warm room |
| Elastic (strain) energy | Stored due to deformation of a stretched or compressed object | Compressed spring, stretched elastic band |
| Nuclear energy | Stored in atomic nuclei, released by fission or fusion | Nuclear reactor, the Sun |
| Electromagnetic energy | Energy carried by electromagnetic waves (light, infrared, etc.) | Sunlight, radio waves |
Energy transfers
Energy is transferred between stores by:
- Mechanical work — a force moves an object (e.g. pushing a box)
- Heating — thermal energy flows from hotter to cooler (conduction, convection, radiation)
- Electrical work — charge flows through a component
- Radiation — electromagnetic waves carry energy (light, infrared)
Common energy transfer chains:
- Falling object: GPE → KE (→ thermal on impact)
- Burning fuel in car: Chemical → Thermal → KE (+ thermal waste)
- Solar panel: Electromagnetic → Electrical
- Stretched spring released: Elastic → KE
- Pendulum: KE ⇌ GPE (back and forth, slowly losing to thermal)
- Nuclear power station: Nuclear → Thermal → KE (steam) → Electrical
Sankey diagrams
A Sankey diagram shows the flow of energy through a system:
- Energy enters from the left as a single wide arrow.
- Arrows branch off to show useful output and wasted energy.
- The width of each arrow is proportional to the amount of energy it represents.
- The total width is conserved (energy is conserved overall).
Conservation of energy
Principle of conservation of energy: Energy cannot be created or destroyed. The total energy of a closed system remains constant — it is only converted from one form to another.
\[ \text{Total energy in} = \text{Useful energy out} + \text{Wasted energy} \]
"Energy is lost" is imprecise language. Energy is never destroyed — it is merely transferred to a less useful store (usually thermal energy due to friction, air resistance, or heating). Always say energy is transferred to thermal energy, not that it "disappears."
To describe an energy transfer: state the starting store, the energy transfer process, and the final store(s). For example: "The chemical energy in the battery is transferred electrically to the bulb, where it is converted to light and thermal energy."