Energy stores, transfers and conservation of energy

Energy stores

Energy is stored in different ways. You need to know all eight stores:

Energy transfers

Energy is transferred between stores by four pathways:

1. Mechanically — by a force doing work (e.g. pushing, pulling)
2. Electrically — by an electric current (e.g. motor, heater)
3. By heating — from hot to cold (conduction, convection, radiation)
4. By radiation — electromagnetic waves (light, infrared, sound waves)

Conservation of energy

The law of conservation of energy: energy cannot be created or destroyed, only transferred from one store to another. The total energy in a closed system remains constant.

In real systems, energy is always dissipated (spread out) to the thermal store of the surroundings. This energy becomes less useful — it is "wasted" but not destroyed.

Sankey diagrams

A Sankey diagram shows energy transfers visually:

• Arrow width is proportional to the amount of energy.
• The main arrow shows useful energy out; smaller arrows branch off as wasted energy.

Example: A light bulb transfers 100 J of electrical energy: 5 J → light (useful), 95 J → thermal (wasted). Efficiency = 5/100 × 100% = 5%.

Efficiency

$$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100%$$

Or using power: $$\text{efficiency} = \frac{\text{useful power output}}{\text{total power input}} \times 100%$$

Efficiency is always ≤ 1 (or ≤ 100%). A value above 1 would violate conservation of energy.

Reducing energy dissipation

• Lubrication — reduces friction between moving parts (less thermal dissipation).
• Insulation — reduces thermal energy transfer to surroundings.
• Streamlining — reduces air resistance (less work done against drag).