Conservation and Dissipation of Energy (P1.2)

Energy conservation law

The law of conservation of energy: energy cannot be created or destroyed — only transferred between stores or dissipated to the surroundings.

In every energy transfer, some energy is dissipated (spread to thermal store of surroundings) — this is often called "wasted" energy, though it is not destroyed.

Sankey diagrams

Sankey diagrams represent energy transfers:

• Arrow thickness is proportional to the amount of energy.
• Input energy on the left; useful output and wasted energy on the right.

Example: a light bulb with 100 J input: 5 J light output, 95 J heat (wasted) → efficiency = 5%.

Efficiency

efficiency = useful output energy ÷ total input energy

OR

efficiency = useful output power ÷ total input power

Multiply by 100 for percentage. Efficiency ≤ 1 (or ≤ 100%).

Worked example: A motor receives 200 J and does 150 J of useful work:
efficiency = 150/200 = 0.75 = 75%

Reducing energy dissipation

• Lubrication: reduces friction between moving parts → less thermal energy wasted.
• Insulation: reduces thermal conduction → less energy lost to surroundings.
• Streamlining: reduces air resistance → less kinetic energy lost.
• Improving thermal conductivity: relevant for heat exchangers.

Thermal conductivity

Thermal conductivity describes how readily a material conducts heat. Good thermal conductors (metals) have high conductivity; insulators (wool, air, expanded polystyrene) have low conductivity.

Heat flow through a material depends on:

• Temperature difference across the material (ΔT)
• Thickness of the material
• Thermal conductivity of the material
• Surface area

Building insulation: reduces rate of heat loss in winter → less energy needed for heating → lower fuel bills + carbon footprint. Examples: loft insulation, cavity wall insulation, double glazing, draught-proofing.

Common exam errors

1. Saying efficiency > 1 or > 100% is possible — it can never be.
2. Confusing power efficiency with energy efficiency — both use the same formula (P or E).
3. Saying thermal energy is "lost" — it is dissipated to the surroundings, but the total energy is conserved.