Lesson focus: dissipationWhen energy spreads out into the surroundings, usually as heat, and becomes less useful. spreads energy into surrounding thermal stores, reducing the amount available for a chosen purpose.
Calculate efficiencyThe fraction of the energy supplied that is transferred usefully: efficiency = useful energy output ÷ total energy input. and explain how design changes reduce unwanted transfers.
Build on earlier learning
Energy adds upBalanced energy accounts now support quantitative efficiency comparisons.
You will be successful when you can…
- Define dissipation.
- Calculate efficiency.
- Interpret percentages and decimals.
- Suggest targeted design improvements.
Words that carry the science
- dissipation
- Spreading energy into surrounding stores so it becomes less available for a purpose.
- useful transfer
- Energy transfer that achieves the intended purpose.
- efficiency
- Useful output divided by total input.
- lubrication
- Reducing friction between moving surfaces using a substance such as oil.
- insulation
- Material or design that reduces energy-transfer rate.
Dissipation reduces usefulness
01Dissipation reduces usefulness
Friction, electrical resistance, sound and unwanted heating can cause dissipationWhen energy spreads out into the surroundings, usually as heat, and becomes less useful., spreading energy into thermal stores of the surroundings.
Total energy remains conserved, but the dispersed energy is less concentrated and harder to use again.
02Efficiency compares output with input
efficiencyThe fraction of the energy supplied that is transferred usefully: efficiency = useful energy output ÷ total energy input. = useful energy transferred ÷ total energy supplied. Multiply by 100 for a percentage.
efficiencyThe fraction of the energy supplied that is transferred usefully: efficiency = useful energy output ÷ total energy input. cannot exceed 1 or 100% for a complete energy account. A result above 100% signals mismatched units, boundaries or measurements.
03Improvements must target a pathway
lubricationUsing oil or grease to reduce friction between moving parts. reduces mechanical transfer by friction. Thermal insulationA material that slows down heat moving in or out, like a woolly jumper or loft insulation. reduces transfer by heating, and streamlined shapes reduce drag.
An improvement can introduce trade-offs: thicker insulation adds material, lubrication needs maintenance and reducing ventilation can create overheating.
Sankey diagrams and efficiency
A Sankey diagram shows where the energy transferred by a device goes. The width of each arrow shows the amount of energy. Energy is never destroyed: useful energy + wasted energy = input energy. Wasted energy usually ends up heating the surroundings (it is dissipated).
Yellow: energy in. Green: useful energy. Red: wasted energy.
Controls
Efficiency = useful energy out ÷ total energy in (× 100 for a percentage). Typical efficiencies are approximate.
Calculate motor efficiency
A motor receives 800 J and transfers 520 J usefully.
- Write efficiency = useful ÷ total.
- Substitute 520 ÷ 800.
- Calculate 0.65.
- Convert to 65% and check it lies between 0 and 100%.
How to make the reasoning visible: The motor is 65% efficient and dissipates 280 J.
Compare device accounts
Calculate before judging.
| Device | Input / J | Useful / J | Efficiency |
|---|---|---|---|
| A | 200 | 150 | 75% |
| B | 500 | 350 | ? |
| C | 120 | 96 | ? |
| D | 900 | 540 | ? |
Complete the efficiencies.
Which device dissipates the most energy and which is most efficient?
Check your understanding
Answer from memory, then check against the suggested answer.
Efficiency equation?
Maximum possible efficiency?
What does dissipation mean?
A device has 0.42 efficiency. Express this as a percentage.
Why can the most efficient device still dissipate more joules?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A machine receives 2.4 kJ of energy and transfers 1.8 kJ usefully.
Calculate the efficiency of the machine as a percentage.
(3)Calculate the energy dissipated.
(1)Total for this question: 4 marks
Command word: Show the equation, substitution, answer and unit for the energy difference.