Lesson focus: Energy is conserved in a closed systemThe object or group of objects you choose to study when following where energy goes. even when it spreads into less useful stores.
Construct energy accounts and interpret proportional Sankey diagramA diagram with arrows whose widths show how much energy goes where..
Build on earlier learning
Food and fuelsCalorimetry showed that energy missing from the water reached other parts of the surroundings.
You will be successful when you can…
- State conservation of energy.
- Identify stores and pathways.
- Balance an energy account.
- Read and check a Sankey diagram.
Words that carry the science
- system
- The object or group of objects chosen for an energy analysis.
- energy store
- A way energy is held in a system, such as chemical, kinetic or thermal.
- transfer pathway
- A process transferring energy, such as mechanically, electrically, by heating or by radiation.
- conservation of energy
- Total energy in a closed system remains constant.
- Sankey diagram
- A diagram whose arrow widths represent energy quantities.
Choose the system before accounting
01Choose the system before accounting
An energy account begins by defining the systemThe object or group of objects you choose to study when following where energy goes.. A torch systemThe object or group of objects you choose to study when following where energy goes. might include the battery, lamp and nearby surroundings.
Stores describe where energy is accounted for. Pathways describe how energy moves between stores: mechanically, electrically, by heating or by radiation.
02Total energy remains constant
Energy cannot be created or destroyed. For a closed system, total energy before equals total energy after.
Energy that seems “lost” has usually spread into thermal stores of the surroundings where it is harder to use for the intended purpose.
03Sankey arrows show quantities
The input arrow splits into useful and dissipated outputs. Their widths must be proportional, and output values must add to the input.
The useful output depends on purpose. Heating from a lamp is usually unwanted, while heating from a kettle is the intended transfer.
Energy stores of a bouncing ball
As the ball falls, energy is transferred from its gravitational store to its kinetic store. Each bounce transfers some to the thermal store of the ball and surroundings. The total never changes.
Controls
Air resistance is ignored. The share of energy kept at each bounce is illustrative.
Balance a lamp account
A lamp receives 100 J electrically and transfers 18 J usefully by visible radiation.
- Define the lamp and surroundings as the system.
- Input = 100 J.
- Dissipated output = 100 − 18 = 82 J.
- Draw output arrows in an 18:82 width ratio.
How to make the reasoning visible: 100 J enters the account and 100 J leaves the device into useful and dissipated pathways.
Complete energy accounts
Every row must balance.
| Device | Input / J | Useful / J | Dissipated / J |
|---|---|---|---|
| lamp | 100 | 18 | ? |
| motor | 500 | 360 | ? |
| kettle | 2400 | ? | 300 |
| speaker | 80 | 12 | ? |
Complete the missing quantities.
Why can the same heating transfer be useful in one device and dissipated in another?
Check your understanding
Answer from memory, then check against the suggested answer.
What is conserved?
What does arrow width represent?
Name one transfer pathway.
A motor receives 250 J and raises a load by 175 J. Account for the rest.
Why does “energy was wasted” need further explanation?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A device receives 600 J of energy but transfers only 420 J usefully.
Calculate the energy that is not transferred usefully. Explain why it is not destroyed.
(2)Where does this energy go? Explain what “dissipated” means.
(2)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., balance the numbers and connect the remainder to real stores or pathways.