A trade-off answer compares options on measurable quantities, states the assumptions the comparison depends on, and gives a conclusion that begins with “if”. It should read as physics reasoning, not as a campaign.
This lesson completes the route through power and energy resources, building on availability versus reliability.
How do I build the answer?
- Name the demand: how much energy or power is needed, in what units and for how long?
- List each option’s numbers: what does each one supply, use or require? (area, fuel, output)
- State assumptions that the numbers rest on, including any given in the question.
- Compare and conclude conditionally: “If assumption A holds, option X needs less … ; if not, …”
- Name one thing that would change the conclusion.
Worked example
A small island needs 1200 kWh of electrical energy each day.
Option A is a solar farm. Option B is a diesel generator. (Invented example data and assumptions, for practice only.)
Assumptions stated: panels deliver an average of 0.8 kWh per square metre per day. The generator delivers 3 kWh of electrical energy per litre of diesel. Burning one litre releases 2.7 kg of carbon dioxide.
Option A, area needed: 1200 ÷ 0.8 = 1500 m².
Option B, fuel needed: 1200 ÷ 3 = 400 litres per day.
Option B, carbon dioxide released: 400 × 2.7 = 1080 kg per day.
Sensitivity check: if cloud lowers the panel yield to 0.6 kWh per m², the area rises to 1200 ÷ 0.6 = 2000 m².
A conditional conclusion: “If 1500 m² of land is available and the panels deliver 0.8 kWh per m² per day, solar meets the demand without fuel. Diesel needs 400 litres a day and releases about 1080 kg of carbon dioxide. However, solar only supplies in daylight, so without storage it does not meet evening demand. The generator can run whenever it is switched on. The better choice depends on land, storage and fuel delivery.”
Every number in that paragraph came from a stated assumption, so the reasoning can be checked.
The mistake to watch for
A common slip is an unsupported verdict.
Mistaken answer: “Solar is the right option because it is clean and diesel is bad.”
There are no numbers, no assumptions and a value judgement in place of a comparison.
The correction is to replace “right” and “bad” with quantities: the area required, the fuel used, the carbon dioxide released, and when each can supply. Then add the condition under which your conclusion would reverse.
Check yourself
1. Using the island example, how much panel area is needed if the yield is 0.75 kWh per m² per day?
Show answer
1200 ÷ 0.75 = 1600 m².
2. How many litres of diesel would the generator need for 900 kWh, at 3 kWh per litre?
Show answer
900 ÷ 3 = 300 litres.
3. Rewrite this into a physics statement: “Wind power is unreliable so we should not use it.”
Show answer
One model answer: “If output follows wind speed, it may fall below demand at times, so without storage or a second source the wind farm would not always meet the demand.” It states the condition and the measurable issue, and gives no instruction.
Where this leads next
Try the power and energy resources practice set, which includes a short trade-off question. You can also go back to module overview to plan revision.
Long written answers are where a teacher helps most, by checking your wording and assumptions in online one-to-one Physics tuition.