A good explanation answer has three short parts: a claim (what you conclude), evidence (specific numbers from the data) and reasoning (the physics idea that links them). Together they fit in three or four sentences. This is the closing skill in physics investigations and explanations.
How do you build the three parts?
- Claim: answer the question directly in one sentence, using the quantities from the question.
- Evidence: quote two or three values with units, or a ratio, gradient or difference you calculated.
- Reasoning: name the physics idea (a law, a particle model, an energy transfer) and say how it produces the pattern.
- Limit (if asked or useful): say the range tested or one limitation, so the claim is not stretched.
Worked example
Invented data. A student measures the resistance R of the same wire at different lengths L.
| L / cm | 20 | 40 | 60 | 80 |
|---|---|---|---|---|
| R / Ω | 2.1 | 4.0 | 6.1 | 7.9 |
Question: Explain how the resistance of the wire depends on its length.
Claim: The resistance of the wire is directly proportional to its length.
Evidence: When the length doubles from 20 cm to 40 cm, the resistance rises from 2.1 Ω to 4.0 Ω. At 80 cm, which is four times 20 cm, the resistance is 7.9 Ω, about 3.8 times 2.1 Ω. R ÷ L is about 0.10 Ω/cm for every reading (0.105, 0.100, 0.102 and 0.099).
Reasoning: In a longer wire of the same material and thickness, the electrons collide with more ions as they drift along the wire. This gives proportionally more opposition to the current.
Limit: The tests cover 20 cm to 80 cm only, and the small differences in R ÷ L are consistent with measurement scatter.
Checking the numbers: 2.1 ÷ 20 = 0.105, 4.0 ÷ 40 = 0.100, 6.1 ÷ 60 = 0.1017 and 7.9 ÷ 80 = 0.0988. All round to about 0.10 Ω/cm.
The mistake to watch for
A student writes: “Resistance increases as length increases because the resistance gets bigger.”
Weak answer: claim only, with circular reasoning and no numbers.
“Resistance gets bigger” repeats the claim. It is not evidence or a reason.
The correction is to add a quoted pair of values and a mechanism. For example: “Resistance rises from 2.1 Ω to 7.9 Ω as length rises from 20 cm to 80 cm, because electrons collide with more ions in a longer wire.”
Check yourself
1. Invented data. Cups of hot water with 0, 1, 2 and 3 layers of insulation fall by 28, 19, 14 and 11 °C in ten minutes. Write a claim with evidence.
Show answer
Claim: More layers of insulation reduce the temperature fall, but each extra layer helps less. Evidence: The fall is 28 °C with no layers and 19 °C with one layer, a reduction of 9 °C. From two layers to three layers it only drops from 14 °C to 11 °C, a reduction of 3 °C. (Reductions: 9, 5 and 3 °C.)
2. “The insulation traps air, which is a poor conductor.” Which of the three parts is this, and what is missing?
Show answer
This is reasoning only. The claim (insulation reduces cooling) and the evidence (quoted temperature falls) are missing.
3. A student writes: “This proves that all wires have resistance proportional to length.” Rewrite it more carefully.
Show answer
“The data support the claim that resistance is directly proportional to length for this wire over 20 cm to 80 cm.” The rewrite limits the claim to the wire and range that were tested.
Where this leads next
Bring the five skills together in the physics investigations practice set. If the evidence step feels shaky, revisit interpreting the gradient with units and systematic and random limitations. The bounds and rounding explainer helps you decide how many figures to quote.
Explanations improve fastest when someone reads them back to you line by line. That is the kind of feedback our teachers give in online one-to-one Physics tuition.