I cannot connect a diagram to an explanation

A good explanation points at the diagram in every sentence. It names a component or a path, says what happens there, and gives the physics reason. General rules that could go with any diagram score less than statements tied to this one.

The reasoning board lets you practise choosing a relationship from a diagram, and the bounds and rounding explainer checks how precise your numbers should be.

Why does the link break?

Students often revise by learning rules: “current is the same in series”, “p.d. is the same across parallel branches”. Rules are the right raw material, but the exam question shows a specific diagram and asks about it.

Between the rule and the diagram is one step that is easy to skip: tracing. Follow the path of the current through the picture, then say what you see.

A three-part pattern

1. Claim: state the result the question asks about.
2. Evidence from the diagram: point to the part of the picture that matters. Name the branch, lamp, switch or meter.
3. Reasoning: give the physics rule that connects the evidence to the claim, in your own words.

Worked example: two lamps

A 6.0 V cell is connected to two identical lamps, A and B, in parallel. Each lamp has resistance 12 Ω.

An ammeter is in the wire between the cell and the point where the branches split. Assume the cell and ammeter have negligible resistance.

Question: what does the ammeter read, and what happens to lamp A and the ammeter reading if lamp B is removed?

Step 1, current in each branch. Each branch is connected directly across the 6.0 V cell. Current = V ÷ R = 6.0 ÷ 12 = 0.50 A in each branch.

Step 2, ammeter reading. It is in the main wire, so it carries the current of both branches: 0.50 + 0.50 = 1.0 A.

Step 3, remove lamp B. Lamp A’s branch is still directly across the cell, so it still has 6.0 V across it and still carries 0.50 A. The ammeter now reads 0.50 A.

Check: total resistance of two 12 Ω resistors in parallel is 6.0 Ω, and 6.0 ÷ 6.0 = 1.0 A. With one lamp, 6.0 ÷ 12 = 0.50 A. Both agree.

Written explanation: “Lamp A stays lit at the same brightness. Its branch is connected directly across the cell, so the potential difference across it is still 6.0 V and the current through it is still 0.50 A. Removing lamp B only removes the second path, so the ammeter reading halves to 0.50 A.”

Every sentence points at the diagram: lamp A’s branch, the cell, the ammeter.

The mistake to watch for

A student writes:

Lamp A stays lit because it is in parallel.

The word “parallel” is correct, but it is a label, not a reason. The answer never says what parallel means for this lamp, nor what happens to the ammeter. It would earn part of the marks at most.

The fix is to finish the sentence “because it is in parallel, so…” with the consequence in the diagram: a direct connection across the cell, the same p.d., the same current.

Check yourself

1. The same cell and two 12 Ω lamps are now in series with an ammeter. Find the current, and say what happens to lamp A if lamp B is removed.

2. In the parallel circuit, lamp B is replaced with a 6.0 Ω lamp. Find the current through each lamp and the ammeter reading.

3. Where should a voltmeter be placed to measure the potential difference across lamp A, and why?

Where this leads next

Practise the meter skill in explaining meter placement on a diagram and the branch reasoning in analysing current in series and parallel branches. For writing the answer itself, see developing a concise claim-evidence-reasoning answer. The potential difference, resistance and circuits module collects the topic.

If your explanations still read like rules instead of answers, a teacher can rebuild them with you in online one-to-one Physics tuition.