To compare two diagrams, choose the same features for both and put the results side by side. The comparison is the difference you state, not the two descriptions.
This lesson is part of SPM subjects and learning resources. It builds on tracing a circuit on paper.
What does a comparison grid look like?
Choose three or four features, then complete one cell for each diagram.
| Feature | Diagram A | Diagram B | Difference |
|---|---|---|---|
| Current paths | |||
| Voltage across a lamp | |||
| Brightness | |||
| If one lamp fails |
The last column turns two descriptions into a comparison.
Worked example: two lamps in series and in parallel
Two original diagrams, described so you can sketch them. In Diagram A, a 6 V battery, a switch and two identical lamps are in a single loop. In Diagram B, the same battery and switch feed two identical lamps, each on its own branch.
| Feature | Diagram A (series) | Diagram B (parallel) | Difference |
|---|---|---|---|
| Current paths | One loop | Two branches | B gives the current a choice of paths |
| Voltage across each lamp | About 3 V each, shared | 6 V each | B gives each lamp the full voltage |
| Brightness | Dimmer | Brighter | Follows from the voltage |
| If one lamp fails | Both go out | The other stays on | A has one path, B has two |
Written answer. In Diagram A, the lamps share the 6 V, so each receives about 3 V and is dimmer. In Diagram B, each lamp is connected across the full 6 V, so it is brighter. If one lamp fails, the loop in A is broken and both lamps go out, whereas in B the other branch still forms a complete path, so that lamp stays on.
Each sentence mentions both diagrams, which is what makes it a comparison.
The mistake that costs marks
The common slip is to describe A fully and then B fully, and never say how they differ.
| Weak | Stronger |
|---|---|
| A has one loop. B has two branches. | A has one loop, whereas B has two branches, so B keeps working if one lamp fails |
| Lamps in B are bright | Lamps in B are brighter than in A, because each receives the full 6 V |
The fix is to use “whereas” or “in contrast” inside the sentence and to finish with the reason.
Keeping the comparison fair
Compare only when the conditions match: the same battery, identical lamps and the same number of parts. If the diagrams differ in other ways, say so, or the comparison is not fair.
Stating the conditions in one line, such as “assuming identical lamps and an ideal battery”, shows careful reasoning.
Check yourself
Using the diagrams above, compare the total current drawn from the battery in A and B, assuming each lamp has a resistance of 6 Ω and the battery is ideal.
Answer
Diagram A: total resistance = 6 + 6 = 12 Ω, so current = 6 ÷ 12 = 0.5 A.
Diagram B: each branch has current 6 ÷ 6 = 1 A, so the total is 1 + 1 = 2 A.
The parallel circuit draws four times the current of the series circuit. This is because each lamp receives the full 6 V instead of sharing it.
What to study next
Test the method on the original mixed practice. For the reasoning behind each part, see explaining the role of labelled components.
For a teacher to go through comparison answers with you, see online one-to-one Electrical and Electronic Engineering Studies tuition.