AQA · GCSE · Combined Science · Higher

Comparing currents in parallel branches

Q06.2 2 marks

Figure 10 shows the circuit with two identical filament lamps.

Compare the currents I $_1$ , I $_2$ and I $_3$

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Marking points

1 State that the current in the main branch equals the sum of the currents in the two parallel branches: $I_1=I_2+I_3$ .
2 State that the two branch currents are equal because the lamps are identical: $I_2=I_3$ .
3 State an equivalent comparison such as $I_1=2I_2$ or $I_1=2I_3$ .

Full-mark answer

The identical parallel branches carry equal currents, so $I_2=I_3$ . The main current is the sum of the branch currents, so $I_1=I_2+I_3=2I_2=2I_3$ .

Why this answer loses marks

$I_1=I_2=I_3$

The branch currents are equal to each other, but the main current is their sum, so it is larger.