Help with RSGB mock exam question please

[KathParticipant]

Hello Pete,

I’m struggling with question 9 on the RSGB mock test three. It is a circuit diagram.

Both C and D look correct to me. The answer key shows that D is correct.

When I build it on DC circuit builder it shows both are the same.

Please can you explain why C is incorrect? I’m clearly missing something.

Thank you,
Kath

[petesKeymaster]

No problem, and yes, this is a messy one.

You can remove R1 and R2 as red herrings.

Essentially, you’re left with the standard “parallel” drawing from the course – this one:

https://www.essexham.co.uk/courseimg/circuit-parallel_5dee3982d6999.png

Where a power supply feeds more than one component (e.g. two lamps, or in this case, 2 resistors), the voltage is the same, and the supplied current is the sum of the currents through the individual components.

Just like the example in the course, each lamp has a direct connection, so each gets the full voltage. Half of the circuit’s total current flows through each of the two bulbs.

Correct answer is D – the current over R3 and R4 will be different (as they are different values), but the voltage across the circuit will be the same. It it was a 3 volt battery, all four resistors would have 3v going across them, but different currents.

Does that help?

Pete

[KathParticipant]

Hi Pete,
Yes, it makes perfect sense to me now.
Thank you very much!
Kath

[petesKeymaster]

Cool. Happy to help.

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