Brightness of lamps

Hi there

I would like some help with further understanding this question.

I understand the marking scheme.
But my thought process was that because there are three lamps connected in parallel on circuit 2 compared to two on circuit 1 (realising all parallel components will have the same voltage), respectively, the circuit will be brighter - regardless of a minor change in current from changing the total resistance of the circuits.

Am I making sense?

Help would be appreciated.


Hi @mekanz1

Great question. Could you please clarify for me which circuit you thought would be brighter?

This is a brilliant opportunity to give each bulb a resistance value and determine the Power of each circuit. This can be a really useful tool in your exam.

Let us assume each bulb has a resistance of 1 ohm:

Note: If you haven’t seen the P=V^2/R equation before, it comes from combining V=IR and P=IV. This also gives another useful equation, P=I^2R

If you would like to test your understanding, have a go at describing what happens to the brightness of the left most bulb (let’s all it bulb A), and the brightness of the parallel bulbs (B and C) when the extra bulb is added. Equations may prove useful.

Hi Doug
I meant to say ‘circuit 2 will be brighter’

Thanks for your example. I think you’re right - it’s easier to understand when using example values to calculate the scenario.

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Nice one, I think I now perhaps understand your question a little better than I orginally had. You’re quite right that circuit 2 will be brighter, and that all parallel components will have the same voltage, although this voltage changes as you add or remove components. The supply voltage is constant.

To earn credit for this question, you would need to clearly link the prediction that circuit 2 will be brighter with a supporting reason. This could be that by adding the extra bulb the total resistance has decreased, or that the current has increased (or ideally both!). Just adding a bulb to a circuit doesn’t necessarily make that circuit brighter. A good example of this would be adding a bulb in series, which would increase the total resistance and decrease the current, decreasing brightness.

You seem to have a decent grasp of the concepts - why not try out some of these circuits questions. You’ll notice the question about how adding or removing a component changes a value in the circuit is a reasonably common one. Let me know if you need a hand with any other questions.

Thanks Mr Walker. I’ll definitely give these Qs a go.
Thank you for your prompt responses.

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