Electricity Question (from AQA AS 2018 Paper 1)

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iStudent26
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Hi,

I'm sitting the AQA Physics Paper 1 on Monday and wasn't sure if I approached question 6.1 about determining which 2 lamps are at their normal brightness in the right way & there's no mark scheme to check with. If anyone understands how to answer/ approach this question please may you explain it to me. Link: http://tutorzone.co.uk/wp-content/up...May18-AM-1.pdf

What I've thought of so far:
The branches with AB and CD must both have 9V across them as they are in parallel and the terminal p.d. is 9V.
The operating voltage (for normal brightness) of A& C is 6V and B& D is 3.5V so they can't both receive their operating voltage.
The power of A& C (at normal brightness) is 6W so they require 1A for normal brightness.
The power of B& D (at normal brightness) is 4.1W so they require 1.171...A for normal brightness.
"The lamps will all receive the same current as each 'branch' of the parallel circuit has the same resistance and voltage. A and C require less current to be at their normal brightness than B and D so A and C must be the 2 lamps at normal brightness".
But, how can I know the distribution of the voltages across each branch between the lamps without knowing their resistances? A and C may not receive their operating voltage of 6V...

I would be really grateful for any help because I'd like this to be clear in my mind ahead of Monday!! (:
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uberteknik
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(Original post by iStudent26)
Hi,

I'm sitting the AQA Physics Paper 1 on Monday and wasn't sure if I approached question 6.1 about determining which 2 lamps are at their normal brightness in the right way & there's no mark scheme to check with. If anyone understands how to answer/ approach this question please may you explain it to me. Link: http://tutorzone.co.uk/wp-content/up...May18-AM-1.pdf

What I've thought of so far:
The branches with AB and CD must both have 9V across them as they are in parallel and the terminal p.d. is 9V.
The operating voltage (for normal brightness) of A& C is 6V and B& D is 3.5V so they can't both receive their operating voltage.
The power of A& C (at normal brightness) is 6W so they require 1A for normal brightness.
The power of B& D (at normal brightness) is 4.1W so they require 1.171...A for normal brightness.
"The lamps will all receive the same current as each 'branch' of the parallel circuit has the same resistance and voltage. A and C require less current to be at their normal brightness than B and D so A and C must be the 2 lamps at normal brightness".
But, how can I know the distribution of the voltages across each branch between the lamps without knowing their resistances? A and C may not receive their operating voltage of 6V...

I would be really grateful for any help because I'd like this to be clear in my mind ahead of Monday!! (:

You are in luck. I answered this question for someone else earlier today.

https://www.thestudentroom.co.uk/sho....php?t=5404818

Alternately:

Work out the actual resistance of each lamp from the given nominal operating conditions using P = VI and ohms law. (6 and 3 ohms)

Once you have these, by inspection the series resistance paths form two potential dividers so you can then work out the p.d. developed across each lamp (6V and 3V), or, the current in each series path (1 Amp).

Then use P = V2/R or I2R to find the power developed in each.

Finally compare that in-circuit actual power with the nominal power given in the table to determine which of the lamps operate at full brightness.
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zimple
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The question states that changes in resistance are negligible.
So you can use I=P/V to find the normal current for each bulb and then use R=V/I to find each resistance.
The pd then splits in the ratio of the resistance for the series resistors.
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Pangol
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Lots of great and correct answers here, but they do seem to involve a bit more work that I would have thought necessary.

- use R = V^2/P to work out the resistance of each lamp
- use a potential divider aregument to find the pd across each lamp (you only need to consider the top or bottom branches)
- see if the pds that you find are the operating pds for the lamps. Only one is!

I don't think that there's any need to conisder currents until the part of the question that specifically asks you to find one.
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iStudent26
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(Original post by uberteknik)
You are in luck. I answered this question for someone else earlier today.

https://www.thestudentroom.co.uk/sho....php?t=5404818

Alternately:

Work out the actual resistance of each lamp from the given nominal operating conditions using P = VI and ohms law. (6 and 3 ohms)

Once you have these, by inspection the series resistance paths form two potential dividers so you can then work out the p.d. developed across each lamp (6V and 3V), or, the current in each series path (1 Amp).

Then use P = V2/R or I2R to find the power developed in each.

Finally compare that in-circuit actual power with the nominal power given in the table to determine which of the lamps operate at full brightness.
(Original post by zimple)
The question states that changes in resistance are negligible.
So you can use I=P/V to find the normal current for each bulb and then use R=V/I to find each resistance.
The pd then splits in the ratio of the resistance for the series resistors.
Thank you both for the help I didn't notice the significance in "changes in resistances are negligible"...which was very important. Lesson learnt that they don't usually add info for the sake of it! When it says lamps rated at say 12V, 24W in questions does that always mean that's when they're operating at 'normal' brightness & that their voltage and power ratings have to be satisfied for this to happen?
P.S. uberteknik I would give you rep, but I gave some to you too recently apparently...you've been very helpful this week. Thanks again!
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zimple
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(Original post by iStudent26)
Thank you both for the help I didn't notice the significance in "changes in resistances are negligible"...which was very important. Lesson learnt that they don't usually add info for the sake of it! When it says lamps rated at say 12V, 24W in questions does that always mean that's when they're operating at 'normal' brightness & that their voltage and power ratings have to be satisfied for this to happen?
P.S. uberteknik I would give you rep, but I gave some to you too recently apparently...you've been very helpful this week. Thanks again!
Yes the values given refer to the normal full brightness levels of the bulb/device.
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iStudent26
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(Original post by zimple)
Yes the values given refer to the normal full brightness levels of the bulb/device.
Thanks, I haven't been sure of what 'rated' meant for a while!
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