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# Need help with A2 physics unit 4 multiple choice exam paper questions

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1. Hi,

Wondering if someone could explain to me & show the working required to get the correct answer to the following questions (correct is circled in red), from the AQA PHY4A 2010 & 2012 past papers

Questions: 4), 24), 14), 16) (see attachments)

Attachment 552674552676Attachment 552674552676552700
Attached Images

2. Bump, I would be very grateful if someone could help me out ASAP if they could, ask the exam is tommorrow, thanks.

Not sure if this is in the right section of the forum, but not sure how to move threads?
3. (Original post by JohnnyDavidson)
Bump, I would be very grateful if someone could help me out ASAP if they could, ask the exam is tommorrow, thanks.

Not sure if this is in the right section of the forum, but not sure how to move threads?
For the first one, the circumference of the earth is 1.5 x10^8 x 1000 x 2pi, and the time the earth takes to travel this distance is 365 x 24 x 60 x60. Then we're just using V = s/t to find the speed.
The second one, in order to induce an EMF in the secondary coil, there must be a change in the magnetic flux of the primary coil. The magnitude of this induced EMF is equal to the rate of this change, so when the switch is turned off suddenly the change is more rapid than when the current is building up initially, meaning the magnitude of the induced EMF is greater.
The last question, they're essentially asking you how kobg it takes for the voltage of the capacitor to drop to 90% of its original value. They tell you the initial current and voltage, so you can find the resistance and therefore
T = RCln0.9
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4. (Original post by pineneedles)
For the first one, the circumference of the earth is 1.5 x10^8 x 1000 x 2pi, and the time the earth takes to travel this distance is 365 x 24 x 60 x60. Then we're just using V = s/t to find the speed.
The second one, in order to induce an EMF in the secondary coil, there must be a change in the magnetic flux of the primary coil. The magnitude of this induced EMF is equal to the rate of this change, so when the switch is turned off suddenly the change is more rapid than when the current is building up initially, meaning the magnitude of the induced EMF is greater.
The last question, they're essentially asking you how kobg it takes for the voltage of the capacitor to drop to 90% of its original value. They tell you the initial current and voltage, so you can find the resistance and therefore
T = RCln0.9
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Hi, many thanks for explaining that, but I just have a few questions on your explanations for them.

24) (the second one) - but surely if the emf induced is maxed at this value, then induced current will be at its smallest value (from P = IV)?

14) also would you be able to explain this one, it's on the third page with 16) (I think you missed this one out)

Thanks
5. (Original post by JohnnyDavidson)
Hi, many thanks for explaining that, but I just have a few questions on your explanations for them.

24) (the second one) - but surely if the emf induced is maxed at this value, then induced current will be at its smallest value (from P = IV)?

14) also would you be able to explain this one, it's on the third page with 16) (I think you missed this one out)

Thanks
I don't think we assume that power is conserved in this situation, it would be V = IR, so a greater emf produces a greater current.
Yeah, I should have said, I'm not sure how to answer that one, so I'm sorry about that. I'm going to link you to a couple of threads for this exam, you could post it there and you will get more help.

http://www.thestudentroom.co.uk/show....php?t=4030039

http://www.thestudentroom.co.uk/show....php?t=4046575

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