# OCR A2 Physics exam question - Capacitors, need some help.

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#1
Past paper: http://www.ocr.org.uk/Images/58049-q...of-physics.pdf

Mark scheme: http://www.ocr.org.uk/Images/63477-m...ysics-june.pdf

Q1 part d (ii)

I'm doing a capacitor question and I don't fully understand the last part of the question.

When capacitors are in parallel the charge on each capacitor is different and Q = Q1 + Q2..

but in that question, they calculated V using the charge for one of the capacitors, that is what I don't understand.

they did V = Q / total C

Thanks
Reda
0
6 years ago
#2
(Original post by Reda2)
Past paper: http://www.ocr.org.uk/Images/58049-q...of-physics.pdf

Mark scheme: http://www.ocr.org.uk/Images/63477-m...ysics-june.pdf

Q1 part d (ii)

I'm doing a capacitor question and I don't fully understand the last part of the question.

When capacitors are in parallel the charge on each capacitor is different and Q = Q1 + Q2..

but in that question, they calculated V using the charge for one of the capacitors, that is what I don't understand.

they did V = Q / total C

Thanks
Reda
have you paid enough attention to the order in which the switches are being opened and closed.

1. initially S1 and S2 are open and both capacitors are discharged
2. then S1 is closed, the 4.5μF capacitor charges up to 6.3V ( carrying 2.835x10-6 C )
3. then S1 is opened disconnecting the battery from the 4.5 μF capacitor (the 4.5 μF capacitor remains charged to 6.3V)
4. then S2 is closed - which forces the parallel 4.5μF and 1.5μF capacitor to share the 2.835x10-6 C

you're asked what the PD across the parallel capacitors at the end of step 4 is...
which is equivalent to asking what the PD across a single 6μF capacitor carrying 2.835x10-6 C is
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