AQA Physics Unit 4 - Calculating Capacitance

Hi, my physics teacher gave us a booklet of questions on chapter 6 (capacitors) and one of the questions was:
`A thunder cloud and the Earth beneath it can be considered to form a parallel plate capactior. The area of the cloud is 8.0 Km^2 and it is 75km above the earth. Calculate the energy stored if the pd is 200 kV."
I have the mark scheme so I know what the answer is, but it used the equation C= (ε₀A)/d.
I have never seen this equation before and it is possible the booklet contained questions from an old specification - does anyone know if this is the case? Or should I have been able to figure it out from equations in the current spec?
Also, there was a question on capactiors in parallel - I am pretty sure this question was the old spec, but I just wanted to make sure?
Thanks :smile:

Reply 1

natninja

21

Original post by Emma09

Hi, my physics teacher gave us a booklet of questions on chapter 6 (capacitors) and one of the questions was:
`A thunder cloud and the Earth beneath it can be considered to form a parallel plate capactior. The area of the cloud is 8.0 Km^2 and it is 75km above the earth. Calculate the energy stored if the pd is 200 kV."
I have the mark scheme so I know what the answer is, but it used the equation C= (ε₀A)/d.
I have never seen this equation before and it is possible the booklet contained questions from an old specification - does anyone know if this is the case? Or should I have been able to figure it out from equations in the current spec?
Also, there was a question on capactiors in parallel - I am pretty sure this question was the old spec, but I just wanted to make sure?
Thanks :smile:

To derive that formula you should be familiar with surface integrals... though the surface integral of a constant scalar is just kA.

but you have to integrate the field elements across the area and then find the field, then you have to get the potential.

for a conducting plate you find that the field is the charge density/epsilon0

=q/Aepsilon

then you get the potential being the integral of that with respect to the seperation and then finally use C=Q/V. learn that result though, it may come up (shouldn't though - as you said, it isn't on the formula sheet and you can't derive it with A-level knowledge)

You DO need to know about capacitors in series and parallel

series: 1/Ct=1/C1+1/C2.....+1/Cn

parallel: Ct=C1+C2....+Cn

sorted :smile:

Reply 2

Emma09

OP

Thank you!!
Hopefully it won't come up then, but at least I know how to deal with it if it does.
It's weird I managed not to come across capacitors in series and parallel, but at least I know now, thanks :smile:

AQA Physics Unit 4 - Calculating Capacitance

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