OCR Physics A G485 - Frontiers of Physics - 18th June 2015

Can anyone explain the whole arcsecond thing and how they could question us on it? Makes absolutely no sense to me

Reply 1102

Could anyone write me some principles of capacitors in series and in parallel, please? I don't fully understand how the charge/p.d. changes on each one in both cases. Capacitors are by far my weakest topic. June 11 Q2 is a perfect example of where I'm completely stumped. Part (b) and part c.ii-iii really confuse me.

Reply 1103

fnatic NateDestiel

18

http://www.ocr.org.uk/Images/63477-mark-scheme-unit-g485-fields-particles-and-frontiers-of-physics-june.pdf

Anyone explain 6aii for me

ty

Reply 1104

leroythelost

10

Can someone please explain the concept behind the movement of electrons in circuits with capictors

Reply 1105

jamesbird18

4

Original post by leroythelost

Can someone please explain the concept behind the movement of electrons in circuits with capictors

Electrons move from negative to positive. Hence they move from one plate to the other. An equal amount of electrons will move from one side to the other is current is constant.

Reply 1106

L'Evil Fish

OP

18

Original post by Elcor

Could anyone write me some principles of capacitors in series and in parallel, please? I don't fully understand how the charge/p.d. changes on each one in both cases. Capacitors are by far my weakest topic. June 11 Q2 is a perfect example of where I'm completely stumped. Part (b) and part c.ii-iii really confuse me.

Q = CV
C prop 1/V because charge is same on both in series

So bigger capacitance, voltage is less

Think of it as opposite to resistors

Rest is just plugging in formulae
____

c) ii) voltage I'd 6 because it's charged to that, and then it's just exponential decay

Just CV² ratio over both

(edited 8 years ago)

Reply 1107

sarcastic-sal

Original post by fnatic NateDestiel

http://www.ocr.org.uk/Images/63477-mark-scheme-unit-g485-fields-particles-and-frontiers-of-physics-june.pdf

Anyone explain 6aii for me

ty

Basically you use the exponential relationship equation and the value of λ calculated in (ai)

(edited 8 years ago)

Reply 1108

randlemcmurphy

11

Original post by Elcor

Could anyone write me some principles of capacitors in series and in parallel, please? I don't fully understand how the charge/p.d. changes on each one in both cases. Capacitors are by far my weakest topic. June 11 Q2 is a perfect example of where I'm completely stumped. Part (b) and part c.ii-iii really confuse me.

I remember getting stumped in part b.

I think of it this way:
The capacitors are in series so the charge across them both is the same.
So we can say: V₁₅₀C₁₅₀=V₄₅₀C₄₅₀
The voltage is shared between the two based upon there capacitance.
However we know that the total voltage is 6.0V. So 6=V₁₅₀+V₄₅₀
We can rearrange this to get: V₄₅₀=6-V₁₅₀
You can now sub this into the equation above and solve for V₁₅₀

Reply 1109

Original post by L'Evil Fish

Q = CV
C prop 1/V because charge is same on both in series

So bigger capacitance, voltage is less

Think of it as opposite to resistors

Rest is just plugging in formulae
____

c) ii) voltage I'd 6 because it's charged to that, and then it's just exponential decay

Just CV² ratio over both

Thanks, that makes sense.

Sorry I meant part iii-iv, why would the charge being the same on both capacitors make the ratio constant? I thought energy=1/2QV and since the time constant=CR and their capacitance is different, they have different time constants and hence the p.d. across either one would decay at different rates?

Reply 1110

kate8

Original post by leroythelost

Can someone please explain the concept behind the movement of electrons in circuits with capictors

Electrons move around the circuit from the negative terminal onto the first plate. As negative charges gather on this plate they repel an equal number of negative charges from the other plate. This leaves one plate positively charged and the other negative as on the other plate as the elections flow around to the positive terminal of the cell. Electons stop flowing when the p.d of the capacitance is equal to the p.d. of the soucre

Reply 1111

sarcastic-sal

Totally forgotten that we needed to know about smoke detectors... :colondollar:

Reply 1112

Original post by randlemcmurphy

I remember getting stumped in part b.

I think of it this way:
The capacitors are in series so the charge across them both is the same.
So we can say: V₁₅₀C₁₅₀=V₄₅₀C₄₅₀
The voltage is shared between the two based upon there capacitance.
However we know that the total voltage is 6.0V. So 6=V₁₅₀+V₄₅₀
We can rearrange this to get: V₄₅₀=6-V₁₅₀
You can now sub this into the equation above and solve for V₁₅₀

Thank you!

Bloody hell, I really hope the only capacitor question is question one and it's piss easy, like just bunging into the formulae and its really obvious whether they're in series or parallel.

Reply 1113

I'll give the examiner a blowjob if there's a massive MRI question

Reply 1114

sarcastic-sal

Original post by Elcor

I'll give the examiner a blowjob if there's a massive MRI question

Came up two exams in a row though (June 12 and Jan 13) so probs not gonna make an appearance. You never know though.

Reply 1115

L'Evil Fish

OP

18

Original post by Elcor

Thanks, that makes sense.

Sorry I meant part iii-iv, why would the charge being the same on both capacitors make the ratio constant? I thought energy=1/2QV and since the time constant=CR and their capacitance is different, they have different time constants and hence the p.d. across either one would decay at different rates?

Both the voltages would decay at the same rate, because it's the total capacitance of a circuit that dictates the rate of it

So it's just their capacitance that leads to the ratio

Reply 1116

WishingChaff