# Capacitors

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#1
If I use the equations Q=CV and I=deltaQ/delta t, and sub the numbers in, I get 9.78 x 10^-4, which they're saying is wrong. What am I missing - do I need to consider the zero resistance, or the AC voltage source? If so, how?
Last edited by user342; 1 year ago
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1 year ago
#2
(Original post by user342)
https://isaacphysics.org/questions/o...ting_capacitor
If I use the equations Q=CV and I=deltaQ/delta t, and sub the numbers in, I get 9.78 x 10^-4, which they're saying is wrong. What am I missing - do I need to consider the zero resistance, or the AC voltage source? If so, how?
can I see your full workings
cause I can't picture what you've done (so I can't see where you have gone wrong)
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#3
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1 year ago
#4
(Original post by user342)
differentiate both sides with respect to t (as seen in hint 3)
then sub in the numbers
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#5

So like this?
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1 year ago
#6
yep nice job
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#7
Was about to reply and say it wasn't giving me the right answer but realised my calculator was in degrees not radians🙃 thank you 😁
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1 year ago
#8
(Original post by user342)
https://isaacphysics.org/questions/o...ting_capacitor
If I use the equations Q=CV and I=deltaQ/delta t, and sub the numbers in, I get 9.78 x 10^-4, which they're saying is wrong. What am I missing - do I need to consider the zero resistance, or the AC voltage source? If so, how?
You might be trying to revise something that's not on the syllabus if you're doing A levels.
capacitors in AC circuits are something that goes hand in hand with calculus (and complex numbers)

here's the way to work out the instantaneous current as a function of time for a capacitor in an AC circuit... the guy is assuming you know how to do a bit of calculus on a trig function otherwise it's not going to make a lot of sense... but you could still use the formula he writes at 2:23 even if you don't understand it

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#9
(Original post by Joinedup)
You might be trying to revise something that's not on the syllabus if you're doing A levels.
capacitors in AC circuits are something that goes hand in hand with calculus (and complex numbers)

here's the way to work out the instantaneous current as a function of time for a capacitor in an AC circuit... the guy is assuming you know how to do a bit of calculus on a trig function otherwise it's not going to make a lot of sense... but you could still use the formula he writes at 2:23 even if you don't understand it

How would resistance in a capacitor in AC be defined then? Is there resistance, and if so how does it work?
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1 year ago
#10
(Original post by user342)
How would resistance in a capacitor in AC be defined then? Is there resistance, and if so how does it work?
Well in this case we talk about the reactance Xc which is 1/ (C omega)

You can use it a bit like resistance but you also need to be careful because although you've got AC voltage and AC current which looks like you could multiply together to get AC power, the phase relationship between current and voltage means there's no power being converted in the capacitor and it doesn't get heated like a resistor would.
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#11
(Original post by Joinedup)
Well in this case we talk about the reactance Xc which is 1/ (C omega)

You can use it a bit like resistance but you also need to be careful because although you've got AC voltage and AC current which looks like you could multiply together to get AC power, the phase relationship between current and voltage means there's no power being converted in the capacitor and it doesn't get heated like a resistor would.
So would you get power in a dc circuit?
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1 year ago
#12
(Original post by user342)
So would you get power in a dc circuit?
Depends exactly what the set up is tbh.

When you initially charge up a capacitor off a battery then energy is transferring from the battery to the capacitor in an amount of time... So that's power.

If you leave a capacitor connected to a battery forever then no current flows (after the initial charge up) .. And no current means no power.

Pulsating DC is still a type of DC but it'd have some similarities to AC since its always changing over time.
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