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Capacitance question

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For part 3aii) Why cant i use Q=IT then sub this Q into E=1/2QV?

Thanks
Original post by Zenarthra
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For part 3aii) Why cant i use Q=IT then sub this Q into E=1/2QV?

Thanks


Because current is defined as the rate of change of charge I =dQ/dt

Q = IT is therefore saying

Q = (dq/dt)*T

Q = Q
Reply 2
Original post by uberteknik
Because current is defined as the rate of change of charge I =dQ/dt

Q = IT is therefore saying

Q = (dq/dt)*T

Q = Q


But using Q=It cant i find the amount of charge supplied in 10 hours and hence it must be the charge stored and i can find the Energy stored using E=1/2QV
Original post by Zenarthra
But using Q=It cant i find the amount of charge supplied in 10 hours and hence it must be the charge stored and i can find the Energy stored using E=1/2QV


For both battery and capacitor:

Since voltage is defined as Joules/Coulomb, V=E/Q hence E = QV...(i)

also I is defined as the rate of change of charge I=Q/t hence Q=It.......(iii)

Substituting:

Energy = QV = VIt

However:

The voltage across a capacitor is an exponential decay function wrt time as the charge is depleted.

The voltage across the battery remains almost constant as its charge is depleted.

In both cases the total energy supplied is a function of both current and voltage and by inspection alone, the voltage is NOT the same for both cases as a function of time.
(edited 9 years ago)
Original post by Zenarthra
But using Q=It cant i find the amount of charge supplied in 10 hours and hence it must be the charge stored and i can find the Energy stored using E=1/2QV
Have you understood the fundamental difference for the voltage and currents involved in each case?

a) In the case of the battery, the current and voltage (Joules per Coulomb of charge) remains fairly constant throughout the duration of the discharge.

b) Whereas for the capacitor, both the current and voltage follow an exponential decay throughout the duration of the discharge.

Q=It applies in both cases, but the current I is not the same for both cases and therefore the energy equation is not the same either.


Please confirm you understand the difference.
Reply 5
Original post by uberteknik
Have you understood the fundamental difference for the voltage and currents involved in each case?

a) In the case of the battery, the current and voltage (Joules per Coulomb of charge) remains fairly constant throughout the duration of the discharge.

b) Whereas for the capacitor, both the current and voltage follow an exponential decay throughout the duration of the discharge.

Q=It applies in both cases, but the current I is not the same for both cases and therefore the energy equation is not the same either.


Please confirm you understand the difference.


Ahh, yes i understand now.

THANK YOU!! :biggrin:
Reply 6
Original post by uberteknik
Have you understood the fundamental difference for the voltage and currents involved in each case?

a) In the case of the battery, the current and voltage (Joules per Coulomb of charge) remains fairly constant throughout the duration of the discharge.

b) Whereas for the capacitor, both the current and voltage follow an exponential decay throughout the duration of the discharge.

Q=It applies in both cases, but the current I is not the same for both cases and therefore the energy equation is not the same either.


Please confirm you understand the difference.


Would this be a correct illustration for what you said?
tt.png
Original post by Zenarthra
Would this be a correct illustration for what you said?
tt.png
Exactly. :smile:
Original post by Zenarthra
heee.png

For part 3aii) Why cant i use Q=IT then sub this Q into E=1/2QV?

Thanks


Q=It only holds for a constant current as dQ=I.dt

The current into a capacitor is governed by and exponent so the relation would involve integration.

You can use C=Q/V to find Q though.
Reply 9
Original post by uberteknik
Exactly. :smile:


Thanks :biggrin:!!
Reply 10
Original post by Zenarthra
Thanks :biggrin:!!


Heehee hello :smile:
What would be the answer to bii?
When they said ,for cell ,current is a constant, I should use q=IT isnt it?? Why incorrect ?

Posted from TSR Mobile
Reply 11
Original post by Lamalam
Heehee hello :smile:
What would be the answer to bii?
When they said ,for cell ,current is a constant, I should use q=IT isnt it?? Why incorrect ?

Posted from TSR Mobile


hehe ni ho, you can use Q=It and then substitute into E=QV but not E=1/2QV. :smile:
Reply 12
Original post by Zenarthra
hehe ni ho, you can use Q=It and then substitute into E=QV but not E=1/2QV. :smile:


oh E=QV comes from E=IVt ! thanks!! :smile:

gayau on A level :smile:
Reply 13
Original post by Lamalam
oh E=QV comes from E=IVt ! thanks!! :smile:

gayau on A level :smile:


No problem. :smile:

die ga ga yau!! :biggrin:

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