# capacitors mcq help

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Thread starter 1 month ago
#1
Hi
So I made a mistake somewhere in my workings which I have typed up below:
E=0.5CV^2 [1]
E=0.5C(V-2)^2 [2]

400*10^-6=V^2-4V+4
V^-4V+3.9996=0
V1 give 2.02V
V2 gives 1.98

To solve for capacitance:
2E/V^2 but when I sub in both numbers into equ 1&2 i don't get a correct answer.

Can someone point out where I went wrong and what should I do next
Last edited by Sclaremonths; 1 month ago
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1 month ago
#2
Where you did 400×10^-6=V^2-4V +4. You got rid of the 0.5C. So you're setting energy equal to voltage².
You could do 1600×10^-6 = 0.5CV²
and 400×10^-6 =0.5C(V-2)². See what you can do from here
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Thread starter 1 month ago
#3
(Original post by tej3141)
Where you did 400×10^-6=V^2-4V +4. You got rid of the 0.5C. So you're setting energy equal to voltage².
You could do 1600×10^-6 = 0.5CV²
and 400×10^-6 =0.5C(V-2)². See what you can do from here
i thought since 0.5c is a constant we would get rid of it and since this is a ratio we couldn't apply simultaneous equations to this?
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1 month ago
#4
(Original post by Sclaremonths)
i thought since 0.5c is a constant we would get rid of it and since this is a ratio we couldn't apply simultaneous equations to this?
You cant just "get rid" of a constant. You would have divide both sides side 0.5C. Which wouldn't get you anywhere. You have 2 equations with 2 unknowns so it is best to some them simultaneously
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