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# Physics capacitors homework help!!! watch

1. Alright, here's the question:

A 2.2 microfarads capacitor is charged to a pd of 6.0 V and then discharged through a 100 kiloohm resistor. Calculate:
a) The charge and energy stored in this capacitor at 6.0 V
I worked the charge out to be 1.32 x 10^-4 C which is correct.
The energy is 3.96 x 10^-4 J which is also correct

b) the pd across the capacitor 0.5s after the discharge started,
Um I used V = V0 e^(-t/RC) because I used it for the question before this and I got the right answer but some reason I get 4.78 V and the answer at the back of the book is 0.62 V??? What have I done wrong?

4) An uncharged 4.7 microfarads capacitor is charged to a pd of 12.0 V through a 200 ohm resistor and then discharged through a 220 kilo Ohm resistor. Calculate:
- The time taken for the pd to fall from 12.0 V to 3.0 V

Thanks for any feedback and help!
2. For part B you've just plugged in the numbers wrong

V = 6* e^-((0.5)/(100*10^3 * 2.2*10^-6))

Part 4: set V to 3 and rearrange for t. Remember to use the right discharge resistor.

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