- #1

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the switch S is closed and the capacitor discharges through resistor R = 8700 Ohms. What's the initial charge of the capacitor?

I'm doing q0 = iRC. I'm stuck on finding what the current through the circuit is.

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- Thread starter eurekameh
- Start date

- #1

- 210

- 0

the switch S is closed and the capacitor discharges through resistor R = 8700 Ohms. What's the initial charge of the capacitor?

I'm doing q0 = iRC. I'm stuck on finding what the current through the circuit is.

- #2

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- #3

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What do the symbols 0- and 0+ mean?

- #4

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- #5

- 1,506

- 18

0.5 x Q x V; 0.5 x C x V^2 ; 0.5 Q^2 /C

You could use the third one directly to find the charge, Q, on the capacitor.

You can now also calculate the voltage across the capacitor, this is the voltage at t = 0.

When S is closed the voltage is connected across the 8700Ω resistor so you should be able to calculate the initial current.

The current (and voltage) decrease exponentially ( I x e^-t/T ; and V x e^-t/T )

T is the time constant of the C R combination

- #6

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- #7

- 1,506

- 18

You have probably met eV (electron volt) as a unit of energy when the charge is that of an electron (e = 1.6 x 10^-19 C)

A really useful thing to know is that Voltage means Joule per Coulomb.

1 Volt means 1 Joule of energy given to 1Coulomb of charge when it passes through 1Volt

It will help you in lots of electricity problems if you translate 'Volt' to be 'Joule per coulomb'

- #8

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Thanks. Units do indeed go a long way.

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