- #1

vysero

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## Homework Statement

With a 12V battery, a 5.00µF capacitor and a 8x105Ω resistor determine the following:

a) The time constant of the circuit

b) The maximum charge on the capacitor

c) The maximum current in the circuit

d) The charge on the capacitor as a function of time, q(t)

e) The current in the circuit as a function of time, I(t)

f) The time until the charge on the capacitor is 75% of it’s maximum value

A long time later the capacitor starts fully charged. At this new t=0, starting with a fully charged capacitor, the switch is moved to position b. Determine the following:

g) The charge on the capacitor as a function of time, q(t)

h) The current in the circuit as a function of time, I(t)

i) The time for the capacitor to reach 15% of it’s maximum value

## Homework Equations

τ = RC

q = CV

V = IR

Charging:

q = CV(1-e^(-t/RC))

i = (V/R)e^(-t/RC)

V = (q/C) = V(1-e^(-t/RC))

Discharging:

q = qₒe^(-t/RC)

i = -(qₒ/RC)e^(-t/RC)

## The Attempt at a Solution

a) RC = (5x10^-6)(8x10^5) = 4 seconds

b) I am not sure what the question is asking. Obviously it wants the maximum charge but does it want that maximum when t = 0 or at some other point? I said the answer was zero but I am not sure.

c) Once again it depends on the time. Right after t = 0 all the current is on the resistor but before that at t = 0 there is no current in the circuit. So for t = 0 the answer is zero but just after zero say like .01 sec the current is all on the resistor: R = V/I = 12/(8x10^5) = 1.5x10^-5 A.

d) Same problem not sure what time I should be looking at. At t = 0 the answer is zero.

e) At t = 0 it is zero again.

f) Not sure how to do this part.

The next few questions I think will be better if left alone until I get the previous set. Any help would be appreciated, mostly I need to know if what I am assuming the questions are asking is correct. Also, any other help would be appreciated.