Halp! RC-circuits equation

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Teh Russians
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Halp! RC-circuits equation

Postby Teh Russians » Sun Mar 22, 2009 7:10 pm UTC

OK here it is:

For the circuit shown in the figure above, switch S has been open for a long time. At time t = 0 the switch is then closed. Let R = 1.20 MΩ, V = 42.0 V, and C = 2.00 µF.
(a) What is the battery current just after switch S is closed?

(b) What is the battery current a long time after switch S is closed?

(c) The switch has been closed for a long time. At time t = 0 the switch is then opened. Find the current in µA through the 600 kΩ resistor as a function of time. (Use t as necessary.)

I got a and b, they were easy, but I have no idea about this function. I don't think it's (V0e^(-t/RC))/R because there's another resistor there...
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Re: Halp! RC-circuits equation

Postby MotorToad » Sun Mar 22, 2009 11:08 pm UTC

Hrm, smells like homework... Suffice to say to say that the capacitor has zero resistance for AC and infinite resistance for DC. Anything more than that is me doing your homework. :-\
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Re: Halp! RC-circuits equation

Postby Grav » Mon Mar 23, 2009 2:47 am UTC

Find the thevenin equivilent circuit of the two resistors, then plug in the capacitor in series with that circuit. Use KVL to find the transient response for voltage across the capacitor. Once you have the equation for the voltage across the capacitor (it will be a function of time), then just use ohm's law on the original circuit, realizing that the resistor you're analysing is in parallel with the voltage you just solved for. Done.
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Re: Halp! RC-circuits equation

Postby IHOPancake » Mon Mar 23, 2009 9:45 pm UTC

Current will only flow if there is a complete circuit. Once the switch is opened, the only complete circuit that can be made is the one involving only the capacitor and the 600k resistor. You can use the standard RC equation for part C.
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