# Homework Help: Calculating the time constant?

1. Mar 28, 2012

### theBEAST

1. The problem statement, all variables and given/known data
I attached a picture of the RC circuit. The question asks to find the current as a function of time for light bulb 3 (R3). I know that I=Ioet/RC where Io is the initial current that runs through R3 but what about the R in the time constant RC? Is this resistance the total resistance of the circuit or is it just the resistance in R3?

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2. Mar 28, 2012

### technician

Do R1 and R2 have any effect on the current through C?

3. Mar 28, 2012

### theBEAST

Hmmm no, I think I kind of get it now, so the capacitor only affects anything on the outer loop. So the resistance I would use is just R3 right?

(PS I forgot to mention the the switch is closed)

Now let's say instead of a switch we had an extra resistor R4. Would that mean the R in t=RC would be R3+R4 since the capacitor is affected by both resistors that are in series?

4. Mar 28, 2012

huh? :O