How is the Time Constant Calculated in an RC Circuit?

[theBEAST]

Homework Statement

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=I_oe^t/RC where I_o 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?

 

[technician]

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

 

[theBEAST]

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

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?

 

[theBEAST]

delete

huh? :O