How is the Time Constant Calculated in an RC Circuit?

AI Thread Summary
In an RC circuit, the time constant (τ) is calculated using the formula τ = R*C, where R represents the resistance affecting the capacitor's charging or discharging. For the current through a specific resistor like R3, only the resistance directly in series with the capacitor is considered, meaning R3 alone is used if no additional resistors are in series. If an extra resistor, R4, is added in series with R3, then the total resistance for the time constant becomes R3 + R4. The resistors R1 and R2 do not influence the current through the capacitor if they are not in the same loop. Understanding these relationships is crucial for accurately determining current as a function of time in RC circuits.
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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=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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Do R1 and R2 have any effect on the current through C?
 
technician said:
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?
 
technician said:
delete

huh? :O
 

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