Capacitance - Bulbs - time

[mspeedwagon]

I have a general question about circuits. Say you have a circuit. You have a voltage source, a switch, and a capacitor. You closed the switch and it takes 10 secs for the light bulb to go from bright to off. You flip the switch again, and you get the same result (charging/discharging the capacitor).

Now, you add another light bulb (two total) and it takes 5 seconds for the same thing to happen. You add a third and it takes 3.3 secs to go from bright to off etc.

Can you tell if this circuit is in series or in parallel? I'd think they are in parallel, but I can't articulate why.

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[Gauss44]

I have a general question about circuits. Say you have a circuit. You have a voltage source, a switch, and a capacitor. You closed the switch and it takes 10 secs for the light bulb to go from bright to off. You flip the switch again, and you get the same result (charging/discharging the capacitor).

Now, you add another light bulb (two total) and it takes 5 seconds for the same thing to happen. You add a third and it takes 3.3 secs to go from bright to off etc.

Can you tell if this circuit is in series or in parallel? I'd think they are in parallel, but I can't articulate why.

The greater the resistance from light bulbs, the less the current. The less current, the longer for them to shut off, vice versa. In parallel = less time; in series = more time.

 

[MD Odyssey]

The time constant for an RC circuit looks like this:

Where C is the capacitance and R is the net resistance of the circuit. If you put another light bulb of equal resistance in parallel with the capacitor, then the net resistance is half that of the single bulb, so the time constant gets cut in half. On the other hand, if you put it in series with the original resistor the time constant doubles. So, based upon the change in the time constant, you can determine whether or not the two bulbs were put in series or parallel.

It's a good exercise to go back and prove this to yourself instead of trying to memorize the relationship. It's relatively easy to do and might prove useful to you.

 

[regeneration]

The greater the resistance from light bulbs, the less the current. The less current, the longer for them to shut off, vice versa. In parallel = less time; in series = more time.

I think this is incorrect. Resistors in parallel would DECREASE the total effective resistance in the circuit. Thus, since I=V/R, current would INCREASE, meaning that the total time to dissipate the charge on the capacitors is LOWER. It also means that the brightness in each light bulb will be HIGHER, since P = V^2/R, and the effective R is decreasing.