00:01
All right, so let's say we have a graph of the voltage on a capacitor being charged up over time, and it looks kind of like this in the interval that we're given, maybe a little bit more asymptotic, there we go.
00:17
And we want to know if the battery has a voltage of 8 volts, and the capacitor is 73 microphabts, what's the value of the resistor in the system? this rc circuit.
00:33
So if we notice, it takes about six seconds to charge it up to a voltage of 5 volts.
00:43
And the voltage of a charging capacitor as a function of time, we can write as v .0 times 1 minus e to the minus t over tau.
00:55
And so what we have, if we take the ratio of these two, this will be 5 volts at this interval we just measured, divided by 8 volts, which is the initial voltage.
01:04
This is equal to 1 minus e to the minus, and this is 6 seconds over tau.
01:10
Now, tau, if we find that out, that's our time constant...