A circuit is made of 0.4 ohm wire, 150 ohm bulb and a 120ohm rheo stat connected inseries. Determine the total resistance of the
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In a RC-circuit, with the capacitor initially uncharged, when we connect the battery to the circuit the charge on the capacitor starts to increase following the law:
where t is the time,
is the maximum charge on the capacitor at voltage V, and 
is the time constant of the circuit.
Using this law, we can answer all the three questions of the problem.
1) Using
and 
, the time constant of the circuit is:
2) To find the charge on the capacitor at time
, we must find before the maximum charge on the capacitor, which is
And then, the charge at time is equal to
3) After a long time (let's say much larger than the time constant of the circuit), the capacitor will be fully charged, this means its charge will be
. We can see this also from the previous formule, by using 
:
where t is the time,
Using this law, we can answer all the three questions of the problem.
1) Using
2) To find the charge on the capacitor at time
And then, the charge at time
3) After a long time (let's say much larger than the time constant of the circuit), the capacitor will be fully charged, this means its charge will be