An RC circuit isa circuit with both a resistor (R) and also a caprincetoneclub.orgcitor (C). RC circuits are freqent element in electronic devices. They likewise play an important role in the infection of electric signals in nerve cells.

A caprincetoneclub.orgcitor have the right to store energy and a resistor placed in collection with it will regulate the price at which that charges or discharges. This produces a properties time dependence that turns out to be exponential. The vital princetoneclub.orgrameter that describes the time dependence is the "time constant" R C . The far-sighted student can guess this simply by observing that R C has the size of time: (1 Ohm) x (1 Farad) = (1 second).

You are watching: In the steady state, what is the charge q of the capacitor?

We will certainly confine our researches to the adhering to circuit, in i m sorry theswitch deserve to be moved in between positions a and b.


Let us start by reviewing part facts around caprincetoneclub.orgcitors: The charge on a caprincetoneclub.orgcitor cannot adjust instantaneously. The existing is given by ns = DQ / Dt. For this reason the change in charge DQ = i Dt goes come zero as the time interval Dt goes come zero. The present flowing into a caprincetoneclub.orgcitor in the stable state that is got to after a long time interval is zero. due to the fact that charge builds up ~ above a caprincetoneclub.orgcitor quite than flowing through it, charge can construct up till the point that the voltage V=Q/C balances out the exterior voltage pushing fee onto the caprincetoneclub.orgcitor.

When a caprincetoneclub.orgcitor of caprincetoneclub.orgcitance Cis in series with a battery that voltage Vband a resistor that resistance R, thevoltage drops have to be:


which is a statement that thevoltage acquired going across the battery need to equal the voltagedrop across the caprincetoneclub.orgcitor to add the voltage drop throughout theresistor. One equation wherein the rate of adjust of a quantitity (DQ/Dt)is proportional to the amount (DQ) will constantly have anexponential solution. We consider two instances:

Discharging the caprincetoneclub.orgcitor: The caprincetoneclub.orgcitor originally is linked (switch in position a) because that a long time, and also is then disconnected by moving the switch to b in ~ time t = 0. The caprincetoneclub.orgcitor climate discharges, leaving the caprincetoneclub.orgcitor without fee or voltage after a long time.

Charging the caprincetoneclub.orgcitor: The move is in position b for a lengthy time, enabling the caprincetoneclub.orgcitor to have actually no charge. In ~ time t = 0, the move is adjusted to a and also the caprincetoneclub.orgcitor charges.

Discharging Charging

Here, Q0,V0and I0 describe the charge, voltage and also current of thecaprincetoneclub.orgcitor in the prompt after the switch is thrown. The time t is the characteristictime that the decay, t= RC. When confronted with an RCproblem, the best strategy is the following:

Decide what the charge across the caprincetoneclub.orgcitor to be just prior to the switch to be thrown. Due to the fact that the charge have the right to not adjust instantly, this is the charge just after the move is thrown.

Decide what the fee is long after the switch is thrown.

Pick the exponential kind for the fee Q(t) to meet the exactly initial and final charges.

The existing through a caprincetoneclub.orgcitor must constantly decay and also end up in ~ zero. The initial current can generally be ascertained through the help of Ohm"s law, V=RI.

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The properties time t =RC speak you the the charging/discharging isslower v a bigger resistor or caprincetoneclub.orgcitor. This makes sense,because a larger resistor impedes the flow of current; thusslowing the charging/discharging, and a larger caprincetoneclub.orgcitor holdsmore charge; for this reason requiring more time to charge.

Every educated person should have a great feeling for exponential functions. (Sketches of the charge Q(t)for charging and also discharging caprincetoneclub.orgcitors could be presented here.)