RC Circuit Lab

 This video shows a series RC circuit, with a 47,000 microfarad capacitor (a large capacitor!) connected to a 10-ohm resistor and a power supply. The capacitor is hooked up to an electronic voltmeter, which can be read out in real time. We can watch the time-dependent behavior of the voltage of the capacitor, which is the same behavior (exponential rise and decay) as the charge being stored by the capacitor. 

Note that as the voltage of the capacitor rises, and the total voltage of the power supply stays constant, then the voltage across the resistor must be decreasing; this means the current in the circuit is decreasing. This is Kirchhoff's voltage rule, V = Vres + Vcap, at all times. And if Rres decreases, then the current must be decreasing because of Ohm's law, I = V/R. 

We first see what happens with a 10-ohm resistor, and then a 200-ohm resistance. Note how much longer it takes the capacitor to charge...this is due to the time constant of the circuit, RC. We can control how quickly or slowly the circuit takes to charge and discharge, which is a key piece of knowledge to have for modern electronics (since everything is basically time dependent). Check it out! 

To see the mathematical derivations we will do in class, check out this one for charging a capacitor in a series circuit; or this one for a discharging capacitor. There is also one for how to think about it if the resistor and capacitor are in parallel.