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# RC step response - intuition

An RC circuit responds to a step of voltage with a smooth transition between the starting and ending voltage.  The RC step response is a fundamental behavior of all digital circuits.  Created by Willy McAllister.

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• In this circuit, what is the direction of the current? First, it goes from left to right starting at source voltage and it stops at Capacitor. Then, it goes in backwards such as right to left? Is the direction changing? In RC Natural Response, I think it would go backward when the switch turns off.
• Hello Yws,

There many things happening in this video.

First, Willy assumes you have seen the previous video covering the natural response of an RC circuit. Here we have a "source less" circuit which is to say there is no voltage supply. We assume an initial condition where the capacitor has magically been charged to some voltage. Starting at "time zero" the current flows out of the capacitor until all of the energy has been turned to waste heat in the resistor.

That was the smaller circuit on the right...

On the left we have a new circuit. This time it includes an external source. Also, we assume the capacitor starts out with zero voltage (more advanced analysis will start the capacitor with a non-zero voltage). In this circuit the current flows from the source to the capacitor. As it does this the resistor get hot. Later after the capacitor is fully charged the current stops flowing and the resistor cools off.

Regards,

APD