Analysing circuits with two different power supplies can be tricky. There are a few approaches one can take. In this video, I show firstly how Kirchhoff’s Laws can be used with simultaneous equations to brute-force answers, and how to take a slightly more elegant approach by using the principle of superposition.
Superposition only works with linear components, such as fixed resistors. If your power supply has internal resistance, just represent that as another fixed resistor outside the supply.
The basic idea is to replace each power supply with a wire (or short circuit each supply in turn), analyse the circuit, then combine the effect of each power supply by adding together the currents.
In the example in the video I calculate the current through the 50Ω resistor, but I could have determined the current through any of the resistors. The advantage of the 50Ω resistor is that the currents were in the same direction, so they could simply be summed. When the currents are in opposite directions, the sum must take into account direction by having backwards flowing currents as negative.
In the video, I said that you couldn’t use superposition when there are diodes. I realised later on that this was untrue, and recorded a follow-up video here:
Incidentally, current is a scalar, so it doesn’t actually have a direction, but it is more convenient to talk about the current ‘direction’ than to talk about the direction of the effective average drift velocity of positive charges, or the direction opposite the average drift velocity of electrons.
We all know what we mean.
Physics with Keith 