Why does adding a resistor in parallel decrease the total resistance?
I think I probably talked about this to everybody, but it’s an important result and it bears repeating. Essentially, adding a parallel resistor adds another path in the circuit through which current can flow. In the diagram above, current flows out of the battery and has a choice between either the branch between points A and B, or the branch between points C and D. We can use Ohm’s Law (the topic of next week’s lab) to find the amount of current through each branch. Ohm’s Law says that the current through any two points is the change in potential between the two points divided by the amount of resistance between them: I=V/R In this circuit, we can just imagine that both resistors are the same. Between points A and B, there is a potential difference defined by whatever voltage the battery is at. But there is the same change in potential between points C and D, because they too are connected directly to the terminals of a battery. Consequently, the current in both branches is the same:
