A fuse burns out if the current in a circuit is too large. This prevents the equipment
connected to the circuit from being damaged by the excess current. For example, if the
ammeter in the previous problem were replaced by a half-ampere fuse, the fuse would
blow and the circuit would be interrupted.
Fuses rarely come up on SAT II Physics. If a question involving fuses appears, it will
probably ask you whether or not the fuse in a given circuit will blow under certain
circumstances.
Kirchhoff’s Rules
Gustav Robert Kirchhoff came up with two simple rules that simplify many complicated
circuit problems. The junction rule helps us to calculate the current through resistors in
parallel and other points where a circuit breaks into several branches, and the loop rule
helps us to calculate the voltage at any point in a circuit. Let’s study Kirchhoff’s Rules in
the context of the circuit represented below:
Before we can apply Kirchhoff’s Rules, we have to draw arrows on the diagram to denote
the direction in which we will follow the current. You can draw these arrows in any
direction you please—they don’t have to denote the actual direction of the current. As
you’ll see, so long as we apply Kirchhoff’s Rules correctly, it doesn’t matter in what
directions the arrows point. Let’s draw in arrows and label the six vertices of the circuit:
We repeat, these arrows do not point in the actual direction of the current. For instance,
we have drawn the current flowing into the positive terminal and out of the negative
terminal of , contrary to how we know the current must flow.