Circuits
Most SAT II Physics questions on circuits will show you a circuit diagram and ask you
questions about the current, resistance, or voltage at different points in the circuit. These
circuits will usually consist of a power source and one or more resistors arranged in
parallel or in series. You will occasionally encounter other circuit elements, such as a
voltmeter, an ammeter, a fuse, or a capacitor. Reading the diagrams is not difficult, but
since there will be a number of questions on the test that rely on diagrams, it’s important
that you master this skill. Here’s a very simple circuit diagram:
Zigzags represent resistors, and a pair of parallel, unequal lines represents a battery cell.
The longer line is the positive terminal and the shorter line is the negative terminal. That
means the current flows from the longer line around the circuit to the shorter line. In the
diagram above, the current flows counterclockwise. Often, more than one set of unequal
parallel lines are arranged together; this just signifies a number of battery cells arranged
in series.
Example
In the diagram above, = 6 V and R = 12. What is the current in the circuit and what is
the power dissipated in the resistor?You don’t really need to refer to the diagram in order to solve this problem. As long as
you know that there’s a circuit with a six-volt battery and a 12 -ohm resistor, you need
only apply Ohm’s Law and the formula for power.
Since I = V/R, the current is:
The power is:
Resistors in Series
Two resistors are in series when they are arranged one after another on the circuit, as in
the diagram below. The same amount of current flows first through one resistor and then
the other, since the current does not change over the length of a circuit.