V is the voltage across the part of the circuit you’re looking at, I is the current flowing through that
part of the circuit, and R is the resistance in that part of the circuit. Ohm’s law is the most important
equation when it comes to circuits, so make sure you know it well.
When current flows through a resistor, electrical energy is being converted into heat energy. The rate
at which this conversion occurs is called the power dissipated by a resistor. This power can be found
with the equation
This equation says that the power, P , dissipated in part of a circuit equals the current flowing through that
part of the circuit multiplied by the voltage across that part of the circuit.
Using Ohm’s law, it can be easily shown that IV = I 2 R = V 2 /R . It’s only worth memorizing the first
form of the equation, but any one of these could be useful.
Resistors in Series and in Parallel
In a circuit, resistors can either be arranged in series with one another or parallel to one another. Before
we take a look at each type of arrangement, though, we need first to familiarize ourselves with circuit
symbols, shown in Figure 19.2 .
Figure 19.2 Common circuit symbols.First, let’s examine resistors in series. In this case, all the resistors are connected in a line, one after the
other after the other: