• First, enter all the given information into your chart. If resistors haven’t already been given names (like
“R 1 ”), you should name them for easy reference.
• Next simplify the circuit to calculate R (^) eq , if possible.
• Once you have two values in a row, you can calculate the third using Ohm’s law. You CANNOT use
Ohm’s law unless you have two of the three values in a row .
• Remember that if two resistors are in series, the current through one of them equals the current through
the other. And if two resistors are in parallel, the voltage across one equals the voltage across the
other.
Kirchoff’s Laws
Kirchoff’s laws help you solve complicated circuits. They are especially useful if your circuit contains
two batteries.
Kirchoff’s laws say:
At any junction, the current entering equals the current leaving.
- The sum of voltages around a closed loop is 0.
The first law is called the “junction rule,” and the second is called the “loop rule.” To illustrate the
junction rule, we’ll revisit the circuit from our first problem. (See Figure 19.5 .)
Figure 19.5 Circuit illustrating Kirchoff’s junction rule.According to the junction rule, whatever current enters Junction “A” must also leave Junction “A.” So
let’s say that 1.25 A enters Junction “A,” and then that current gets split between the two branches. If we
measured the current in the top branch and the current in the bottom branch, we would find that the total
current equals 1.25 A. And, in fact, when the two branches came back together at Junction “B,” we would
find that exactly 1.25 A was flowing out through Junction “B” and through the rest of the circuit.
Kirchoff’s junction rule says that charge is conserved: you don’t lose any current when the wire bends
or branches. This seems remarkably obvious, but it’s also remarkably essential to solving circuit
problems.
Kirchoff’s loop rule is a bit less self-evident, but it’s quite useful in sorting out difficult circuits.
As an example, we’ll show you how to use Kirchoff’s loop rule to find the current through all the
resistors in the circuit.