Use your calculator to get
Taking the reciprocal and rounding to 1 significant figure, we get
R (^) eq = 4 Ω.
So we can redraw our circuit like this:
Next, we calculate the equivalent resistance of the entire circuit. Following our rule for resistors in
series, we have
R (^) eq = 4 Ω + 5 Ω = 9 Ω.
We can now fill this value into the V-I-R chart.
Notice that we now have two of the three values in the “Total” row. Using Ohm’s law, we can calculate
the third. That’s the beauty of the V-I-R chart: Ohm’s law is valid whenever two of the three entries in a
row are known .
Then we need to put on our thinking caps. We know that all the current that flows through our circuit
will also flow through R 1 (You may want to take a look back at the original drawing of our circuit to
make sure you understand why this is so). Therefore, the I value in the “R 1 ” row will be the same as the
I in the “Total” row. We now have two of the three values in the “R 1 ” row, so we can solve for the third
using Ohm’s law.