Problem 2.15   Source:   Problem elaborated by the author of the site.

Calculate the resistance that the circuit offers to the V voltage source in the circuit shown in Figure 02-15.1.

Solution of the Problema 2.15

As we see in the Figure 02-15.2, initially we will calculate the equivalent resistance of the circuit that is circled in yellow. In the right branch we have two resistors in series, one of 4 ohms and another one of 16 ohms, totaling 20 ohms, because in a series circuit to find the equivalent resistance value we must add the value of all the resistors that make up the circuit.

In the other branch we have two resistors of 40 ohms in series, totaling 80 ohms . Now we get two resistors in parallel: one of 20 ohms and one of 80 ohms. Recalling the equation of the parallel of two resistors, we have:

Then, substituting for the values found and performing the calculation we arrive at:

Similarly, for the parallel circuit that is connected to the source negative and making the calculations we find an equivalent resistance of 4 ohms that added to the resistor of 10 ohms that is in series with it results a resistor of 14 ohms. So to finish, we get the circuit below:

Note that we have reduced the entire circuit to a much simpler circuit. To calculate the total resistance simply add all the values of the resistors that are in series and find the value sought. Like this: