The actual computation of the open-circuit voltage is best illustrated by examples;
there is no substitute for practice in becoming familiar with these computations.
To summarize the main points in the computation of open-circuit voltages,
consider the circuit of Figure 3.33, shown again in Figure 3.46 for convenience. Recall
that the equivalent resistance of this circuit was given by RT = R3 +R1 R2.
To compute vOC, we disconnect the load, as shown in Figure 3.47, and immediately
observe that no current flows through R3, since there is no closed circuit
connection at that branch. Therefore, vOC must be equal to the voltage across R2,
as illustrated in Figure 3.48. Since the only closed circuit is the mesh consisting of
vS, R1, and R2, the answer we are seeking may be obtained by means of a simple
voltage divider: