Because Voc is affected by the load resistance RL. Consider an extreme case where RL is so low as to be zero Ohms; in other words, a straight piece of wire. In such a case, the the junction of R1 and R2 will be pulled down to zero volts because the straight piece of wire is connected to the zero volts of the battery. Increase the resistance of RL from a straight piece of wire and the voltage will appear. Taking the other extreme, the only time you can expect the output voltage to be Voc when a load is connected is when that load resistance, RL is very much grater than R2.
Basically, the reason why you can't use Voc is because the calculated value of the LHS assumes R2, but the right hand side replaces R2 with the parallel combination of R2 in parallel with RL, and that will always be slightly less than RL and hence the voltage drop across that parallel combination will also be less.
· 7 years ago