In circuit analysis problems it is convenient to use the short-circuit c rent and open-circuit voltage to specify Thévenin and Norton circuits. The circuits in Figure 3-38(a) show that when the load is an open circuit the interface voltage equals the Thévenin voltage; that is P since there is no voltage across RT when P. Similarly, the circuits in Figure 3-38(b) show that when the load is a short circuit the interface current equals the Norton current; that is P since all the source current P is diverted through the short-circuit load.