During a Lab we had a simple purely resistive circuit hooked up to a 10 VDC power supply with an adjustable voltage output. In order to determine the power dissipated by the circuit we were asked to determine the source voltage while the circuit was DISCONNECTED... we were not told why... In the Lab write up we are asked to explain clearly why we had to do this... The only thing I can think of is that with the circuit disconnected no current would be running through the secondary side of the step down transformer in the power supply which would give you the unloaded voltage output....when the circuit is connected current would start flowing throught he secondary and maybe the CEMF affects the apparent voltage output? I have no idea.... Can someone please explain why this gives a more accurate reading for power dissipated?