hi That's the question. I have done an experiment. The voltage of a free DC power supply was set to 5V. When it was connected to a circuit, its voltage dropped to 3V. What happened? Isn't the 2V is lost in the internal resistance of the power supply. However,the power supply will continuously supply 5V.So why do the voltage will drop to 3V.And, in a simple circuit,the voltage will be 0V when it comes to the end after passing through those resistance,so why the voltage will drop to 3V. Thanks everyone!