# Simple power equation question

Suppose I have a circuit with voltage 4V and current 1A. By $P=I~V$ the total power across the circuit is 4W.

I have a $20~\Omega$ resistor and want to calculate the power loss over the resistor. By $I^2 R$ that is 20W.

This doesn't make sense to me. If the total power across the circuit is 4W, how can I lose 20W of power over the resistor. Where is my understanding wrong here?

• The current flowing through the resistor would be 0.2 A(V / R = I ), so the power loss across it would be 0.8 W. "Suppose I have a circuit with voltage 4V and current 1A" are you describing a power supply with this statement. May be you should provide a schematic. – karthik Jay Aug 30 '17 at 4:07
• A $20\:\Omega$ resistor would require $20\:\textrm{V}$ to cause $1\:\textrm{A}$ through it. So your voltage is wrong. You don't get to pick parts from circuit 1 and different parts from circuit 2 and ask "why?" – jonk Aug 30 '17 at 4:53
• @karthikJay yes I'm describing the output of my power supply – user Aug 30 '17 at 5:09
• What do you mean "you are describing the output of your power supply"? Are you describing what is written on a PS label, or do you have a more sophisticated PS with DMM displays, or are you measuring the output with a Voltmeter and a separate Ampermeter? – Ale..chenski Aug 30 '17 at 5:52
• If your power supply can deliver UP TO 1 Amp at 4 volts, this does not mean that it WILL deliver 1 Amp. The current drawn from the power supply will be determined by the load resistance, provided that the supply can produce the required current. A power supply does not force its advertised current through the load. – Peter Bennett Aug 30 '17 at 6:18