 Now we look at power in the terms of electrical circuits So power is a rate of energy transfer and we could write the equation out in words as power equals energy over time Now power is often given the symbol of capital P A lot of textbooks use a script P, but that's hard for me to write by hand. So I just use a regular capital P in terms of units Energy is always measured in joules time is measured in seconds And so my unit of joule per second gives me my unit for power, which is a watt capital W Now this is true for power no matter what context it's in whether it's mechanical power or electrical power When we talk about electrical power The particular type of energy that we're using has to do with charges and Electric potential energy using our potential difference So substituting that in I get that my power is equal to Q times delta V which is my voltage over delta T and Rearranging this just a little bit. I could recognize it in terms of the amount of charge in a specific amount of time And that amount of charge in a specific amount of time that flows through an electrical circuit is also known as the current So substituting that in we get sort of our standard equation here that power is the current times the voltage So again power is my P current is my I and Delta V is my voltage So this is our standard equation for power in an electrical circuit Now in terms of units well power is supposed to be in watts Current is measured in amps and voltage is measured in volts So that means a watt must be equal to an amp volt Now just to get this back into our familiar terms we can remember that an amp is a Coulomb per second And we can also remember that a volt was originally defined as a Jewel per Coulomb So those Coulomb's can cancel out and that leaves us with our jewel per second, which is our familiar unit for watts So now some alternate equations So I said that P equals I delta V or current times voltage was our standard equation But if we remember from Ohm's law that voltage can be related to the current in the resistance And plugging that in I would have that my power is equal to the current squared times the resistance So if you know your current in your resistance, you can use that instead of your current in voltage Similarly my current can be defined as a voltage per resistant and Plugging that in I could see that my power is also equal to the voltage squared over the resistance So these three are all acceptable Representations for the power in an electrical circuit