P=IV, P=I^2R, P=V^2/R, what are the distinctions between these equations?

[johnwandering]

I originally thought that these were just different variable equations to the same power generation, but just learned they are not.

P=IV is for the P of the emf source
P=I^2 R is for the P of the Resistors

I was wondering if these were correct?

What does that leave P=V^2/R??
Also, is Power generated by the capacitor?

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[howlovely]

All three of those equations are equivalent -- you start with P = IV and then substitute from Ohm's Law (V = IR) to get the other two.

I'm a bit confused by what you mean by the "power of the resistors" and the "power of the EMF". Power is generated by the EMF and dissipated in the circuit through resistors. Whenever I'm asked about power, I just use whichever of the three equations I have enough information for. (Perhaps that's what you're trying to say?)

Capacitors do not generate power by themselves. Voltage flows through capacitors when they are in a circuit, and gradually charge builds on each side of the capacitor, creating a electric potential difference (difference in voltage) between each side. The voltage across a capacitor will equal the voltage of the EMF when it is fully charged. If you were then to remove the EMF from the circuit but nothing else, the capacitor would then discharge, acting as an EMF until there is no potential difference across its sides.

 

[johnwandering]

You are missing a pivotal concept, the equations are not equivalent when you consider resistors separately.

A circuit I had on an exam recently:

V=50V
I=1x10^-3
Req=unknown
R1=100 ohms

What is the power going through the resistor R1?
You cannot solve it via P=IV, which will give you the wrong answer, because that considers Req.


You have to use P=I^2R. You can't even use P=V^2/R

The answer is 1x10^-4

 

[milski]

The equations are equivalent - they all can be derived from each other using Ohm's law.

You have to use I,R and V for the appropriate resistor, not Req.

Power is just work rate over time. In the case of emf, the work is being done by the emf, in the case of a resistor, the work is being done on the resistor (and normally dissipated as heat).

 

[TipToad]

The equations are ALL equivalent.

For example:

P = IV. Well, what is V? V = IR. Therefore, P = I(IR) = I^2R.

P = IV. V = IR, so I = V/R. Therefore, P = (V/R)V = V^2/R

Which equation you use just depends on which variables you are given.

 

[deleted357625]

how do you know which equation to use depending on the question. im sorta lost
i see how the power equations are equivalent due to substitution of ohms law.

 

[Yonko Shanks]

Physics 1 in fall yay.... lol my energy was drained just doing reading the comments

 

[Belle Melodie]

Physics 1 in fall yay.... lol my energy was drained just doing reading the comments

I'm almost scared to take physics now.

 

[jHustle]

how do you know which equation to use depending on the question. im sorta lost
i see how the power equations are equivalent due to substitution of ohms law.

It depends on the question stem and what variables are given. Resistors in parallel have the same voltage, but different currents; and resistors in series have the same current but different voltages. If the question asks you about a specific resistor or current in a parallel resistor for instance, then you most likely use the P=(I^2)R, but if the question asks you about the overall power or current, then you would most likely use P=IV.

Oh yeah and

(I have no idea what I'm doing dog.jpg)