# Power P=IV

• B
There are three Formulas for Power, ( P=IV, P=I^2R, P=V^2/R). Can someone tell me when to use the right forumlas?

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ProfuselyQuarky
Gold Member
How has your textbook explained it?

Gajan1234
How has your textbook explained it?
It is quite confusing in the book as they have used different formula for different question. But I know that P=I^2R is used to find the heat

Dale
Mentor
What are your thoughts? What is different about the three equations and when might those differences be important?

ProfuselyQuarky
Gold Member
It is quite confusing in the book as they have used different formula for different question. But I know that P=I^2R is used to find the heat
Okay, well, do you know what each variable stands for?

What are your thoughts? What is different about the three equations and when might those differences be important?
I think that P=IV is mainly used to find the power of the whole circuit whereas P=I^2R is used to find how much energy is given out by the component per second.
But not sure if I am right.

Okay, well, do you know what each variable stands for?

I for current
R resistance

I understand the fact that it is simple substitution and the formulas are literally same. But when it come to practice questions it is used differently in different context

Dale
Mentor
I think that P=IV is mainly used to find the power of the whole circuit whereas P=I^2R is used to find how much energy is given out by the component per second.
But not sure if I am right.
OK, that is a good start.

What is different about P=I^2 R that makes it better for individual components than P=IV? Or what do components have that might lead you to use P=I^2R.

OK, that is a good start.

What is different about P=I^2 R that makes it better for individual components than P=IV?
Well we are looking at difficulty of current going through due to resistivity.

Dale
Mentor
Well we are looking at difficulty of current going through due to resistivity.
Right. So would it be possible to use P=I^2 R if you don't know the resistance?

ProfuselyQuarky
Gold Member
But when it come to practice questions it is used differently in different context
Not really It just depends on what you're looking for--and the only way to know what you're looking for would be to know what each part means. Perhaps this picture will help: click here

Gajan1234
Right. So would it be possible to use P=I^2 R if you don't know the resistance?
Well it is possible if you know the voltage

ProfuselyQuarky
ProfuselyQuarky
Gold Member
Well it is possible if you know the voltage
That's correct! See, you can use different equations (in this case, Ohm's Law) to find the missing parts you need. So where's the trouble?

Gajan1234
Dale
Mentor
Well it is possible if you know the voltage
Yes, exactly. There are only two formulas here: P=IV and V=IR. You have two equations in four variables. You can always substitute one equation into the other to eliminate any one variable.

That is all those equations do. You are then left with one equation in three variables. Use the one that fits.

If you look ag question 3c you can not use all the formulas. Only one will give you the right answer.
Yes, exactly. There are only two formulas here: P=IV and V=IR. You have two equations in four variables. You can always substitute one equation into the other to eliminate any one variable.

That is all those ewuayions do. You are then left with one equation in three variables. Use the one that fits.

Yes, exactly. There are only two formulas here: P=IV and V=IR. You have two equations in four variables. You can always substitute one equation into the other to eliminate any one variable.

That is all those ewuayions do. You are then left with one equation in three variables. Use the one that fits.

Dale
Mentor
If you look ag question 3c you can not use all the formulas. Only one will give you the right answer.
What are the knowns and what are the unknowns? Which formula fits?

ProfuselyQuarky
ProfuselyQuarky
Gold Member
What are the knowns and what are the unknowns?
And based on that, which formula for power matches what you know?

And based on that, which formula for power matches what you know?
P=v2/r

P=v2/r
This gives me the wrong answer

ProfuselyQuarky
Gold Member
This gives me the wrong answer
What numbers did you use?

What are the knowns and what are the unknowns? Which formula fits?
Well I can use the P=v^2/R formula but it gives me the wrong answer

What numbers did you use?
P=6^2/9

ProfuselyQuarky
Gold Member
P=6^2/9
Why did you use "6" for volts? The picture's not that clear, but doesn't it read "3"?

Gajan1234
My suggestion is to use P = IV. Even though you get the same results this equation is more general. For instance for an inductor you can still use P=IV but not the other ones.

Dale
russ_watters
Mentor
There are three Formulas for Power, ( P=IV, P=I^2R, P=V^2/R). Can someone tell me when to use the right forumlas?
When? Always, always use the right formulas!

In addition to what others have said about the variables provided in the problem, it also matters what the problem is asking you to calculate the power of. If it asks you for the power used by a light bulb, it may be different than if they asked you to find the power loss by the wires going to the light bulb.

Gajan1234