Gajan1234
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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?
Understanding Power Formulas; When to use Them
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ProfuselyQuarky
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How has your textbook explained it?
Gajan1234
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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 heatProfuselyQuarky said:
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What are your thoughts? What is different about the three equations and when might those differences be important?
ProfuselyQuarky
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Okay, well, do you know what each variable stands for?Gajan1234 said:
Gajan1234
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Dale said:
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.
Gajan1234
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I for currentProfuselyQuarky said:
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
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OK, that is a good start.Gajan1234 said:
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.
Gajan1234
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Dale said:
Well we are looking at difficulty of current going through due to resistivity.
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Right. So would it be possible to use P=I^2 R if you don't know the resistance?Gajan1234 said:
ProfuselyQuarky
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Not reallyGajan1234 said:
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
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Dale said:
Well it is possible if you know the voltage
ProfuselyQuarky
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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 said:
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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 anyone variable.Gajan1234 said:
That is all those equations do. You are then left with one equation in three variables. Use the one that fits.
Gajan1234
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If you look ag question 3c you can not use all the formulas. Only one will give you the right answer.
Dale said:
Dale said:
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What are the knowns and what are the unknowns? Which formula fits?Gajan1234 said:
ProfuselyQuarky
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And based on that, which formula for power matches what you know?Dale said:
Gajan1234
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P=v`2/rProfuselyQuarky said:
Gajan1234
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This gives me the wrong answerGajan1234 said:
ProfuselyQuarky
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What numbers did you use?Gajan1234 said:
Gajan1234
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Well I can use the P=v^2/R formula but it gives me the wrong answerDale said:
Gajan1234
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P=6^2/9ProfuselyQuarky said:
ProfuselyQuarky
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Why did you use "6" for volts? The picture's not that clear, but doesn't it read "3"?Gajan1234 said:
Guneykan Ozgul
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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.
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When? Always, always use the right formulas!Gajan1234 said:
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.