When to use Power = I2R vs. V2/R

[JJR22]

I feel like they contradict each other. One says power is proportional to resistance, while the other says power is inversely proportional to resistance.

Can someone explain? When should I use them?

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[MedPR]

It's really just equation rearrangement.
V=IR, I=V/R
P=IV, P=(V/R)*V = V^2/R
P=IV, P=I*(IR) = I^2R

Which one you use will vary by what variables you are given.

 

[JJR22]

It's really just equation rearrangement.
V=IR, I=V/R
P=IV, P=(V/R)*V = V^2/R
P=IV, P=I*(IR) = I^2R

Which one you use will vary by what variables you are given.

Thanks, I am aware of how they are derived. But if a question asks you what would happen to power as resistance increased, would the power decrease or increase? one shows a linear relationship while the other shows an inverse relationship

 

[dsoz]

It depends on whether it is constant voltage or constant current. It should specify and that is what makes the difference.

 

[MedPR]

Thanks, I am aware of how they are derived. But if a question asks you what would happen to power as resistance increased, would the power decrease or increase? one shows a linear relationship while the other shows an inverse relationship

Neither shows a linear relationship.

You couldn't answer the question "what would happen to power as resistance increased" unless they tell you something about I or V. You cannot make statements about a relationship between two things when there are three unknowns involved.

 

[DNA]

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[millimpeter]

Neither shows a linear relationship.

You couldn't answer the question "what would happen to power as resistance increased" unless they tell you something about I or V. You cannot make statements about a relationship between two things when there are three unknowns involved.

So say they tell you V is constant, which would you use?

 

[t5Nitro]

I just remember it by taking the equation with the constant variable and ignore it. Then you can determine what happens to your power by changing your resistance.

 

[millimpeter]

I just remember it by taking the equation with the constant variable and ignore it. Then you can determine what happens to your power by changing your resistance.

Ah okay thanks, I wanna say "duh", but my confidence in this chapter is low...

 

[September24]

One thing helps is if resistors are in parallel or series.

If in parallel: use V^2/R

If in series: Use I^2R