Power transformed when resistance is zero and infinite

[Janiceleong26]

1. Homework Statement

Homework Equations

P=IV,
=I²R
=V²/R

The Attempt at a Solution

For zero resistance, I used P=V²/R formula, and sub. R=0 , power would be infinite. But if I sub. Into P=I²R, power will be zero. The correct answer should be zero. But why do we need to use the second relevant equation I typed above?
For infinite resistance, I used P=V²/R and so will result in power tending to zero.
I am confused on when to we use which equations to calculate power

 

[mfb]

For zero resistance, I used P=V²/R formula, and sub. R=0 , power would be infinite.

Both R and V go to zero, so you have "0/0" which is undefined. You cannot use this formula.

 

[Janiceleong26]

Both R and V go to zero, so you have "0/0" which is undefined. You cannot use this formula.

Oh I see.. Thanks

 

[Janiceleong26]

Both R and V go to zero, so you have "0/0" which is undefined. You cannot use this formula.

But if resistance is infinite, then shouldn't voltage be infinite too?

 

[mfb]

No. Why do you expect this?

 

[Janiceleong26]

No. Why do you expect this?

Because V=IR, V α R, though I feel that it doesn't make sense to have an infinite voltage..

 

[CWatters]

Remember we are talking about the voltage drop across Q.

 

[mfb]

Because V=IR, V α R, though I feel that it doesn't make sense to have an infinite voltage..

V α R is only true at fixed current.
What is the current you expect for infinite resistance?
And, as cross-check, can the voltage at the resistor exceed the source voltage?

 

[Janiceleong26]

V α R is only true at fixed current.
What is the current you expect for infinite resistance?
And, as cross-check, can the voltage at the resistor exceed the source voltage?

Zero current
Oh yeah.. Never thought of that too, thanks!