# Homework Help: Power transformed when resistance is zero and infinite

1. Oct 18, 2015

### Janiceleong26

1. The problem statement, all variables and given/known data

2. Relevant equations
P=IV,
=I2R
=V2/R

3. 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

2. Oct 18, 2015

### Staff: Mentor

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

3. Oct 18, 2015

### Janiceleong26

Oh I see.. Thanks

4. Oct 18, 2015

### Janiceleong26

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

5. Oct 18, 2015

### Staff: Mentor

No. Why do you expect this?

6. Oct 18, 2015

### Janiceleong26

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

7. Oct 19, 2015

### CWatters

Remember we are talking about the voltage drop across Q.

8. Oct 19, 2015

### Staff: Mentor

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?

9. Oct 22, 2015

### Janiceleong26

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