Homework Help: Uncertainty formula in resistor

1. Aug 5, 2010

jinx007

I am having a lot pf problem to attempt this question, please help me, i am stuck..!!!!!!

The power loss P in a resistor is calculated using the formula p=v^2/R

The uncertainty in the potential difference V is 3% and the uncertainty in the resistance R is 2%. What is the Uncertainty in P?

A/ 4%

B/ 7%

C/ 8%

D/ 11%

Hmmm My attempt:

Fractional error = in V, 0.09 and in R 0.2 i subtracted and i got 0.11 that is 11% ANSWER D

And i can be at 98% sure that the answer is not correct as uncertainty cannot be substrate d ..??? please help me to attempt the question !!!!!!!!

2. Aug 5, 2010

arkofnoah

the fractional error in V is 6% (3% x 2), not 9%. if it's a square, the fractional error doubles; if it is a cube, the fractional error triples; and so on...

the fractional error in R is 2%.

ADD them up. If I remember correctly you never subtract any fractional error. it just add them all.

The answer should be C, 8%.