How Is Percentage Uncertainty Calculated in Power Dissipation?

[moimoi24]

Homework Statement

In a simple electrical circuit, the current in a resistor is measured as 2.5 ± 0.05 mA. The resistor is marked as having a value of 4.7 Ω ± 2%. If these values were used to calculate the power dissipated in the resistor, what would be the percentage uncertainty in the value obtained?

Homework Equations

none

The Attempt at a Solution

please help me, what is the solution for this one? thanks

i'm sorry, i post this in the advanced physics too, I'm confused where to post this.. hope you can help me...

 

[PeterO]

Homework Statement

In a simple electrical circuit, the current in a resistor is measured as 2.5 ± 0.05 mA. The resistor is marked as having a value of 4.7 Ω ± 2%. If these values were used to calculate the power dissipated in the resistor, what would be the percentage uncertainty in the value obtained?

Homework Equations

none

The Attempt at a Solution

please help me, what is the solution for this one? thanks

i'm sorry, i post this in the advanced physics too, I'm confused where to post this.. hope you can help me...

I think that when multiplying two quantities, you merely add the percentage errors??

In this case you are multiplying 3 things I x I x R as in P = I²R; so you would add all three percentage errors.
You are given the percentage error in R, and can calculate the percentage error in I from the values given.

Peter

 

[moimoi24]

Can you show me the solution? I'm confused... thanks peter...

 

[PeterO]

Can you show me the solution? I'm confused... thanks peter...

If you wish to calculate the area of a rectangle with length 5.0 ±0.1 cm by 10.0 ± 0.1 cm, and give a percentage uncertainty, then

A = l x w so Area = 50 cm²

Now the percentage error.
5.0 ± 0.1 means an uncertainty of 0.1 in 5 or 1 in 50 or 2%
10.0 ± 0.1 mans an uncertainty of 1 in 100 or 1%

SO the uncertainty in the answer is 3% [add them together]

So Area is 50 cm² ± 3%

That is as close to the solution you seek I will give,