Converting Watts to Ohms: How to Calculate Resistance from Power and Temperature

[wakejosh]

Homework Statement

A 20 ohm platinum wire at 20 C with a temperature coefficient of resistivity of 3.9 x 10 ^-3 /C will have what resistance at 100 C?

I know how to answer this in watts and I get 26 W, but for some reason my only options are in ohms.

how do i convert Watts to ohms? i looked around and could not find any solid conversion unit.

thanks in advance.

 

[neutrino]

"how do i convert Watts to ohms?"

By altering the laws of physics, perhaps?

 

[wakejosh]

thats what I thought.

is there a way to solve this problem and get the answer in ohm?

 

[HallsofIvy]

First go back and review! You can't "answer this is Watts"! I don't know what formulas you are using but the problem asks for "resistance" and the Watt is NOT a measure of resistance. The ohm is the measure of resistance. Asking how to convert Watts to Ohms is like asking how to convert degrees celsius to meters! You can't they don't measure the same things!

You say the wire has a resistivity of "3.9 x 10 ^-3 /C". Shouldn't there be some unit above that "/C"? What is it?

 

[marcusl]

Whoa, you're off in the wrong direction here. Watts are a units of power and have nothing to do with this problem.

What is the meaning of the temp co of resistivity? (Look in your book...)

 

[berkeman]

You should be careful with your units, and use the units to help guide your work on a solution. You should have written the question like this:

Resistance at 20 [C] is 20 [Ohms]

Resistivity coefficient = 3.9*10^-3 [Ohms/C]

Delta Temp = 100 - 80 [C] = ?

Now do you see how to solve it?

 

[wakejosh]

here is the exact problem:

A 20 ohm platinum wire at 20°C with a temperature coefficient of resistivity of 3.9 x 10^-3 /°C will have what resistance at 100°C?

the multiple choice options given are:

14 W
20 W
26 W
28 W


so do you think my professor messed up and meant to put the answers in ohm? actually, looking at my work, I figure it should be 26 ohm, maybe he just messed up on his units? or am I missing something here?

 

[berkeman]

That's a font problem. If you convert a W in most fonts into "symbol", you get the Omega symbol for Ohms. It's a simple typo in the question.

 

[wakejosh]

yea, looking at my work I was solving in ohm the whole time and I get 26 ohm, i just didnt pay attention and I assumed I was solving it in watts because that was the only option i had, It seemed funny to me at the time becuase I figured ohms would be much better since it is a measurement of resistance, but I figured my professor knew what he wanted. bleh :P