Compute Joule Heat in Heating Coils

[BillJ3986]

Homework Statement

Two heating coils have resistances 12.0 ohms and 6.0 ohms, respectively.
a)What is the Joule heat generated in each if they are connected in parallel to a source of emf of 115V?
b) What if they are connected in series.

Homework Equations

Am I doing this question properly?

The Attempt at a Solution

Since the resistors are in parallel. In order to find R effective I use the formula 1/12.0 +1/6.0=1/4 so my R effective is 4.0 ohms. I then needed to find the current, I. So I used I=Emf/R, I=115/4.0=28.8A, I think that is a little high.I use P=I^2(R), P=28.8^2(12)=9953W to find the joule heating in resistor 1? and then I did the same for the second resistor in parallel. P=28.8^2(6.0)=4976W.
For part B I had to find the current so I did I=Emf/r+r and they are in series so I just added them up to 18 ohms for my R effective. I found I, using I=115V/18ohms= 6.39A. P=6.39^2(12)=490W, second resistor p=6.39^2(6)=245W

 

[tiny-tim]

Welcome to PF!

Hi BillJ3986! Welcome to PF!

Yes, that's all correct

but … why did you bother with finding the current in part a) ?

you know the voltage across each resistor, so just use the formula P = V²/R

 

[BillJ3986]

Thank you,

Yeah I'm looking over my notes now and I see that formula. I'm Glad I found this place, hopefully I can get my physics grade up a little higher. Thanks again for your input