# Determine an unknown resistor with a known resistor to get a heat rate value

Discussion in 'Homework Help' started by foleystefan, Oct 4, 2010.

1. ### foleystefan Thread Starter New Member

Oct 4, 2010
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0
It's hard for me to really understand the question I was given, so I'm sorry if it is hard to understand the topic line.

The question is:

What resistance must be connected in series with a 100 Ohm resistor for the 100 Ohm resistor to heat at a rate of 30W when the combination is connected to a 120V source?

It's for an assignment, I'd appreciate any help with this with as much detail in the answer as possible. I tried looking throughout my textbook through everything we covered this far and didn't get an answer that made sense. I think the wording is throwing me more than anything. Thanks for anything you can contribute.

Stefan

2. ### Georacer Moderator

Nov 25, 2009
5,160
1,273
Don't let the word heat throw you off. All you want to do is have a circuit with a source of 120V and two resistors in series, one uknown and one of 100Ω. You also want the 100Ω resistor to consume 30W.

Start from what you know. $Power=I^2 \cdot R$. From this equation you can find the current required to produce the needed power.
You can also find the voltage on the 100Ω resistor. $Power=\frac{V_1^2}{R}$.

So, you know that a current I flows through the uknown resistor and a voltage V1 is applied over the known resistor.

Can you find the rest? Make a schematic, it will help.

3. ### foleystefan Thread Starter New Member

Oct 4, 2010
11
0
To be quite honest, I'm still lost. I'm getting 1.2A as the current over the 100 Ohm resistor, when I use it in the P=I squared X R I get 144W? I really feel like I don't know what I'm doing. I was kinda looking for an explained answer so I can learn how to do this correctly for the test as well. Thanks all the same for the rest though, at least I was using the correct equations.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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As Georacer said, you should draw the circuit - it's far less difficult to resolve.

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5. ### JoeJester AAC Fanatic!

Apr 26, 2005
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This is why we ask you to show your work. You know the power and you know the resistance. Transpose the Power formula to find the current. How did you get 1.2 Amps as the current?

Have you studies Kirchoff's Laws?

Below is the outline I would use to solve the problem. Granted it goes further than the assignment, but it allows for cross checking.

• ###### foleystefan_101004_Homework.pdf
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Last edited: Oct 5, 2010
6. ### foleystefan Thread Starter New Member

Oct 4, 2010
11
0
Thanks for all the help, but I still don't understand what I'm supposed to do. The assignment is due in two hours so I won't get it done now.

I guess I should have said that I feel stupid when doing these problems with circuits, it's the only class I have where I don't really understand what's going on. My teacher just writes and writes and doesn't explain all that well so I'm practically lost in the course. On top of that, there are no tutors or in school help for that subject, so I have to try and figure this out on my own.

Anyways, that's my problem and I shouldn't bring it here. Again, thanks for all the help.

Stefan

7. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,909
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In that case, I recommend you start with Volume 1 of the ebooks at the top of the page and work your way through them. This semester isn't over for a bit and you can catch up to where you should be ok.

What book are you using as a text book?

8. ### Georacer Moderator

Nov 25, 2009
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I suggest you finish this exercise anyway, just for practice. Refer to t_n_k's diagram.

If you know that P=I*R, and you know P and R you can calculate I.
If you know I and you know R you can calculate $V_R$.
If you know $V_R$ and $V_s$ the you can calculate $V_x$.
If you know $V_x$ and I you can calculate $R_x$.

Can you follow this line of thoughts?