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Calculating Resistance In Circuit

  • Thread starter Andy111
  • Start date
26
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1. Homework Statement

A loop circuit has a resistance of R and a current of 1.8A. The current is reduced to 1.3A when an additional 2.7 ohm resistor is added in series with R.

What is the value of R?

2. Homework Equations

I is constant throughout a series circuit
V is shared in a series
Total resistance=the sum of the resistors in series

V=I x R

3. The Attempt at a Solution
I substituted R (in the equation V=IR) with 4.7+R because this would represent the total resistance in the series. I knew that I was 1.3A. I calculated the Voltage on the 4.7 resistor (using V=IR) and got 6V. So then I substitued V total with 6 + V.

I ended up with the following equation. 6+V=1.3(4.7+R). I then tried substituting V (of the unknown resistor) with 1.3R, but then I realized that couldn't possibly work, because the R's would cancel. Then I tried 1.8R instead so I would end up with the equation .5R=.11, R=.22 but that doesn't sound right.
 
Last edited:

Answers and Replies

458
0
You don't have to figure out V. In the first case, you have [tex]\frac{V}{R}=1.8[/tex]

And in the second case, you have [tex]\frac{V}{2.7+R}=1.3[/tex]

Two unknowns and two equations.
 
Last edited:
26
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Okay. I got an answer of 7.02 using those equations, but it was incorrect.
 
26
0
Nevermind, I see where the mistake was. I used 2.7, it's supposed to be 4.7
 

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