Calculating Current in a Wire: Finding the I5 Current

In summary, a freshman in university is considering switching majors to physics. They are having trouble deciding if they want to stay in chemical engineering or go into physics. They have been struggling with a math problem that was introduced in their current course, and they are hoping someone can help them.
  • #1
BadAnima
5
0

Homework Statement



Find the current in the bottom-most wire (the I5 current).

nvs27r.jpg



Homework Equations



[tex]
I=\frac{\Delta V}{R}
[/tex]
Kirchhoff's current and loop rules.

The Attempt at a Solution



Using Kirchoff's current and loop rules, I obtained:

sp97w0.gif


I then substitued my results into my equation for I5 and used Kichoff's rules again to put I5 in terms of I1

2d0jrzb.gif


Then I solved for I1 and used that to solve for I5.

25gucsn.gif


Unfortunately, the answer in my book is 2.1 amps. I've worked this problem multiple times, and the only thing worse than doing all that algebra over and over again is getting a wrong answer each time. Can someone help me out here?

Also, I didn't see an intro section, so I guess I'll take care of that here. My handle is BadAnima, and I'm currently a freshman in university. My major is chemical engineering, but I'm not sure if I want to stay on this track or go into physics.
 
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  • #2
Welcome to Physics Forums.

I would go ahead and find all 5 currents. Then check for consistency in the original two KCL equations and 3 KVL loops equations. (i.e., do your values satisfy I5+I4-I1=0, etc.)

That way you can convince yourself whether the book or your answer is wrong, and go from there. Post back with what you find, I'll be online from time to time throughout today (Eastern USA time) if you need more help.

p.s. I did not find any algebra mistake, but then again there is a lot of algebra so I could be missing the same thing you are. Or the book could be wrong.
 
  • #3
BadAnima said:
I then substitued my results into my equation for I5 and used Kichoff's rules again to put I5 in terms of I1

2d0jrzb.gif

The simplification for I3 looks a bit dodgy. Better check it.
 
  • #4
Redbelly98 said:
p.s. I did not find any algebra mistake, but then again there is a lot of algebra so I could be missing the same thing you are. Or the book could be wrong.

Before I rework the problem again, I have the solution manual entry for this problem. Would it help to post that instead?
gneill said:
The simplification for I3 looks a bit dodgy. Better check it.

I don't see where I went wrong with it. I just traveled around another loop in the circuit.
 
  • #5
BadAnima said:
I don't see where I went wrong with it. I just traveled around another loop in the circuit.

Ah. My bad. I thought you'd solved the equation above it for I3. I guess I expected all the loop equations to be written together, not have another pop up partway through.

If it's any consolation, I solved for I5 using mesh equations and arrived at a result that matches yours, namely 39/94 ≈ 0.415 amps.
 
  • #6
BadAnima said:
Before I rework the problem again, I have the solution manual entry for this problem. Would it help to post that instead?
Not yet, at this point. I think it would be more useful to learn how to verify (and/or debug) your algebra. So: solve for all unknown quantities, then verify whether their values satisfy the original equations you wrote to begin solving the problem.
 
  • #7
Redbelly98 said:
Not yet, at this point. I think it would be more useful to learn how to verify (and/or debug) your algebra. So: solve for all unknown quantities, then verify whether their values satisfy the original equations you wrote to begin solving the problem.

Okay, I found all of the currents individually and checked my equations. They all sum to zero as they should. I'm starting to think that the book is wrong. It wouldn't be the first time.
 
  • #8
Yes, I did the same and found they check out as well.
BadAnima said:
Also, I didn't see an intro section, so I guess I'll take care of that here. My handle is BadAnima, and I'm currently a freshman in university. My major is chemical engineering, but I'm not sure if I want to stay on this track or go into physics.
Hey, feel free to introduce yourself in our General Discussion area, if you want to:

https://www.physicsforums.com/forumdisplay.php?f=14
 

What is the formula for calculating current in a wire?

The formula for calculating current in a wire is I = V/R, where I is the current in amperes, V is the voltage in volts, and R is the resistance in ohms.

How do you calculate the resistance of a wire?

The resistance of a wire can be calculated by dividing the voltage (V) by the current (I). This is known as Ohm's Law: R = V/I.

What factors affect the current in a wire?

The current in a wire is affected by the voltage, resistance, and length of the wire. Other factors that can affect current include the type of material the wire is made of, the temperature, and the presence of any magnetic fields.

What is the difference between direct current (DC) and alternating current (AC)?

Direct current (DC) flows in one direction only, while alternating current (AC) changes direction periodically. In DC circuits, the current remains constant, but in AC circuits, the current oscillates back and forth.

How do you measure the current in a wire?

The current in a wire can be measured using a device called an ammeter. The ammeter is connected in series with the wire, and the current can be read directly from the device's display.

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