Can someone help me with 2 circuit problems...please!!!


by supermenscher
Tags: circuit, problemsplease
supermenscher
supermenscher is offline
#1
Jun28-04, 12:47 PM
P: 47
1. The equivalent resistance between points A and B of the resistors shown below is 38 ohms. Find the value of the resistance R.

The resistor looks like this
55 Ohms

_____\/\/\/\/_____
| |
12 Ohms | |
A | |
.____/\/\/\__ | |______. B
| |
| |
|____\/\/\/\/_____|
R

I have no idea what to do...please help.


2. Determine the magnitudes and directions of the currents through R1 and R2, where R1 = 36 ohms and R2 = 10 ohms. I already found the directions. they are both from right to the left, I just need the magnitudes. The Bs represent batteries, and the + next to them indicates the positive side of the battery. I am also really lost on this one can anyone help?

V1=9.0V R1
--+B--/\/\/\-------
| |
| R2 |
.-----/\/\/\/-------.
| |
| |
|----------B+------|
V2=6.0V
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AKG
AKG is offline
#2
Jun28-04, 03:27 PM
Sci Advisor
HW Helper
P: 2,589
I would suggest using [ code ] and [ / code ] tags:
                             55 Ohms
                       
                      _____\/\/\/\/_____
                      |                 |
       12 Ohms        |                 |
   A                  |                 |
   .____/\/\/\_______ |                 |______. B
                      |                 |
                      |                 |
                      |____\/\/\/\/_____|
                               R

                            
                            V1=9.0V     R1
                               --+B--/\/\/\-------
                               |                  |
                               |         R2       |
                               .-----/\/\/\/-------.
                               |                  |
                               |                  |
                               |----------B+------|
                                          V2=6.0V
EDIT: Well, it looks best in the editor window, slightly better (but not great) in the code tags, and unreadable without those tags.

SECOND EDIT: looks fine now

------------------------------------------------------------

For the first question, you should know how to calculate net resistances when you have resistors in series and parallel. To find the total resistance between A and B, it is:

[tex]R_{total} = 12 \Omega + (55 \Omega || R)[/tex]

[tex]38 \Omega = 12 \Omega + (\frac{1}{55 \Omega} + \frac{1}{R})^{-1}[/tex]

[tex]26 \Omega = (\frac{1}{55 \Omega} + \frac{1}{R})^{-1}[/tex]

[tex](\frac{1}{26 \Omega} - \frac{1}{55 \Omega})^{-1} = R[/tex]

[tex]R \approx 49.310 \Omega[/tex]

[tex]R = 49 \Omega[/tex]

-----------------------------------------------------------

Solve question 2 using the principle of superposition. That is, find the voltages and currents with the circuit first by replacing the top battery with a short circuit, then find the voltages and currents with the top battery, replacing the bottom with a short circuit. Add the numbers from the first calculations to the second calculations, and make sure you take direction/sign into account.
supermenscher
supermenscher is offline
#3
Jun28-04, 06:34 PM
P: 47
Thank you so much, I got them both right and 100 on the assignment :) :) :)


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