Can someone help me with 2 circuit problems please

1. Jun 28, 2004

supermenscher

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

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

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

2. Jun 28, 2004

AKG

I would suggest using [ code ] and [ / code ] tags:
Code (Text):

55 Ohms

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

V1=9.0V     R1
--+B--/\/\/\-------
|                  |
|         R2       |
.-----/\/\/\/-------.
|                  |
|                  |
|----------B+------|
V2=6.0V

EDIT: :yuck: 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:

$$R_{total} = 12 \Omega + (55 \Omega || R)$$

$$38 \Omega = 12 \Omega + (\frac{1}{55 \Omega} + \frac{1}{R})^{-1}$$

$$26 \Omega = (\frac{1}{55 \Omega} + \frac{1}{R})^{-1}$$

$$(\frac{1}{26 \Omega} - \frac{1}{55 \Omega})^{-1} = R$$

$$R \approx 49.310 \Omega$$

$$R = 49 \Omega$$

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

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.

Last edited: Jun 28, 2004
3. Jun 28, 2004

supermenscher

Thank you so much, I got them both right and 100 on the assignment :) :) :)