# Circuits problem Need help by 10:00 tonight (EST).

Bensky
Circuits problem :( Need help by 10:00 tonight (EST).

## Homework Statement

In the circuit shown below, R1=10 ohms, R2=45 ohms, and R3=14 ohms. The power supply delivers a voltage of 25.0 V. a) What is the current through R3? b) What is the voltage across R2?

http://img91.imageshack.us/img91/6443/dia2pj3.png [Broken]

Possible choices for part a:
0.92 A, 1.21 A, 1.79 A, 1.84 A, 2.50 A

Possible choices for part b:
25.0 V, 12.1 V, 12.9 V, 15.0 V, 0 V.

V=IR

## The Attempt at a Solution

To be honest, I have no idea how to do this problem. I don't know where to start. Do I have to find the voltage drop first? I'm just lost, heh.

Any help is appreciated.
Bensky

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Homework Helper
Is R3 in series or parallel with R2? Is R2 in parallel or series with the supply?

Bensky
Is R3 in series or parallel with R2? Is R2 in parallel or series with the supply?

R3 is in parallel with R2, R2 is in series with the supply. (I would assume)

Homework Helper
R3 is in parallel with R2, R2 is in series with the supply. (I would assume)

Then what should be the p.d. across R2?

Bensky
Then what should be the p.d. across R2?

Umm, I have to find the current first for that, right?

Homework Helper
Umm, I have to find the current first for that, right?

Remember how the currents through resistors in series are the same? How does the p.d. vary in resistors in parallel?

Bensky
Remember how the currents through resistors in series are the same? How does the p.d. vary in resistors in parallel?

The P.D. is the same.

Homework Helper
The P.D. is the same.

Right so if R2 is in parallel with the supply...what is it's pd?

Bensky
Right so if R2 is in parallel with the supply...what is it's pd?

25V

Is it really that easy? XD

Homework Helper
25V

Is it really that easy? XD

It is multiple choice...but another way to do this is by kirchhoffs laws which would give you a system of equations to solve with 3 unknowns.. which sounds like way too much to do for a multiple choice question.

So...I am hoping that is the correct thing to do...

Bensky
Ok, thanks...we did not cover Kirchoff's Laws. Do you have any idea how to do the first part of the problem?

Homework Helper
Ok, thanks...we did not cover Kirchoff's Laws. Do you have any idea how to do the first part of the problem?

R3 in parallel with R2...same idea. You will get the pd across R3 and use V=IR.

Dozent100
How to solve:

1. Homework Statement
In the circuit shown below, R1=10 ohms, R2=45 ohms, and R3=14 ohms. The power supply delivers a voltage of 25.0 V. a) What is the current through R3? b) What is the voltage across R2?

First Calculate the total resistance of the R2,R3 parallel circuit:

1/R23 = 1/R2 + 1/R3

.
. . R23 = (R2 x R3)/(R2 + R3)

Then add R2 and R1 This gives the Rt which you plug into V= IR to determine total Current. With the current you can determine the voltage drop across R1. Subtract that Voltage from the supply voltage to determine the voltage Across R2 and R3. Knowing that Voltage you can Calculate the Current through R3 using I=V/R3!

So not only do you need V=IR, but you also need Rt = R1 + (R2 X R3)/(R2 + R3) for a circuit containing two Parallel resistance in series with another resistance.

Edmund

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