# Homework Help: Ohm's Law and the Principles of DC Circuits

1. Jun 10, 2010

### tigerwoods99

1. The problem statement, all variables and given/known data

R1 = 9.29 V= _____ I = _______
R2 = 9.19 V= _____ I = _______
R3 = 8.45 V= _____ I = _______
R4 = 4.47 V= _____ I = _______
R5 = 9.12 V= _____ I = _______
R6 = 4.60 V= 9.15 v I = 1.99 a

1/(r1, r2, r3) = (1/R1+R2) + (1/R3) = 5.7986 equivalent for R1, R2, R3
1/(r4, r5) = (1/R4) + (1/R5) = 2.9997

Picture is attached. How do I got about finding these terms:

2. Relevant equations

1/(r1, r2 ...) = (1/r1) + (1/r2) ...

V = IR

3. The attempt at a solution

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2. Jun 10, 2010

### tiny-tim

hi tigerwoods99!
Yes, those are the correct formulas (I haven't checked the actual numbers).

I don't understand what you're asking

you have the equivalent resistances, so find the currents, and then find the voltages.

3. Jun 10, 2010

### tigerwoods99

I am not sure how to get from equivalent resistances to currents...

4. Jun 10, 2010

### tiny-tim

First add all the equivalent resistances to get the total equivalent resistance for the whole circuit.

That will give you the current, I6, in the undivided part of the circuit.

Then write down the Kirchhoff's rules equations for the other currents, I12 I3 and I45 (one equation for each junction, and one for each loop) …

what do you get?

5. Jun 10, 2010

### Tommo1

Simplify the circuit...
R4||R5 = 3.00 ohms
(R1 + R2)||R3 = 5.80 ohms

Then work back through the circuit using the voltage divider...
V1/VT=R1/RT
This gives R4||R5 = 5.97 V (They both have this voltage as they are in parallel)

To get the currents use the current division equation I1 = IT X R2/RT.

You then repeat this process for the (R1 + R2)||R3 part of the circuit.

6. Jun 10, 2010

### tigerwoods99

we havn't learned about capacitors, all we go is like:

V = IR
Vt = v1 + v2...
It = i1 + 12..
(1/r(x)) = (1/...) + (1/...)

I got I6 done:

R1 = 9.29 V= _____ I = _______
R2 = 9.19 V= _____ I = _______
R3 = 8.45 V= _____ I = _______
R4 = 4.47 V= _____ I = _______
R5 = 9.12 V= _____ I = _______
R6 = 4.60 V= 9.15 v I = 1.99 a

7. Jun 10, 2010

### tigerwoods99

How did you get R4||R5 = 5.97 volts?

8. Jun 10, 2010

### tigerwoods99

9. Jun 11, 2010

### Tommo1

Here's how to do it using only V=IR and series and parallel resistors.

Combine R4 and R5 in parallel
1/Rt=1/R1 + 1/R2
This gives Rt=3.00 ohms

Now add R1 + R2 in series = 18.48 ohms
Then combine that with R3 in parallel to give 5.80 ohms.

[PLAIN]http://www.xphysics.co.uk/x/E1.jpg [Broken]

This is a series circuit so current will stay constant.

Finding the voltage across the 3 ohm resistor V=IR =1.99 x 3 = 5.97 V

Now expand that part back out...

[PLAIN]http://www.xphysics.co.uk/x/E2.jpg [Broken]

Clearly V4=5.97 V and also V5 = 5.97 V.

The currents are I = V/R = 5.97/4.47 = 1.34 A for I4.
And similarly I5 = 0.65 A

Now go to the top part of the circuit and do the same process.

[PLAIN]http://www.xphysics.co.uk/x/E3.jpg [Broken]

V3=11.54 V, I3=1.37 A

Then for the little series part...

[PLAIN]http://www.xphysics.co.uk/x/E4.jpg [Broken]

V1=5.76 V, V2=5.74 V, I1=I2=0.62 A

Note that my current direction is "electron flow" and I'm using European resistor symbols.

There are lots of different ways of doing this sort of problem but this is a simple approach even if a bit long winded.

Hope this helps!

Last edited by a moderator: May 4, 2017