# Need Help With Series-Parallel Ohms Law Circuit Math Problem

1. Dec 23, 2011

### ChainedBoxer

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

Total Volts: 10
Resistor 1 - R1: 1K
Resistor 2 - R2: 4K
Resistor 3 - R3: 4K

Photo That I Made Of The Series-Parallel Circuit:
http://imageshack.us/photo/my-images/266/circuitn.png/

2. Relevant equations

http://en.wikipedia.org/wiki/Series_and_parallel_circuits

3. The attempt at a solution

Total Current: 0.5
Voltage Across R1: 0.167
Current Through R3 In Amps: 0.5
Current Through R2: 0.333
Total Circuit Power In Watts: 3
Power Consumed By R3 In Watts: 1.5

2. Dec 23, 2011

### vela

Staff Emeritus
How'd you come up with those answers?

3. Dec 23, 2011

### ChainedBoxer

I came up with them based on the formulas that I read, but I know they must be wrong because they are not adding up to what the numbers should be based on the multiple choice questions that I was given.

((NOTE: Where the question "What Is the Current Through R1 In Amps?", it should read "R2" instead.))

Known Circuit Values:

Total Voltage: 10 Volts
Resistor 1 - R1 - In Series - Value: 1K
Resistor 2 - R2 - In Parallel With R3 - Value: 4K
Resistor 3 - R3 - In Parallel With R2 - Value: 4K

Questions:

1. What Is The Total Current Equal to In Amps?

2. What Is The Voltage Across R1?

3. What Is The Current Through R3 In Amps?

4. What Is The Current Through R2 In Amps? (I First Listed It As R1 In The Pic, But It Should Be R2).

5. What Is The Current Value Through R1 In Watts?

6. What Is The Total Power Consumed In Watts?

7. What Is The Power Consumed By R3 In Watts

Can anyone help me out here?

4. Dec 23, 2011

### Staff: Mentor

How are you going about determining this:

1. What Is The Total Current Equal to In Amps?

5. Dec 23, 2011

### vela

Staff Emeritus
We want to see your actual calculations so we can see where you are going wrong.

6. Dec 23, 2011

### ChainedBoxer

Are you referring to question 5 "5. What Is The Current Value Through R1 In Watts?"? That should be Amps. My mistake.

And I will see if I can post some of my notes here, to help you figure out what I am doing wrong. I have a bunch of them, but I don't know if I can write them in a way to actually explain what steps that I have been taking so far.

7. Dec 23, 2011

### Staff: Mentor

Start at the beginning. I'd like to see how you determine the current flowing from the 10v source.

8. Dec 23, 2011

### ChainedBoxer

For the 10 Volts and Current, I created a chart based on what I found on a site dealing with this type of math. I can't create the chart in the thread, but I can try and list some of the math problems that I used. Since all Voltage is equal in a parallel circuit, the top of the chart had 10 Volts listed as all of the answers. The Amps came out to, R1-10, R2-2.5, R3-2.5, for a total of 15. The Resistance on the chart had the Resistance as R1-1, R2-4, R3-4, for a total of 0.666. I know the chart came out to be wrong, because the answers didnt match any of the choices. Can you tell me what I did wrong? This chart worked on all of my Series math problems, but the version for Parallel didn't match up.

9. Dec 23, 2011

### ChainedBoxer

There is one other problem that I have been having problems with. Its a True or False question. Can anyone here help me find the answer?

If all Resistors in a Parallel circuit are equal size, then the Current through each is Equal? True or False?

10. Dec 23, 2011

### Staff: Mentor

That's true, and is a fact that you can use when considering the two 4kΩ

11. Dec 23, 2011

### Staff: Mentor

Do you know Ohm's Law? Can you answer questions such as: if a 100 ohm resistor has 0.5 amps flowing through it, what reading would a voltmeter give if you connected it across the two terminals of that resistor?

12. Dec 23, 2011

### ChainedBoxer

Thanks! That is what I originally had as the answer, but I wasn't sure. The book mention something to this effect, but it was written in a way where I couldnt tell if True was correct.

How would I use this to help me solve the two 4K resistors? And the "K" means 1000 right? So its actually 4000 or .1000?

13. Dec 23, 2011

### ChainedBoxer

Yeah I know Ohms Law, but I am having a problem figuring out Series-Parallel math. I x V = 50 Volts. At least I think that is right.

14. Dec 23, 2011

### Staff: Mentor

Yes. I prefer a lower-case "k" for 1000.

I'm not sure what "it" is that you refer to. But I'm puzzled by that decimal point.

15. Dec 23, 2011

### ChainedBoxer

I dont know what it your referring to. And I havent been doing this type of math for very long and I am 10+ years out from high school; so I don't remember everything from that time math wise. So I wasnt sure if K in this type of math meant 1000, or it meant something else.

So have I posted enough to where you can start helping me figure out this math? Like tell me which formulas to use for each question?

16. Dec 23, 2011

### Staff: Mentor

The "formula" for any number of resistors in parallel is easiest remembered as:

1/R = 1/R1 + 1/R2 + 1/R3 + ...

So work out the sum of the reciprocals, then take the reciprocal of that.

For 2 resistors in parallel, this boils down to:
R = (product of the resistances) / (sum of the resistances)

Does that help?

17. Dec 24, 2011

### ChainedBoxer

How do I figure out the resistor that is in Series, with the two from Parallel? You said to use the formula for the two resistors in parallel, but there is also a resistor in Series. How do I add them all up?
How do I find the Total Current? The Voltage Drop over R1? The current through R1, R2 and R3? Find Total Watts? The Watts consumed by R3?

18. Dec 24, 2011

### Staff: Mentor

Once you have worked out the resistance of the 2 in parallel, you can then treat that pair as a single resistance. It is in series with R1, so you add your answer to the resistance of R1 to arrive at the total circuit resistance. For resistances in series, their values simply add together.

Last edited: Dec 24, 2011
19. Dec 24, 2011

### ChainedBoxer

Thanks for your help so far! I'm learning a good amount of things from this thread. Last night and into the morning I have been trying to better figure out these questions and I came up with some possible answers. Can you please tell me if I got any of them wrong and I will do them over again until I get them right.

2. What Is The Voltage Across R1? (I Came Up With C Being The Right Answer.)
A - 0.5
B - 2
C - 0.0033
D - 3

3. What Is The Current Through R3 In Amps? (I Came Up With B Being the Right Answer.)
A - 2
B - 3.3
C - 6
D - 1.5

4. Total Current Through R2 In Amps (I Came Up With A Being The Right Answer).
A - .00165
B - 0.5
C - 0.333
D - 0.167

5. What Is The Current Value Through R1 In Amps? (I Came Up With D Being The Right Answer)
A - 2
B - 3.3
C - 0.333
D - 0.167

6. What Is The Total Power Consumed In Watts? (I Came Up With C Being The Right Answer)
A - 1.5
B - 3
C - 0.333
D - 0.167

7. What Is The Power Consumed By R3 In Watts? (I Came Up With C Being The Right Answer)
A - 0.5
B - 1.5
C - .01089
D - 5

20. Dec 24, 2011

### ChainedBoxer

Also, how do I convert circuit values like this "6,67.0,00166= 0,0011W"? I know how to convert numbers like 60/1000, but not the one with commas in them.