# Homework Help: Series circuit resister help needed

1. Sep 13, 2011

1. The problem statement, all variables and given/known data
Here are pictures of the questions:

Q1:
http://i.imgur.com/3s9Ma.jpg

Q2:
http://i.imgur.com/0YChq.jpg

3. The attempt at a solution
Q1: As you can see, I have put down 1V above the 5V and 6V to the left. Is this right?

Q2: I have no idea what I've done.

2. Sep 13, 2011

### Staff: Mentor

Sorry, the questions are not clear. What are you given, and what are you asked to find?

Also, you seem to show places where there are different voltages on the top and bottom of a pair of parallel connected resistors. If two resistors are in parallel, they will have the same voltage across them.

3. Sep 13, 2011

Right, sorry, I though it was clear enough that I have to find the voltages that were unknown. If the voltages of two parallel resisters are the same, then in the first picture, there are two parallel resisters, so they'll be 5V each, does that mean that the one I filled in 6V should be 1V?

4. Sep 13, 2011

### Staff: Mentor

Do all the resistors have the same value of resistance?

5. Sep 13, 2011

Yes they do.

6. Sep 13, 2011

### Staff: Mentor

If they are all the same resistance value, and the configuration is one resistor in series with two parallel resistors, then you do not get 1V and 5V.

The total will add up to the 6V source voltage that you show, but the voltages across the resistors are not 1V and 5V.

What math are you using to calculate the voltage division?

BTW, the two circuits look the same to me. Are they supposed to be different configurations?

7. Sep 13, 2011

Oh yeah, I just realised I had put a duplicate up. Here's the actual Q2:

http://i.imgur.com/nqPrY.jpg

Well, my teacher said that the total voltage of the circuit is equal to the voltages of the separate resisters added up.

8. Sep 13, 2011

### Staff: Mentor

You are not getting the right answers for the resistor voltages. What equations are you using?

The fundamental equation is V=IR. The voltage across parallel components is the same. The current through series components is the same.

9. Sep 13, 2011

We're only using V=IR, the question ask to complete the voltage readings on the voltmeters. Oh, it's D.C. by the way, if that changes anything. So basically, I know that the voltage changes in a series circuit but I'm not sure what the volt readings should be. Also if two resisters are in a parallel circuit but in an overall series circuit, they have the same voltage readings right? So how does the Q1 circuit work out?

10. Sep 13, 2011

### Staff: Mentor

What is the overall resistance of two identical resistors in parallel?

11. Sep 14, 2011

They're the same right?

12. Sep 14, 2011

### Staff: Mentor

I don't know what you mean by that statement.

If you put two 100 Ohm resistors in parallel, what is the resulting combined resistance?

13. Sep 14, 2011

200 Ohm, but what I don't get is, if you look at the first picture, the voltages of 2 resisters in parallel are the same, the current changes. So, if I label one of the parallel resisters 5V, then is it right?

14. Sep 14, 2011

### Staff: Mentor

No, not 200 Ohms. When you put resistors in series, their values add. So two 100 Ohm resistors in series make a total of 200 Ohms.

What do you get when you put two 100 Ohm resistors in parallel, and why?

15. Sep 14, 2011

To be honest, I don't know, that's kind of why I'm asking. I really need to understand the answers to the questions I'm asking.

16. Sep 14, 2011

### Staff: Mentor

It is strange that your teacher is not giving you the tools to figure out these questions.

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

Now you should be able to tell us what the value of two parallel 100 Ohm resistors is...

17. Sep 14, 2011

Ok, it's a 100 Ohms. Right, so will the voltage be the same as well?

18. Sep 14, 2011

### Staff: Mentor

No. Two 100 Ohm resistors in parallel do not still measure 100 Ohms.

What is the formula from the wikipedia page for the parallel combination of resistors? What do you get when you plug in 100 Ohms for the value of the two parallel resistors?

19. Sep 14, 2011

Is it 1/Req = 1/R1 + 1/R2? If so, 1/Req = 1/100 + 1/100 which is 0.02.

20. Sep 14, 2011

### Staff: Mentor

So if 1/Req = 0.2, what does Req equal?

And knowing what the parallel resistance is compared to the single resistor, what would you expect the voltage drops to be going around that circuit? They do need to add up to the total battery voltage applied, and the voltage drops will ratio with the effective resistances of each stage...

21. Sep 14, 2011

50.

Sorry, but I can't really understand what you said. Are you talking about the circuit in the first picture?

22. Sep 14, 2011

### Staff: Mentor

Sure, let's do the first circuit now. You have a 6V source, and then (going [STRIKE]clockwise[/STRIKE] counter-clockwise) you have a resistor (call it 100 Ohms), and then two parallel resistors (which you now know has a combined resistance of 50 Ohms).

So, since V=IR, what is the current that flows in the circuit? Remember that series resistances add, so the series combination of the 100 Ohm resistor and the 50 Ohm combined parallel resistors adds to what? So what is the current I then, since you have 6V across that total resistance?

Now you have the total I, so you can calculate the voltage drop across the 100 Ohm and 50 Ohm resistance.

Please show us your work and the solution for that first circuit. Then show us how you solve the 2nd circuit.

Last edited: Sep 14, 2011
23. Sep 14, 2011

The current that flows into the circuit is 0.06A. The series combination is 150 Ohms. The current is then 0.04A.

0.06 x 100 = 6V - (0.04 x 50) = 2V

So is the voltage drop 4V?

24. Sep 14, 2011

### Staff: Mentor

I don't know what the first sentence is about, but the rest is correct.

6V/150 Ohms = 0.04A = 40mA

So you get 4V dropping across the single 100 Ohm resistor, and 2V dropping across the two parallel resistors. That adds up to the total of 6V.

Now show us your work on the 2nd circuit. You're almost there!

25. Sep 14, 2011

Single resistor = 100 Ohms
Parallel = 50 Ohm

Current = 9V / 250 = 0.036A

9V - ( 0.036 x 50) = 7.2V drop across first resistor.

7.2V - 1.8V = 5.4V drop across parallel resistors.

5.4V - 3.6V = 1.8V drop across the last resistor.

Hmm, I've gone wrong somewhere but I'm not sure where. Oh yeah, the actual question I asked was about what shall I fill in the voltmeter readings in the diagrams for the ones that I've filled in already, some of them are probably wrong but I'm not sure which ones. Basically what I had to do was fill in the voltmeter readings but I can figure them out.