# How to find the resistence if it is not noted

#### computergirl

Joined Dec 13, 2014
4
I am taking a basic electric class and am stumped. In of my assignments it asks things like figure out RT, IT, IR1, IR2, IR3, VR1,Vr2,VR3, R2, PR1, PR2 and PR3. The circuit has the voltage at 18.399v, R1 at 1000 ohms, R2 is ? and R3 is 470 ohms. How would I figure out the missing ohms? My class is self taught and the "teacher" doesn't respond. SO I decided to see if someone that has knowledge on this could help. I believe this circuit is parallel.
I am not looking for answers. I have to learn this but am extremely stumped on how to figure the R2 and RT out. I looked in a book and see stuff like this but when you crunch the problem the answers offered are not what I come up with so I know I am doing this wrong. If it is already been answered please don't yell at me. I looked on the book that is offered but I don't know what to ask to get the right section.
Thank you for your help in understanding this and I am sure I will be asking a lot due to me just starting this electric thing.

#### bertus

Joined Apr 5, 2008
20,317
Hello,

A schematic drawing of the circuit would help.
You can upload the schematic using the "Upload a File" button under the reply window.

Bertus

#### computergirl

Joined Dec 13, 2014
4
Hello,

A schematic drawing of the circuit would help.
You can upload the schematic using the "Upload a File" button under the reply window.

Bertus
I don't have access to the file right now I want to post it with different values. I will post one tomorrow. Thank you for responding. The diagram is a rectangle with the voltage being on the left and the R1 r2 and r3 are on the right side of the rectangle in a line. There is not any cross members or parallel lines.

#### MrChips

Joined Oct 2, 2009
20,197
Circuit analysis relies on Ohm's Law if the circuit elements are linear in behavior.

I = V/R

There are three variables in that equation. If you know two of the three variables, you can determine the unknown variable.

#### MrChips

Joined Oct 2, 2009
20,197
I was waiting to hear your response and then would have suggested that you make sure you understand the difference between series and parallel circuits.

The link to the tutorial that Bertus has provided should make it clear.

Also note that in a series circuit the current flowing through the circuit elements is the same.
In a parallel circuit, the voltage across the circuit elements is the same.

In the information you have provided in your post #1, there is a missing piece of information.

#### computergirl

Joined Dec 13, 2014
4
I will read the e-book. I have a few days to figure this out. I love that everyone helps. I hope to get as good as everyone here and help someone else in their time of need.
Holly

Circuit analysis relies on Ohm's Law if the circuit elements are linear in behavior.

I = V/R

There are three variables in that equation. If you know two of the three variables, you can determine the unknown variable.

#### WBahn

Joined Mar 31, 2012
25,223
One way to approach many, many problems in general is to ask yourself what it is you need to know that would allow you to solve the problem most directly. In this case, the problem is to find the value of a resistance and you can find that most directly if you knew the voltage across THAT resistance and the current through THAT resistance (i.e., Ohm's Law). So you have just turned your original problem into two separate problems: Find the current through the unknown resistance and find the voltage across the unknown resistance. Yes, you have doubled the number of problems you have to solve, but hopefully each new problem is easier (or at least possible) to do. Now you pick one of those and repeat the process, asking what new problems you could craft that, if solved, would let you answer the current one. Eventually you should get to problems that are trivial to solve and then you can start working your way back toward the original problem.

#### computergirl

Joined Dec 13, 2014
4
I figured it out and will post my solution. I want to thank everyone for their kindness. It was so frustrating but I got a 170 out of 200 so I am pleased. The best thing is I did it on my own and didn't have someone tell me the answer. Thank you again. I just want you to know I went to Ohms law which really helped me a lot. All of your input has helped immensely.
Holly

#### JoeJester

Joined Apr 26, 2005
4,184
In the homework section, people rarely "give you the answer". They normally make you show your work and point you in the correct direction when they see you made an error.