# Homework Help: Find the current?

1. Jan 23, 2016

### Marcin H

1. The problem statement, all variables and given/known data
Did I do number 14 correctly? I have a feeling it's not that simple.

2. Relevant equations
V=IR
i1=i2+i3+...In

3. The attempt at a solution
on picture

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• ###### Screen Shot 2016-01-23 at 3.52.22 PM.png
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2. Jan 23, 2016

### Staff: Mentor

Looks good to me.

3. Jan 23, 2016

### SammyS

Staff Emeritus
That's correct.

4. Jan 23, 2016

### Marcin H

Ok good. I am stuck on #15 now. I'm haven't seen this kind of circuit yet, so I'm not sure how to apply kirchoff's current law. I think A, B, and D are correct, but I'm not sure. I think all the squares in the circuit are power sources, but I'm not sure where to start in this circuit to make my loop and write out my equation.

5. Jan 23, 2016

### Staff: Mentor

Kirchoff's current law just says, for any point, the currents going into the point must equal those going out of the point. (You don't need to know anything about that kind of circuit to apply the law to each point.)

6. Jan 23, 2016

### SammyS

Staff Emeritus
Do you know how to do screen shots, or use the snipping tool ? It's much more likely that you get help if we don't have to open new tabs/windows on our browser.

All that is required for this problem is to apply Kirchhoff's Current Law.

You do not need to know what the squares represent at all.

7. Jan 23, 2016

### Staff: Mentor

Hint: Write an equation for each point where wires connect. (The three dots in the diagram.) Then compare those equations with the choices given.

8. Jan 23, 2016

### Staff: Mentor

It looks like you specified the units of the result as mA. That would be milliamps. Was that your intention?

9. Jan 23, 2016

### Staff: Mentor

Good eye!

10. Jan 23, 2016

### Marcin H

I'm having a hard time with this. How do you know what is going in and what is coming out exactly. All the currents can come out of one point. I was trying to write an equation for the point above box c, but how can I tell what is going in and out. Also, I thought current flows to the spot with least resistance.
Edit: Also, I think I would understand this there were 2 points/junctions, but how do I do this with three?

11. Jan 23, 2016

### Marcin H

Fixed. Thanks!

12. Jan 23, 2016

### Marcin H

Sorry, I'm new here. How did you do that exactly?

13. Jan 23, 2016

### Staff: Mentor

The currents have directions, specified by the arrows.

14. Jan 23, 2016

### SammyS

Staff Emeritus
OK.

One of the first things to learn here is to make it clear as to what post (you did that fine) or what part of a post you are responding to.

If you were asking me, about that thing I did, and that thing was posting those images, see the following. Otherwise, Doc Al and others, will continue working with you.

In (Microsoft) Windows, there is a "Snipping Tool" application. Find it in the Applications folder in the list of programs.

It will take a 'snapshot' of the portion of your monitor's screen, that portion being determined by what you highlight. You may need to use the help feature provided by the Snipping Tool or google for more details.

Once you take a snapshot of whatever is of interest, you simply paste it into the reply window of PF .

15. Jan 23, 2016

### Marcin H

I know, but I still don't see how I can find my equations. This is a very weird circuit. It looks like you can have many possibilities. EX. i1=i3+i4-i2 or i2=i3+i4-i1 (these 2 are the same thing actually). i5=-i3-i4 or i3 = i5+i4 (not sure about this one).

16. Jan 23, 2016

### Marcin H

So I just copy paste a picture from a word document and it will display in a post like in your first post? Can I drag and drop pics from my desktop? Or is there a way to post them the way you did. i usually take a screenshot on my mac and post that.

17. Jan 23, 2016

### SammyS

Staff Emeritus
The Snipping Tool allows you to take a snap shot of a relatively small portion of the screen, rather than snapping the whole screen. I'm not very familiar with the details of what's available on the Mac.
I used the pdf file viewed in adobe reader and snapped a portion of that.

Yes, you can do the same with a word document.

18. Jan 23, 2016

### cnh1995

You have four options. Check each option. You are asked to find which option(s) is(are) correct.

19. Jan 24, 2016

### Staff: Mentor

There are three 'junctions', each marked by a dot. So you can have three equations.

Good! But there's one more.

Now just check to see which of the given equations are consistent with those.

20. Jan 24, 2016

### Marcin H

One more? I can't find another. Re-arraging them i get A,B, and D fro my answers. i1+i2=i3+i4, i5=i1+i2, and i3+i4+i5=0

21. Jan 24, 2016

### Staff: Mentor

Do you agree that the diagram shows three junctions? Each one deserves its own equation.

22. Jan 24, 2016

### Staff: Mentor

Good.

Almost, but not quite.

Good.

23. Jan 24, 2016

### Marcin H

Hmm. So am I missing an equation for the junction above box C? it looks like i1, i2, and i5 would be coming out of that junction. So Does that mean i1+i2+i5=0?

24. Jan 24, 2016

### Staff: Mentor

Exactly!

25. Jan 24, 2016

### Marcin H

Ok I see that now. Thanks! I have another question about Kirchoffs Voltage Law related to a different question. Should I make a new thread or may I ask here?