# Homework Help: Direction of electric current in a conductor

1. Jul 26, 2012

### alicia113

ok so here is the question
number 45 and its second part

and this is what i did

I have a c magnet drawn with the north pole and the bottom opening right. And the magnetic field lines running between the two polls (virtically) And I have a conductor running between the poles perpendicular to the field lines. I don't understand what the coils are suppose to be.
------------------------- S
| *
| ^ *= magnetic field lines (i have 3 in my picture)
| *
| 0 ^ 0= conductor
| *
| *
| *
------------------------- N

can anyone please help me ... i know i didnt draw the right diagram.. i just need the diagram and then i can explain it ... i dont get the coils !

2. Jul 26, 2012

### alicia113

ok my picture went all wanky when i hit submit ... those lines are suppose to be at the ends of the poles !! sorry... i hope you can still see what i mean

3. Jul 26, 2012

### PeterO

The terms primary coils and secondary coils usually are used when referring to transformers. Perhaps the text has just pasted in the reference to the wrong question..
With your diagram, I would expect you to be explaining what current, if any, was induced in the conductor if it moved perpendicular to itself, and to the field - so left or right in your diagram.

4. Jul 26, 2012

### SammyS

Staff Emeritus
Use an editor program such as "Notepad" to create your "picture". Then use [ code ] "tags" to get something closer to the format that you want.

Code (Text):

-------------------------  S
|                          *
|                          ^     *= magnetic field lines (i have 3 in my picture)
|                          *
|      0                   ^     0= conductor
|                          *
|                          *
|                          *
------------------------- N

5. Jul 26, 2012

### alicia113

So in my diagram the current Would e going right.

And that's what I believed that it was incorrect because the next question refers to transformers

6. Jul 26, 2012

### PeterO

You are right, the NEXT question refers to transformers. Question 45 just reads wrong. As written it is a garbage question, worthy only of a garbage answer - ie: give no answer to it, and explain why.

7. Jul 26, 2012

### alicia113

I dont need 45 I already did it

But could you explain to me why the current flows right like I get it but at the same time I don't .. ;s

8. Jul 26, 2012

### alicia113

Would a good explaination be Because of te right hand rule the current has to flow right ?

9. Jul 26, 2012

### PeterO

10. Jul 26, 2012

### alicia113

That's what I ment sorry. I don't need help with 46 the transformers. My apologies I read it wrong

11. Jul 26, 2012

### alicia113

I don't understand how lenzes law or the motor law

12. Jul 26, 2012

### alicia113

I dont understand how lenzes law or the motor law is in this. I only understand how the right han rule shows direction of current.

13. Jul 26, 2012

### PeterO

NO, a good explanation would be that the original question makes no mention of coils, so explaining the role of the current in the primary and secondary coils is a ridiculous request.

If you are considering your original diagram, the current is not to the right.

Your diagram looks to be like a C-magnet [we usually call them horse-shoe magnets] drawn in the plane of the page, with a straight conductor passing into/out of the page, deep inside the jaws.

If you now move that conductor to the right, there will be a current induced out of the page, along the conductor.
If you then move the conductor back to the left, there will be an induced current into the page

Any induced current will be along the conductor, so unless the conductor is drawn passing from left to right, the current could not be to the right [or the left]

14. Jul 26, 2012

### alicia113

So this question number 45.. Can not be Anwsered is what you are saying and a drawing can not be drawn for it?

15. Jul 26, 2012

### PeterO

If you just ignore all reference to coils, your diagram, and my explanation fit the situation described quite well.

If coils must be involved, the question is rubbish.

16. Jul 26, 2012

### alicia113

Ok so I leave my diagram the way it is .. But the current dose not flow to the right correct ?

17. Jul 26, 2012

### alicia113

So where is my current .. I'm kind of lost now :s

18. Jul 26, 2012

### alicia113

Sorry I'm a little lost

19. Jul 27, 2012

### PeterO

I won't answer that question - but I will ask you this:

In which element/component shown in your diagram, was the current to the right going to flow?

20. Jul 27, 2012

### alicia113

Ok lenzes law states

An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.

So it was going up and out of page?0

21. Jul 27, 2012

### PeterO

In what component was that current flowing? and what direction of magnetic field did/does it give rise to?

22. Jul 27, 2012

### alicia113

I am getting so confussed here ... I have no Idea what you are talking about .. :(

23. Jul 27, 2012

### PeterO

Ok lenzes law states

An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.

So it was going up and out of page?

... didn't really show me you knew what change in original magnetic flux was happening, And I wanted to check you knew how the current was able to go up out of the page.
I was seeking clarification of your understanding.

24. Jul 27, 2012

### alicia113

As you probably can see I can not really grasp the concept. Could you please explain the change in magnetic flux and what is happening .. Is it because he force of the conductor is acting upon it causing It to change ?? Or could you reword what I mean into the proper way?

25. Jul 27, 2012

### PeterO

I am assuming that your conductor is passing into/out of the page [which is why we just see the end of it in the picture]

When that conductor is moved to the right - so as to move it out from "jaws" of the magnet - we analyse the force induced on any charges in the rod.
Like all matter, there are many protons and many electrons in that conductor.
For simplicity, we usually just consider the protons, the positive charges.
Since they are moving to the right [the whole conductor is moving to the right, and none of it is left behind], they constitute a current to the right.
Using the Right hand rule, we model this as an open right hand on the page.
The fingers pointing up represent the field [from N to S as you showed]
The outstretched thumb, pointing right, represents the current.
The force on those charges comes out of your palm, so out of the page.
If those positive charges in the conductor could move, they would constitute a current out of the page, through the moving conductor.

The fact that the protons are trapped in the nuclei, and can't move at all, does not affect our argument.

All the electrons in the conductor are negative, so would experience a force into the page [they are the opposite of positive charges, so the induced effects are in the opposite direction].
If the electrons could move, they would move into the page. But some of the electrons can, and do, move that way [into the page]
But, negative charges moving into the page equates to a conventional current out of the page, so there is a current out of the page - just as predicted by analysing what effect the induced force on the positive charges would do.

So this is not really a case of change in magnetic flux in a loop, but one of charges moving in a magnetic field.
Changing magnetic flux is what we get in a transformer, and I think this question has found its way into a transformer section by mistake - and then had references to primary and secondary coils added to complete the error.