# Deflection of compass needle

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1. Oct 21, 2015

### heavystray

hi, i know this question is a bit unnecessary but i just want to know if i got my concept right
so, here's the question
if you have a compass pointing northwards placed between two parallel wires carrying opposite current, does the needle deflect?

my answer is the needle won't deflect since the forces acting on it are opposite but the answer given is the needle deflects eastwards.
here's a better look of the question.

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2. Oct 22, 2015

### mukundpa

For deflection of the needle a torque is required which is constituted by a couple of two forces equal in magnitude and opposite in direction but not in same lime.
If the two parallel wires are above and bellow the compass, the forces on the poles will be equal and opposite but not acting in the same direction.

3. Oct 22, 2015

### heavystray

That means in the case above (compass between two parallel wires carrying opposite current) the needle does not deflect right?

4. Oct 22, 2015

### mukundpa

If the needle and the wires are in same horizontal plane, needle is exactly midway and the needle is free to rotate in a horizontal plane only. I this case the torque on the needle will try to rotate the needle in vertical plane but due to pivot it is not possible.

5. Oct 22, 2015

### heavystray

I'm sorry but i don't understand what your're trying to say. What do you mean by the the same horizontal plane? Do you mean that they are parallel with each other?

6. Oct 22, 2015

### heavystray

To make it simpler, i guess my diagram is a bit confusing. Assume that the compass needle is parallel to both wires, what would the direction of deflection be? My reference book said the direction is eastwards but i don't understand why

7. Oct 22, 2015

### mukundpa

The basic thing is that the two wires creates a magnetic field. The field midway the wires will be in the direction normal to the plane of the wires. this field will create a torque which try to rotate the needle. if the wires are not in the same horizontal plane as that of compass the field will have a component in horizontal direction and the needle will be deflected.
I think you should go to the text again to learn about torque acting on a magnetic dipole in a magnetic field.

8. Oct 22, 2015

### heavystray

thank you very much, but we haven't learn about torque and all that. we just learn about basic electromagnetism

9. Oct 22, 2015

### BvU

Dear Heavy, dear muku,

I have a simple problem with the picture in post #1, arising from the fact that it is 2 dimensional.
For a question like this it is pretty important to know whether

the plane of the wires and the plane of the compass coincide (I would say: no deflection, as heavy concludes as well),
or

the wires are above and below the compass and the compass plane is perpendicular to the plane of the two wires (yes deflection !, contrary to what muku claims in post #2)
the second interpretation is clearly what the book is counting on. With the northward current above the compass and the southward below. What remains is to find out which way the needle will point !

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10. Oct 22, 2015

### heavystray

That's the original question. But can you explain to me how to determine the direction of deflection?

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11. Oct 22, 2015

### BvU

Good picture. Clearly above and below, so let's continue.

Yes, I could do that, but that would rob you of the opportunity to find out for yourself. So: what is the direction of the b-field from the top wire at the location of the needle? Idem bottom wire ?

12. Oct 22, 2015

### heavystray

what's b-field? i would say that the needle will deflect eastwards? i just understand it just now (if i understand)
so my reason is because, first, the compass is below the wire carrying south-north current. so the needle will deflect eastwards
and then the compass is above the current carrying north-south current, and the needle would deflect eastwards too.

but the needle won't deflect if it's placed between wires carrying current in the same direction, since the forces would be opposite?
one would be eastwards and the other is westwards? haha i don't really know i hope you can explain it better, thanks!

13. Oct 22, 2015

### BvU

Sorry, b-field means magnetic field.

Oops, there you go!
1. the picture suggests without much doubt that the compass is below the wire carrying north-south current !
2. the so you use as if its completetly straightforward. Well, now that it's turned around 180 degrees, you'll have to explain the intermediate steps, so we can find the flaw there is apparently lurking in one of those steps
So: what is the direction of the magnetic field lines due to the the top wire at the location of the needle?

 and I do agree that in the (b) picture there will be no deflection of the needle. To conclude that, you have used information that wasn't in your picture in post #1. Can you guess what I'm referring to ?

14. Oct 22, 2015

### heavystray

Oh okay but first what is north south or south north current means? I thought that if current pointing to the north it's a north south current haha. Sorry i didn't learn these terms

Oh please i feel so stupid, what could you possibly referring to?

15. Oct 22, 2015

### BvU

No need to feel stupid, no need to feel desperate at all .
In your post #10 I noticed that the current in the top wire was running in a direction opposite to the direction the upper wire in your drawing in post #1.

We can safely assume the compass is pointing in a northward direction, and the book picture confirms that. Let's stick with the book picture from now on to avoid further confusion. There I see that the arrow at P is pointing south, so I use the term north-south for the current in the top wire. (Usually we read from left to right and from origin to destination).

We skip posts 12, 13, 14 and I simply ask again: in what direction is the magnetic field due to the upper wire (P) in the book picture ?

(not a trick question and I think you had it right already all the time).

16. Oct 22, 2015

### heavystray

Oh sorry for the confusion, i've drawn it wrongly

I guess by using the right hand grip rule the direction would be anticlockwise and since the compass is placed below the wire, the direction would be from left to right?

17. Oct 22, 2015

### BvU

Spot on. Very well. Now all we have to understand is what way a compass needle points. Does it point to the north pole of a magnet, so against the magnetic field lines, or does it point with the field lines (which means that our North pole in the arctic is actually a magnetic south pole ) ?

 there is yet another yes/no possibility: that the needle tip of the compass that says "N" and points to the north pole is actually the south pole of the magnet that constitutes the compass needle !

Last edited: Oct 22, 2015
18. Oct 22, 2015

### heavystray

Haha yes i thought about that too.

When you say the north pole of magnet, does it refer to the direction of b field? From left to right, means the north is on the left side since magnetic field flows from north to south? I just want to make sure i got it right

If i got it correct, then it'll point with the magentic field lines (since north pole will repel each other, right?)

19. Oct 22, 2015

### BvU

Yes the north pole of a magnet is where the field lines come out of the thing.
And the compass needle that points to the north is the north pole of that small magnet.
So the geographic north pole is a magnetic south pole (see here)

Somewhat confusing, but you get used to it.

You've got it correct, it'll point with the field lines and yes north poles don't attract other north poles.

So : !

20. Oct 22, 2015

### heavystray

Haha yeay, finally ! Thank you very much! I appreciate it.

I'll read the article later since i've got lots more to revise

Last edited: Oct 22, 2015
21. Oct 22, 2015

### heavystray

Wait, sorry another question..
When you ask whether the needle will deflect to the north pole of a magnet or not, the magnet refers to the wire right? ( since it'll become a magnet when current flows through it)

22. Oct 22, 2015

### BvU

No I refer to some arbitrary other magnet.

The current-carrying wire is definitely not a magnet ! It doesn't, for example, attract iron !

23. Oct 22, 2015

### heavystray

24. Oct 22, 2015

### BvU

A straight current carrying wire as such is not a magnet. For a current loop or a coil you can distinguish a north pole side and a south pole side (and attract iron).

We don't have to assume there is another magnet in your exercise. I just wanted to bring in that a compass needle that points to the north is a magnetic north pole and that the arctic is in fact a magnetic south pole.

is somewhat philosophical. I don't know what the 'function' is. But we can sure do all kinds of calculations with a magnetic field strength.

25. Oct 22, 2015

### heavystray

oh i understand now, if the 'n' on the needle means the south pole of a magnet, it'll attract the north pole of another magnet. i hope i got it correct this time

sorry, i should rephrase my question.
so, first you asked me what is the direction of the b-field acting on the needle by the top wire,
and i say left to right,
does that mean the needle will deflect to the right?