# Alignment of the compass needle

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1. Nov 17, 2015

### Pulzz

If a compass is placed above a current carrying wire, why does the compass needle align itself in the direction of the magnetic field produced by the wire?

2. Nov 18, 2015

### Drakkith

Staff Emeritus
Because the needle is magnetic? I'm a little confused by your question. Why wouldn't the needle align itself with the magnetic field?

3. Nov 18, 2015

### Pulzz

The magnetic field produced by the current does not have a north or south pole, so what would the compass needle be attracted by?

4. Nov 18, 2015

### Pulzz

I know this question might be really rudimentary and stupid for y'all but any help would be really appreciated

5. Nov 18, 2015

### Drakkith

Staff Emeritus
A compass only points towards the magnetic poles of a magnet because it orients itself along the magnetic field lines that loop from the north pole to the south pole. A current-carrying wire also has magnetic field lines, but they run in circles around the wire instead of from one 'pole' to the other. See the image below:

A compass needle would orient itself so that the 'north' indicator on the needle pointed along the direction of the arrows, like the image below shows.

6. Nov 18, 2015

### arul_k

Considering that magnetic field lines are imaginary, why would the compass needle align itself along an imaginary line?

In the case of a bar magnet one could reason that the compass needle is influenced by the forces emanating from the north and south poles of the magnet, but in the case of the wire there are no forces emanating from N/S poles, so why do we obtain this circular field line pattern?

7. Nov 18, 2015

### Drakkith

Staff Emeritus
The lines are imaginary, but they represent real effects of the magnetic field, just like the elevation lines on a topographic map are imaginary but represent a real change in elevation.

I'm not sure how to explain it. Perhaps someone else here can elaborate on the details.

8. Nov 18, 2015

### sophiecentaur

Lines of gravitational force (the Field lines) are just as imaginary - the geometry happens to be a bit less complicated than for the field round a wire. If you drop a stone, it follows those "imaginary lines" of the g field but I doubt that you would have a problem with that. The lines are only a graphical representation of an effect.
The "why" question is no more valid here than anywhere else in Science. All we can say for sure is that it's how things have been observed to work.

9. Nov 18, 2015

### rootone

Although the field lines are 'imaginary' in the sense of being just an aid to visualisation, the field itself is real, and it's effect predictable.
An analogy would be a weather map which depicts weather systems using isobars.
A forecast map may show an approaching depression as a roughly circular set of lines, but when the storm arrives we don't see lines in the sky,
we see the usual consequences of a low pressure system, wind and rain.

Last edited: Nov 18, 2015
10. Nov 18, 2015

### gjonesy

The "right hand rule" can determine which direction the field is traveling. you can point your right thumb along the direction of the current traveling through the wire, the direction of the magnetic field can by found by curling your index finger around the wire. The EM field is moving in spiral around the wire in the direction of the current.

11. Nov 19, 2015

### sophiecentaur

The poles are only places where the field is most concentrated (highest). They are an even more 'imaginary' concept than the field lines, themselves. In no way are they points of origin for the lines.

12. Nov 19, 2015

### arul_k

The difference is that the gravitational and electric field lines originate from the body causing the force, and thus represent the direction of accleration a body would experience in the presence of these fields. The compass needle experiences no accleration along a magnetic field line.

Only a magnetic monopole would experience an accleration, but the magnetic monopole is another imaginary concept

The "why" question is the essence of all enquiry. If we accept "how things have been observed to work" as an answer, we would still be in the stone age!!!

13. Nov 19, 2015

### arul_k

What about the fact that like poles repel and unlike poles attract. I haven't come across any text that states the magnetic poles of a permanant magnet are imaginary.
Are you refering to magnetic monopoles?

Since B filed lines are assumed to be continous loops there can be no point of origin of the field lines, however, by convention the lines are assumed to eminate at the north pole and terminate at the south pole of a magnet

14. Nov 19, 2015

### rootone

I think it's more of 'how', not 'why'.
Science attempts to make accurate theories or models of how things interact in the manner they are observed to.
Once we have a good model of how things happen we can often predict what the outcome *ought to be* if we set up an experiment, thus either validating or invalidating our model.
'Why' things interact in the way they do leaves room for any kind of speculation, and that would be philosophy rather than science.

Last edited: Nov 19, 2015
15. Nov 19, 2015

### OmCheeto

You explained it in post #5.

Pulzz, check out my most awesome experiment, where I discovered, that, um, that's just the way it is.

How does [a clamp-on] ammeter work?

We actually never got down to the details of how a real one works. I imagine there's a spring in there somewhere. Fun thread.

16. Nov 19, 2015

### sophiecentaur

I don't expect you have read that the polesof a magnet are located at points either.
If I am in the stone age then I am in good company (listen to the Richard Feynman "Why" talk.
Imo, people who really believe there are proper answers to "why" then they are in cloud cuckoo land. I haven't yet read of one, anywhere in the field of established Science.

Last edited: Nov 19, 2015
17. Nov 20, 2015

### arul_k

Since you are so well read, I would once again request you to mention the the references that state "magnetic poles are imaginary" and poles of a magnet are located at "points"

I have heard Feynmans talk, he dosen't object to "why" questions. You only need to type "Why" in PF search to find out how many members of PF are "in a cloud cuckoo land".

Since it appears to be more a question of semantics, allow me to rephrase my question in post #6

In the case of a bar magnet one could reason that the compass needle is influenced by the forces emanating from the north and south poles of the magnet, but in the case of the wire there are no forces emanating from N/S poles, so how is it that we obtain this circular field line pattern?

18. Nov 20, 2015

### sophiecentaur

This is his famous comment on "why?". I am happy that he feels the same way as I do about it. Starting at around 1 minute in. If you actually read the posts in the "why" hits on PF, you will usually find that the responses to the Why question tend not to show a lot of sympathy from the majority of PF posters. When someone demands "why" it is because they want a specific but easy answer. There never is one as one answer leads to another question - ad infinitum. Science is the journey and not the arrival at some celestial city of knowledge. The more of it that you know, the more you realise that about Science.

Anywhere you care to look for a definition of a "magnetic pole" is is always defined in terms of an approximate region where the "lines of force" are closest together or the field is strongest. That is an open statement and is as far as they go. It isn't the place that a compass will point to; the (imaginary) lines of force follow curves which pass through a region - at one end or the other (or whatever) but they don't point at it. The term is vague and simply describes what a casual observer will experience in a kitchen table-top experiment; nothing more. Why would I need a reference that uses the word "imaginary", to justify the term I used? Would you say that a place 1mm / 1cm / 1m from the place that someone would call the Pole of a magnet would still be the pole?
Stick to Maxwell's equations which describe the pattern of things and you can use them to predict the outcome of any classical situation you care to invent (subject to time and computing power). Maxwell doesn't tell you why.

19. Nov 23, 2015

### rude man

Ah, no. All of electricity and magnetism incl. the Maxwell relations, are the result of empirical evidence.

And gravity is yet to be explained at all. Big Nobel if you can do it.

We are not relegated to the stone age and yet all we know is empirical.

20. Nov 23, 2015

### sophiecentaur

The field from a bar magnet doesn't 'stop' at the faces with N and S painted on them. Those lines carry on through the magnet (as with a solenoid) to form a continuous loop. The Poles are just where the external field is strongest. In the case of a current carrying wire, the lines are continuous loops which do not bunch together to form poles. Any book (or website) on elementary electromagnetic theory will show you that, along with all the relevant diagrams. Don't try to get all your knowledge from asking questions on PF (especially when you argue with the answers). It is easier not to get offended with what's written in a book because you can;t get into an argument with it.