# The direction of a magnetic field?

1. Sep 19, 2015

### sphear

my questions are rather simple and stupid and i won't delve into theory like the last times

how do we know the magnetic field lines are pointing inward on south and outward on north instead of inward on north and outward on south?

or for this rule it's really the same question, how do we know the direction of force is up? how do we know the direction of force isn't really down and the right hand rule is reversed?

i'm just asking for the scientific explanation because i have no clue on this, ty

2. Sep 19, 2015

### Staff: Mentor

Because that is how we defined it.

3. Sep 19, 2015

### sphear

does that mean it's irrelevant which way it goes or does that mean we just don't know? i mean it IS irrelevant either way I guess, i wouldn't know, the magnetic field should go in one side and out the other in a north/south magnet, isn't it important to know which is which just for the sake of knowing?

...thank you good sir...

4. Sep 19, 2015

### Staff: Mentor

It is simply a matter of convention. Just like we decided that an electron is negative and therefore electric field lines go in to an electron.

5. Sep 19, 2015

### sphear

i meant to ask this before, but i kind of forgot

so it's just random then... news to me

still i thought there was truth to the magnetic field lines at least as far as magnets were concerned, two lines pointing the same direction= attraction and the opposite when pointing opposite directions

is it just that the lines are not real and just for illustration purposes and don't represent the true forces? if the lines do represent real forces and not just for illustration, isn't it unscientific to just say the lines point away from the north and in the south when we don't really know?

6. Sep 19, 2015

### Staff: Mentor

The "line" of the magnetic field is well-defined, but the choice of the "forwards" and "backwards" directions is arbitrary and has to be defined by convention, that is, it has to be agreed on by everyone who uses magnetic fields, and it has to be applied consistently in all situations. Historically, the first application of magnetism was in compasses for finding direction, and so I suspect that the direction of $\vec B$ at a given location was originally defined as the direction that the north-seeking end of a compass needle points when placed at that location.

If most of civilization (or at least the part that first started to use magnetic compasses) had arisen in the Earth's southern hemisphere, we might very well have used the south-seeking end of a compass needle to define the direction of $\vec B$, instead.

7. Sep 19, 2015

### Staff: Mentor

The lines are like contour lines on a topographic map that represent the change in elevation. The lines themselves are simply lines on a piece of paper. Instead of lines, I could use a color-coded system where red represents lower elevation and slowly changes to blue as the elevation changes. In fact, this is very much how the magnetic field is represented in many illustrations. The magnetic field is real but the lines are just lines on a picture that allow us to represent the properties of the field. Unlike contour lines on a map, magnetic field lines also require arrows because of the way the magnetic field works. By convention, we draw the arrows as pointing away from the north pole of a magnet and towards the south pole.

8. Sep 19, 2015

### Staff: Mentor

I definitely would not describe it as random. That implies that it might change from observation to observation. It is a convention. Conventions are arbitrary, not random.

That holds regardless of the convention.

The magnetic field is not a force field. The lines of the magnetic field do not represent forces, they are perpendicular to the force.

I already told you that we do really know. We know because we defined it. This isn't a fact of nature waiting to be discovered and only measured with some uncertainty or error. This is a human invention, completely known and clearly decided.

Would you consider it unscientific to say that electric field lines point away from positive charges? Or would you consider it unscientific to say that electrons are negatively charged?

Last edited: Sep 19, 2015
9. Sep 20, 2015

### UncertaintyAjay

For the right hand rule- there is a reason. It's called the cross product. F= qV×B is the vector form of the equation. Force is essentially the cross product of velocity and magnetic field which is why in the diagram, its upwards,

10. Sep 20, 2015

### Staff: Mentor

If the direction of $\vec B$ had originally been defined the opposite of the way it is now understood, then we would have (later) defined the magnetic force law as $\vec F = -q \vec v \times \vec B$. (Assuming of course that we used the same convention for + versus - charges, which was originally a random guess by Benjamin Franklin.)

Last edited: Sep 21, 2015
11. Sep 20, 2015

### UncertaintyAjay

Ahhh, can't escape convention

12. Sep 21, 2015

### sphear

Ok, well I have been having computer trouble and it's the first day i've slept well in a while so i just realized that i phrased this question all wrong so please let me start over, also maybe i didn't phrase it wrong and the answer is convention

I realize that we can choose by convention that we CALL the north side the magnetic field leaves the magnet when we could CALL that the south side

but my main question was how do we KNOW, scientifically, that the north side has the magnetic field exiting on the side of the magnet we call north

shouldn't you be able to scientifically test is by the way magnets react with charged particles? see my question was much stupider than you would think

Or this one here:

How do we know which direction the the charge along the wire is moving scientifically? How do we know which direction, scientifically that the lines on the LHC image are pointing? see stupid questions kind of, but i'm hoping someone will bare with me

13. Sep 21, 2015

### Staff: Mentor

No. The problem is that we have to know whether the test particle we're using is positively charged or negatively charged - and that's the same conventional choice, because we determine whether the test particle is positively or negatively charged by looking at the direction it deflects in the magnetic field.

Gotta take a look at this: https://xkcd.com/567/

14. Sep 21, 2015

### Staff: Mentor

The magnetic field doesn't exit the magnet. Nor does it enter the magnet. The direction of the arrows just lets us calculate the direction of the force a charged particle will feel when moving through the magnetic field (among a few other things).

Last edited: Sep 21, 2015
15. Sep 21, 2015

### Staff: Mentor

That is a convention.

Calling one side of a magnet north and the other south is a convention. It could be reversed without changing the outcome of any experiment.

Calling an electron negative and a proton positive is a convention. It could be reversed without changing the outcome of any experiment.

Having field lines leave the north pole and enter the south pole is a convention. It could be reversed without changing the outcome of any experiment.

16. Sep 30, 2015

### sphear

should i not be asking another question related to my main question because this thread is a little old? it isn't locked yet so i was thinking it would be ok
it took me too long to get the convention thing, yeah i used to know that years ago but i forgot

why does the aluminum core rotate in this video?:

the copper coil is creating a north to south magnetic field in the aluminum that matches its own field, the magnetic field always seems to be perpendicular and 90 degrees to the action, like it's perpendicular to the flow of electrons in a wire

so the action in the aluminum is along the east and west axis because it's spinning in that direction? for example in an iron bar is the magnetic domain going east to west (more electrons pointing/spinning) along the east and west axis and that produces a perpendicular and 90 degree magnetic field in the north to south axis?
ty

Last edited: Sep 30, 2015