# B Magnetic field strenght - confusion here

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1. Apr 7, 2016

### holiwis123

I'm not sure how magnetic field strenghts and dipoles work. I know that the strenght decreases by 1/r^3, but
- isn't magnetic field strenght a constant inherent to the magnet?
- if it is not constant, what is?
- how is all this related to magnetic damping force?

2. Apr 7, 2016

### blue_leaf77

That can't be true, the magnetic field of a localized current distribution is a function of space. If you have a magnet, do you think that it can attract anything with equal strength, regardless of the distance from this magnet?
I thought you know that it decreases by a factor of inverse cubic.

3. Apr 7, 2016

### holiwis123

okay right, makes sense. Then, shouldn't the magnet have some tipe of constant, that divided over 1/r^3 would give the strenght with which it would attract something at r distance?

4. Apr 7, 2016

### blue_leaf77

From what I understand, it sounds like you are looking for some parameters which are typically used to specify the attractability of the magnet. I am not particularly familiar with how a commercial magnet is specified, but I think the magnetic permeability and probably the magnetic moment are among them. By the way, the inverse cubic dependency of the magnetic field of a magnetic dipole is derived for large distances. One cannot immediately apply it to everyday cases of metal objects in the vicinity of a magnet.

5. Apr 7, 2016

### holiwis123

so what would you use? the experiment I'm planning is a magnet going down a slope of aluminium. if the magnet is directly in contact with the slope, it goes really slowly, and as I increase the distance by putting card board layers, the velocity increases. Which dependeny is there in this case?

6. Apr 7, 2016

### blue_leaf77

Aluminium has a very weak magnetic property, in your experiment I doubt the magnetic property is the culprit causing the magnet to slow down. I suspect it's merely due to the nature of the surfaces of the aluminium and the cardboard such that the cardboard has lower coefficient of friction.

7. Apr 7, 2016

### holiwis123

No! It actually works quite well, there is an increase in speed the more cardboard are there. (i.e. if there's only one cardboard, it goes pretty slow, with three is faster, with eight even more)

8. Apr 7, 2016

### blue_leaf77

There are two factors which come to my mind. First is that a relative movement between certain metal, one of which is aluminium, with a magnet can induce a current flowing in the metal. This induced current interact with the magnetic field of the magnet such that a magnetic force is exerted upon the objects (Lens law). I haven't analyzed this case, but if the force is such that it attracts the magnet toward the Al surface, then the sliding magnet will feel more normal force. This necessarily increase the effect of surface friction and thus, the closer the magnet to the surface is, the stronger the friction force.
Second, it's possible that the friction force between the cardboard and Al surface is not a linear function of the mass, as it is usually assumed in typical sliding box problem. In this case, the acceleration will have the form
$$a(m) = \frac{mg\sin\theta - f(m)}{m}$$
The friction force $f(m)$ may be such that $a(m)$ is an increasing function of $m$.

9. Apr 7, 2016

### holiwis123

I forgot to mention I'm using a neodymium magnet and not a normal one. About the "the closer the magnet to the surface is, the stronger the friction force.", the friction force, as the interaction of both materials (cardboard and magnet) is always the same, what changes is another factor called magnetic damping coefficient and that depends on speed, but thanks anyway for the help(: