How Does Magnet Strength Change Over Distance?

[joeyh1234]

Hello Physics Forums World

this is my first post so please be gentle.

I am working on a project and wanted to understand the strength of magnets over distance.

Imagine, if you will, two magnets both with a gauss of 3600, the opposite poles are facing each other. how would I calculate the distance they have to be apart before they start to pull towards each other. Also assume that one of the magnets is in a fixed position. so only one of the magnets would move.

If any further detail is required I will respond as soon as possible.

many thanks
joe

 

[phyzguy]

Welcome! Try reading this Wikipedia entry. The force between two magnetic dipoles falls off as 1/r^4 when the separation is large compared to the size of the magnets. So there is no distance where they " start to pull towards each other". They are always attracting no matter how far apart they are, it is just that the attraction gets less and less as they get further apart. If they are sitting on a surface ( let's say a table), then in order to calculate when one magnet starts to move as you bring them together, you need to know the coefficient of static friction between the magnet and the table. The magnet will start to move when the force of magnetic attraction exceeds the frictional force.

 

[joeyh1234]

Hi, Many thanks for responding.

I will read the wiki entry later, in my project/experiment one of the magenets is fixed to a rotor blade in a mglav track so there wouldn't be any friction other than air resistance.

I suppose i was looking for a formula which I could use to determine the pulling strength of the two magnets over distance so if to begin with at 10 cm apart they was no noticeable pull but at 5 cm it did pull. I want to understand/know the relationship between the north of one and the south of the other (both 3600 gauss).

Many thanks for your earlier prompt response.

thanks
joe

 

[phyzguy]

The formula you are looking for is in the Wiki link I sent you. Again, it is approximately 1/r^4, so the attraction at 5 cm will be 16X larger than the attraction at 10 cm.