# Magnetic fields

## Main Question or Discussion Point

I'm engaged in a project that requires a magnet that can attract from at least a foot away. I mistakenly thought that a larger pull force would also mean a larger field and wasted \$15 on a hook magnet with 130lbs pull force but only attracts from abut 2 inches away.

So what kind of magnet would fit my need? Is it the dimension that determines the size of the field?

Last edited:

## Answers and Replies

Related Other Physics Topics News on Phys.org
As with electric charge, you will find that the field of an infinte current sheet is uniform and independent of the distance a point is from the sheet.
Check out:
http://en.wikipedia.org/wiki/Current_sheet
Of course you cannot hope to make an infinitely big sheet but a larger sheet will result in a larger range.

Thanks for the reply LHarriger but that is way ahead of my current level. That seems like collage stuff and I'm only in 11th grade.

marcusl
Gold Member
Your hook magnet confines the fields to a small annulus between the magnet disk at center and the steel cup it's mounted in. This field drops quickly if your metal object isn't right up against the assembly. Why is it done that way? By confining the field to a tiny volume, its strength (hence pull force) goes way up.

To pull from a distance you need a magnet with "open" geometry, that is the fields extend away from the body. The most open geometry is a bar magnet (technically it's called a dipole), so look for the combination of biggest and strongest bar magnet. Beware that the pull force 1 foot away will be tiny.

Last edited:
Good info marcusl. Would you believe this? I contacted what seems to be a major supplier (http://www.google.com/url?sa=L&ai=BmISI6P24RZPPBJjmpAKMgZGDBay9gBD86dP5Ac_jnQfgzS8IABABGAEgtlQoA1DL0La3-f____8BYMneiIqYpIgTmAHvhgGYAYmXA6oBI29yZy5tb3ppbGxhOmVuLVVTOm9mZmljaWFsK2NmcysyR01MyAEBgAIB&q=http://www.adamsmagnetic.com/magnets.php&usg=__6auz0vf2ug8PWUYoGShR13vStK8=") and they were surprised by my requirement. "That's a strong magnet!" ...and they told me it would have to be custom made and cost several hundred dollars.

Last edited by a moderator:
What are those tubular magnets called that when you put a metal rod inside it will shoot the rod out. I heard they are used in car locks.

marcusl