What magnets to choose

[jewelergirl]

My daughter is doing a 3rd grade science fair project on magnetic levitation. It is a variation of a project we found on the net. We are just starting it--this is her first real science fair project (I have never participated in one either).

Her hypothesis is that she can suspend a metal ring in the air using magnetic force. She wants to explore how heavy the ring can be before the magnetic force will no longer support levitation and the ring falls.

Her design is to have a dowel (12" to 18") set into a base board. At the top of the dowel she wants to nail/glue a light (balsa wood?) platform to extend forward approx 6 inches. In the platoform she wants to glue or somehow attach a magnet. From the base she wants to anchor a piece of light fishing line tied to a ferrous metal ring that will attract to the magnet in the top platform. She wanted to levitate something sizeable for dramatic effect, but it can't be heavy so we came up with the idea of a wire ring. Her hope is that she can use the pull of the magnet to make the ring rise, but the fishing line will tether it to the base so the ring is several inches short of being able to connect with the magnet, causing the ring to appear to be levitating. I hope this descibes it properly.

Our question is in regard to the type of magnet to buy. I will need to order it online so don't have a way to test them out. We looked at the magnets availabe at the local craft store but they were far too weak to do what we need it to. In researching we found neodymium magnets. These look like they would work, but have strong warnings about safety, so it seems like the smaller the magnet the safer. If we use a metal ring that is approx. one ounce in weight, how strong of a magnet do we need to get in order to have enough "pull" for this project? For example--the little fridge magnets we have at home have to be a few millimeters away before they exert much pull- we need a magnet that will pull and hold and ounce from several inches away. I see that the magnets have ratings in pounds, but am not really sure what that refers to in practical terms.

Any help would be greatly appreciated. Thank you

[rmarz]

jewelgirl - You might want to go back to the experiment you first saw for some ideas. I was curious and experimented with some neodymium magnets I had in the garage. While they will suspend any number of ferrous items; washers, nuts, screws etc., the suspension only occurs when the article is about 1/4" or 6mm from the magnet. I used magnets rated between 10 and 50 pounds pull. Pretty much the same result. It seems that the flux density falls off pretty rapidly, even for relatively strong magnets. I don't think your goal of an attraction force of 1 oz over several inches of separation is achievable. Also, the strength of attraction between this critical distance of 6mm and contact with the magnet quickly goes from 1 oz (or the weight of the suspended object) to 10 pounds (the strength of the magnet once it latches on). This suggests that a balsa wood beam, several inches in length would quickly deflect or break at some critical point and a fraction of a millimeter in distance. Another idea would be to use 2 neodymium magnets in repulsion to show the effect strong magnets have. Some of the magnets I had in the box were round with a 1/4" hole like a doughnut. Assuming the polarity is right, they could be placed on a dowel and the top magnet would be repulsed several inches. I took a few photos as an example. The magnets I used were about 9/16" in diameter and about 1/4" thick. The magnets were rated at about 10 pounds pull. The frame I used was cut from a 2x4 piece of wood, much more rigid than the construction you contemplated. The fine nylon fish line actually began to stretch as the washer got closer to the magnet. The repuIsion setup could be a simple dowel and base. I found another source, K & J Magnetics on the web that has a levitation experiment that uses pyrolytic graphite as a diamagnetic material to achieve suspension in equilibrium. Their link is attached, and they could be a source of magnets if you have difficulty finding them locally. Good luck.

http://www.kjmagnetics.com/proddetail.asp?prod=LEV2

Rick Marz

[drenaline]

I don't know really how i got here, but i'm trying to post something to find a magnet expert? If I'm in the wrong place, I'd appreciate any and all advice on where to go - what to do.

My situation: I have a product (patented) relating to golf accessory. There is a 5"W x 3"L zipper pocket to hold wallets, balls, phones, tees, etc. Approximately 4" above the pocket is a 15lbs pull force magnet to hold a metal divot repair tool (easy access to carry to the green and place back on the magnet when done...versus holding in your pocket plus it gives my product an added value). I scrapped the concept about 2 months ago as i was afraid the magnet would wipe out credit cards or ruin phone. After again seeing a magnetic money clip, i started back on the track to research. I just don't understand if my concept will work as is? Is there a threshold of pull force that you shouldn't exceed when dealing with credit cards (i.e don't go over 11 lbs)? Is there a proximity to magnet threshold (i.e. don't get closer than 3" at 11 lbs pull force)? Is there exposue threshold (i.e. 10 sec at 11 lbs within 3" will ruin it)? Type of magnet material matter (rare earth, NiCad)? Is there any level of comfort anyone could confidently give me as to what to do so i don't enter the market and make a terrible mistake (i.e. stay under 11 lbs, 5" away and you'll be fine)? Help or direct me!