neodynium vs. permanent iron magnets

[houran123]

I am doing a science fair experiment with magnets. I am putting the magnets in boiling water, freezing water, and dry ice to see how temperature affects their strength. Some web sites recommend using neodynium magnets for this kind of experiment, and some say to use permanent iron magnets. Can you suggest which type of magnet to use? I would like to use the type that is most affected by temperature so my results will be as clear as possible. Thank you.

[rmarz]

houran123 - A little research on the effects of temperature on magnetic materials might reveal a possible direction for your experiment. Bottom line, alnico or probably what you consider iron magnets, have the highest Curie temperature, or temperature that diminishes magnetic properties. That might not be the best choice as the suggested operating temperature extends to over 500⁰C. Neodymium, on the other hand has an operating temperature of only 80-150⁰C. On that basis, it might be that Neodymium magnetic material that exhibits the greatest change over a temperature gradient in the range you are testing. If you are using boiling water, that is only 100⁰C, so you need to select your material carefully. Perhaps using vegetable oil and a candy or deep frying thermometer can safely get you to a higher temperature for your experiment. Here are a couple references that might help in making your selection. How do you intend to measure the magnetic strength? Be careful, very hot (and cold) stuff is being used here. Gloves and glasses.

http://www.magmamagnets.com/permanent-magnet-stability
http://www.rare-earth-magnets.com/Perma ... ndbook.pdf

Rick Marz

[houran123]

Thank you for your quick reply to my son's query. We were looking at a suggested science experiment here on Sciencebuddies.org that recommended using boiling water to heat the magnet. While your idea of using cooking oil would help us reach a higher temperature, I'm not crazy about the idea of dealing with hot, splattering oil. Might it work to heat the magnet on a baking sheet in the oven instead?
Also, thank you for your input on the type of magnet to use. I think we'll go with neodymium magnets. Oh--and in response to your question about how we are measuring the magnet's strength, the Sciencebuddies experiment suggesting using a big bowl of paper clips and seeing how many each magnet could lift. My son had a different idea--he wants to secure a tape measure to the table and see from how many inches away the magnet can attract the nail. Do you think his way will work?

[rmarz]

houran123 - I understand the concern over the hot oil, my thought was that using a good thermometer would provide good temperature data. The cookie sheet idea is fine, but will you have a precise measurement of temperature? An oven setting might not sufficient. If you can borrow or obtain a digital, infra-red, non contact thermometer, that might be a workable alternative. Even better, a good oven thermometer (bi-metallic) inside the oven is probably much superior to the oven setting knob.

With regards the magnetic strength measurement, I definitely like the paper clip or steel B-B's approach as opposed to the distance required to attract a nail on a surface. Other variables are acting against you there, primarily friction between the nail and surface it is resting on. A better technique if you go that route is to suspend the nail from a string and carefully move the magnet toward the nail. As soon as the magnetic strength exceeds downward effect of gravity, the nail will start to move toward the magnet, then rapidly accelerate to it as the field strength increases. You might try both methods and correlate measurements using the two approaches. More data is better than less data when you are proving a hypothesis. Good luck.

Rick Marz

[houran123]

Rick, thank you very much for all your help!