Good questions, but they have easy answers.
1. Microwaves don't penetrate conductors. The magnet will not heat up, and it will not be affected by the microwaves. Only the conduction electrons on the surface will participate in an microwave activity, and they will basically just reflect the microwaves. Most microwave ovens have a little fan hidden between the inside top surface of the oven and the microwave emitter, so 'stir' the microwaves, to avoid hot spots caused by resonance. The magnet would just look like an extra (and much smaller) stirrer.
2. Having the magnet above the metal box means that there are no eddy currents between the magnet on the motor and the magnet in the glassware. I doubt that this would be much of an issue, but I have not done the experiment, so I gave both methods. Certainly having the magnet sealed in the can would make cleaning and sterilizing easier. But you would still have to open the can to turn the device on and off, and to replace the batteries.
On Tue, Jul 9, 2013 at 3:56 PM, John Griessen <john@industromatic.com> wrote:
On 07/07/2013 01:30 PM, Simon Quellen Field wrote:
Has anyone on the list built their own magnetic stirrer yet?
no, but...
A motor, a bar magnet glued to the axle, a couple batteries, and a non-ferrous metal enclosure to prevent the microwaves from
cooking the batteries, and you are all done. Add a rheostat to control the speed.
What would the interaction of the bar magnet field and the microwaves be like? What I can find on resonances suggests
iron can move and have resonances much higher than the 2.4GHz of microwave oven, so they would interact, but what are the relative strengths of fields? Does the 2.4GHz E & M field peak out higher than the bar magnet, or the other way around?
How much would the bar magnet field bend and rearrange the microwaves?
You could drill a hole in the metal box for the
axle and glue the magnet on the outside of the metal box, then put a glass plate above it to hold the beaker to be stirred.
What would that get you vs. the bar magnet inside a brass can?
Supposing you have a strong magnet rotating under a beaker of water in a
microwave oven that's on for 30 seconds, will you get:
a. better diffuse heating by magnetically deflected microwaves.
b. A hot bar magnet.
c. all of the above.
d. none of the above.
e. a bar magnet with decreased permanent magnetism.
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