- #1
jds10011
- 38
- 0
Hi all,
I teach high school physics, and as usual I welcome the opportunity for someone out there to set me straight. A common (and somewhat silly) demo involves taking a microwave, removing the rotating tray, and microwaving a plate of marshmallows for a brief time to observe the irregular pattern of heating the microwave produces. By claiming the melted spots are anti-nodes, and measuring the distance between them, one theoretically observes half the wavelength of the standing wave set up within. Then, in combination with the frequency printed on the back of the unit (usually 2450 MHz), one can use v=f(lambda) to calculate the speed of light. Obviously this is circular reasoning, and I don't care to belabor that point a great deal, other than to say that it's a neat way to introduce a concept and kids like marshmallows; I'm very clear to the students about this. However, the problem is that I (and a number of others I've found online) consistently measure about 12 cm between melted spots, when it seems one ought to measure 6 cm if the math is to come out correctly. Care to let me know what I'm missing? I'm concerned this may be related to some debate I've heard regarding misconceptions about microwaves and resonance, although some more mundane misconception about the existence of reflectors/fans in the microwave (or how the magnetron works altogether) is also relatively likely.
Thoughts?
A typical description (from a journal) is here: http://www.physics.umd.edu/icpe/newsletters/n34/marshmal.htm
Thanks!
I teach high school physics, and as usual I welcome the opportunity for someone out there to set me straight. A common (and somewhat silly) demo involves taking a microwave, removing the rotating tray, and microwaving a plate of marshmallows for a brief time to observe the irregular pattern of heating the microwave produces. By claiming the melted spots are anti-nodes, and measuring the distance between them, one theoretically observes half the wavelength of the standing wave set up within. Then, in combination with the frequency printed on the back of the unit (usually 2450 MHz), one can use v=f(lambda) to calculate the speed of light. Obviously this is circular reasoning, and I don't care to belabor that point a great deal, other than to say that it's a neat way to introduce a concept and kids like marshmallows; I'm very clear to the students about this. However, the problem is that I (and a number of others I've found online) consistently measure about 12 cm between melted spots, when it seems one ought to measure 6 cm if the math is to come out correctly. Care to let me know what I'm missing? I'm concerned this may be related to some debate I've heard regarding misconceptions about microwaves and resonance, although some more mundane misconception about the existence of reflectors/fans in the microwave (or how the magnetron works altogether) is also relatively likely.
Thoughts?
A typical description (from a journal) is here: http://www.physics.umd.edu/icpe/newsletters/n34/marshmal.htm
Thanks!