Re: [Phys-l] Microwave oven experiment

[Brian Whatcott <betwys1@sbcglobal.net>]

Brian Whatcott wrote:
> Craft, Peter wrote:
>
> Several staff at our school have just conducted an experiment to 
> measure the velocity of
> microwaves using a domestic microwave oven. They followed the well 
> known method of
> placing chocolate in a tray and running the microwave until spots 
> appear in the chocolate
> where the antinodes occur in the standing waves. These should occur at 
> 1/2 lambda or
> around 6-6.5 cm to give the "desired" velocity. In both cases the hot 
> spots
> were well defined at 11.5cm or 1 complete wavelength. The microwave 
> oven states that it
> operates with a frequency of 2450 Hz.
>
> Could anyone advise us if we are making an error or explain why we are not
> getting the expected results.
>
> Thanks
>
> Peter Craft
>
> I suggest that identifying microwave hot spots as antinodes may be the 
> issue.
> If you place a loudspeaker in a room with hard walls, floor and 
> ceiling, there will
> be intensity peaks at particular spots. It would be unwise to identify 
> these
> locations only with the sound wavelengths in use. Details of room 
> structurewould reposition these hot spots, after all.
>
> Brian W
Hmm…on reconsideration, it appears that my first response exemplifies an 
answer of the type sometimes attributed to either management or 
engineers (your preference) – as being true but unhelpful – so I’ll try 
again.
People who design microwaves set out to even out the microwave exposure 
throughout the volume. They use two methods:

1) A microwave stirrer – which consists of a metallic paddle rotating in 
the waveguide.
2) a rotary plinth.

If you can disable the mode-stirrer by disconnecting the geared motor in 
the oven roof, and turn off the plinth, you will have a better 
opportunity to observe intensity peaks.

Brian W