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How to get the frequency of microwave oven

  1. Jun 18, 2012 #1
    I had a phone interview. People asked me how to get the frequency of microwave oven if I do not know. I said measure it. They said, you are not allowed to use any measurement tools. Then I said, read product spec. They said, no, you do not have the spec. Finally I said I do not know. I think they are asking me something related to eddy current according to the questions after that.

    Back to my original question, anybody knows the answer?

  2. jcsd
  3. Jun 18, 2012 #2
    Is the interview over?
  4. Jun 18, 2012 #3
    They invited me for the onsite interview though I wasnt able to answer that question.

    I want to know also because of my curiosity. I asked people from my lab. And my friends told me that I should tell them, ask GOD for the answer if you are not allow to use the measurement tools.
  5. Jun 18, 2012 #4
    Reverse the unknowns and assume you know the speed of light.
  6. Jun 18, 2012 #5
    Here is the answer: look up the molecular absorption peak of the water molecule in the microwave band.

    It's 2450 MHz. The oven is designed to heat water.
  7. Jun 19, 2012 #6
    I c. Thanks. I should have thought of it at that time.
  8. Jun 19, 2012 #7
    Nope. What you should have thought of at the time was googling "frequency used by microwave oven".

    The microwave oven does *not* operate at the microwave absorption peak of water.
  9. Jun 19, 2012 #8
    I did. Most explanations said that it was because that frequency water molecules resanant. I think what they wanted to test me is that whether I understand EM heating.

    So what is your answer?
  10. Jun 19, 2012 #9


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    There is a method, though I'm a bit vague on the details. You sprinkle small marsmallows or chocolate drops all over the plate, and fix the turntable so it does not rotate. Switch the microwaves on and you'll observe the confectionary does not melt evenly across the plate, rather it melts in a regular pattern. In the cavity of the oven, the radiation sets up standing waves and the distance apart of the melt lines will be λ (or maybe it's λ/2, I just forget.)

    It's likely that I will encounter this again in the next month or so, and I'll return to this thread when/if I do so.
  11. Jun 19, 2012 #10
    From Wikipedia: http://en.wikipedia.org/wiki/Microwave_oven
    "Microwave heating is sometimes explained as a resonance of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 GHz.[11] Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz—wavelength 328 millimetres (12.9 in)—also heat water and food perfectly well.[12]"

    If microwave ovens were tuned to the resonant frequency of water, nearly all of the energy of the microwaves would be dissipated in the first few millimeters of the food, burning the outside and leaving the inside of the food cold. By using a non-resonant frequency, the food is heated more evenly.
  12. Jun 19, 2012 #11


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    Staff: Mentor

    Very clever! :biggrin:
  13. Jun 19, 2012 #12
    What is your question?
    You started this post asking how to "get" the frequency of a microwave (ambiguous language). Next you proposed physically measuring it or looking in a product spec. These indicate you are seeking to know what freq is used in a microwave oven right?. Next someone (me) suggests the obvious (Google), and you say that you have already done this.

    If you are actually trying to find out *why* 2.45GHz was chosen, how would you expect to discover this with a "measurement tool"? You would really expect this to be in a product spec?
  14. Jun 21, 2012 #13
    I am sorry if I sound rude. The question I was being asked was "since you do not know the frequency microwave oven operates, now tell me how you can find it out".

    I did googled the question but not in English and the answer got was related to water. When I said I had already googled, what was your answer means the answer I've got from the internet was related to water, I do not know. Can you let me know if you know the right one?

    Anyway, my apologies for being not nice.
  15. Jun 21, 2012 #14


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    2017 Award

    This link is quite informative, I think, if you want to know the whys and wherefores of the business.
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