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Microwave Ovens

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Microwave ovens cook food from inside out. They can cook foods unevenly ... Magnetron tube has tank circuits in it. Streams of electrons amplify tank oscillations ... – PowerPoint PPT presentation

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Title: Microwave Ovens


1
Microwave Ovens
2
Introductory Question
  • If you put a CD in a microwave oven, it will
  • do nothing.
  • burn up the microwave oven.
  • burn up the CD.

3
Observations About Microwaves
  • Microwave ovens cook food from inside out
  • They can cook foods unevenly
  • They dont defrost foods well
  • You shouldnt put metal inside them?!
  • Do they make food radioactive or toxic?

4
Electromagnetic Spectrum
  • Long-wavelength EM waves Radio Microwave
  • Medium-wavelength IR, Visible, UV light
  • Short-wavelength X-rays Gamma-rays

5
Water Molecules
  • Water molecules are unusually polar
  • An electric field orients water molecules
  • A fluctuating electric field causes water
    molecules to fluctuate in orientation

6
Microwave Heating
  • Microwaves have fluctuating electric fields
  • Water molecules orient back and forth
  • Liquid water heats due to molecular friction
  • Ice doesnt heat due to orientational stiffness
  • Steam doesnt heat due to lack of friction
  • Foods liquid water content heats the food

7
Effects of Microwaves
  • Non-Conductors Polarization
  • Mobile, polar molecules orient and heat
  • Immobile, polar molecules do nothing much
  • Non-polar molecules do nothing much
  • Conductors Current flow
  • Good, thick conductors reflect microwaves
  • Poor conductors experience resistive heating
  • Thin conductors experience resistive heating

8
Introductory Question (revisited)
  • If you put a CD in a microwave oven, it will
  • do nothing.
  • burn up the microwave oven.
  • burn up the CD.

9
Interference
  • Identical waves that overlap can interfere
  • Interference is when the fields add or cancel
  • Adding fields are constructive interference
  • Canceling fields are destructive interference
  • Reflects cause interference in a microwave
  • Interference causes uneven cooking
  • Good microwaves stir waves or move food

10
Generating Microwaves
  • Magnetron tube has tank circuits in it
  • Streams of electrons amplify tank oscillations
  • A loop of wire extracts energy from tanks
  • A short ¼-wave antenna emits the microwaves
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