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Microwave Ovens

Explore how microwaves cook food, effect of water molecules, interference in ovens, metal response, and wave generation.

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Microwave Ovens

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  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 don’t defrost foods well • You shouldn’t put metal inside them?! • Do they make food radioactive or toxic?

  4. 3 Questions aboutMicrowave Ovens • Why do microwaves cook food? • How does metal respond to microwaves? • How does the oven create its microwaves?

  5. Question 1 • Why do microwaves cook food?

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

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

  8. Microwave Heating • Microwaves have fluctuating electric fields • Water molecules orient back and forth • Liquid water heats due to molecular “friction” • Ice doesn’t heat due to orientational stiffness • Steam doesn’t heat due to lack of “friction” • Food’s liquid water content heats the food

  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. Question 2 • How does metal respond to microwaves?

  11. 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

  12. 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.

  13. Question 3 • How does the oven create its microwaves?

  14. 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

  15. Summary aboutMicrowave Ovens • They cook food because of its water content • Polar water molecules heat in microwave fields • Thin or sharp metals overheat or spark • The microwaves are produced by a magnetronS

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