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Microwave Ovens

Introductory Question. If you put a CD in a microwave oven, it willdo nothing.burn up the microwave oven.burn up the CD.. Observations About Microwaves. Microwave ovens cook food from inside outThey can cook foods unevenlyThey don't defrost foods wellYou shouldn't put metal inside them?!Do t

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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. 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 doesn’t heat due to orientational stiffness Steam doesn’t heat due to lack of “friction” Food’s 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 Demonstration: Heat “Magic Lights” in Microwave Oven Demonstration: Heat Clear Light Bulb in Microwave OvenDemonstration: Heat “Magic Lights” in Microwave Oven Demonstration: Heat Clear Light Bulb in Microwave Oven

    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. Demonstration: Burn up CD in the microwave oven Demonstration: Heat burning wooden match in the microwave oven to make plasma ballsDemonstration: Burn up CD in the microwave oven Demonstration: Heat burning wooden match in the microwave oven to make plasma balls

    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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