1 / 15

Microwave Ovens

Microwave Ovens. Introductory Question. If you put a CD in a microwave oven, it will do nothing. burn up the microwave oven. burn up the CD. Observations About Microwaves. Microwave ovens cook food from inside out They can cook foods unevenly They don’t defrost foods well

hawkinsr
Download Presentation

Microwave Ovens

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related