Microwave Ovens 1 Microwave Ovens Microwave Ovens 2

Microwave Ovens 1 Microwave Ovens

Microwave Ovens 2 Introductory Question n If you put a CD in a microwave oven, it will A. do nothing. burn up the microwave oven. burn up the CD. B. C.

Microwave Ovens 3 Observations About Microwaves Microwave ovens cook food from inside out n They can cook foods unevenly n They don’t defrost foods well n You shouldn’t put metal inside them? ! n Do they make food radioactive or toxic? n

Microwave Ovens 4 3 Questions about Microwave Ovens Why do microwaves cook food? n How does metal respond to microwaves? n How does the oven create its microwaves? n

Microwave Ovens 5 Question 1 n Why do microwaves cook food?

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

Microwave Ovens 7 Water Molecules Water molecules are unusually polar n An electric field orients water molecules n A fluctuating electric field causes water molecules to fluctuate in orientation n

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

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

Microwave Ovens 10 Question 2 n How does metal respond to microwaves?

Microwave Ovens 11 Effects of Microwaves n Non-Conductors: Polarization Mobile, polar molecules orient and heat n Immobile, polar molecules do nothing much n Non-polar molecules do nothing much n n Conductors: Current flow Good, thick conductors reflect microwaves n Poor conductors experience resistive heating n Thin conductors experience resistive heating n

Microwave Ovens 12 Introductory Question (revisited) n If you put a CD in a microwave oven, it will A. do nothing. burn up the microwave oven. burn up the CD. B. C.

Microwave Ovens 13 Question 3 n How does the oven create its microwaves?

Microwave Ovens 14 Generating Microwaves Magnetron tube has tank circuits in it n Streams of electrons amplify tank oscillations n A loop of wire extracts energy from tanks n A short ¼-wave antenna emits the microwaves n

Microwave Ovens 15 Summary about Microwave Ovens They cook food because of its water content n Polar water molecules heat in microwave fields n Thin or sharp metals overheat or spark n The microwaves are produced by a magnetron. S n