Microwave Ovens 1 Microwave Ovens

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

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

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

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

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

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

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

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

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

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

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

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

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

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