Microwave Ovens 1

Microwave OvensMicrowave Ovens

Microwave Ovens 2

Introductory QuestionIntroductory Question

If you put a CD in a microwave If you put a CD in a microwave oven, it willoven, it will

A.A. do nothing.do nothing.

B.B. burn up the microwave oven.burn up the microwave oven.

C.C. burn up the CD.burn up the CD.

Microwave Ovens 3

Observations About Observations About MicrowavesMicrowaves

Microwave ovens cook food from Microwave ovens cook food from inside outinside out

They can cook foods unevenlyThey can cook foods unevenly They don’t defrost foods wellThey don’t defrost foods well You shouldn’t put metal inside You shouldn’t put metal inside

them?!them?! Do they make food radioactive or Do they make food radioactive or

toxic?toxic?

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3 Questions about3 Questions aboutMicrowave OvensMicrowave Ovens

Why do microwaves cook food?Why do microwaves cook food? How does metal respond to How does metal respond to

microwaves?microwaves? How does the oven create its How does the oven create its

microwaves?microwaves?

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Question 1Question 1

Why do microwaves cook food?Why do microwaves cook food?

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Electromagnetic Electromagnetic SpectrumSpectrum

Long-wavelength EM waves: Radio Long-wavelength EM waves: Radio & Microwave& Microwave

Medium-wavelength: IR, Visible, UV Medium-wavelength: IR, Visible, UV lightlight

Short-wavelength: X-rays & Gamma-Short-wavelength: X-rays & Gamma-raysrays

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Water MoleculesWater Molecules

Water molecules are Water molecules are unusually polarunusually polar

An electric field An electric field orients water orients water moleculesmolecules

A fluctuating electric A fluctuating electric field causes water field causes water molecules to fluctuate molecules to fluctuate in orientationin orientation

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Microwave HeatingMicrowave Heating

Microwaves have fluctuating electric fieldsMicrowaves have fluctuating electric fields Water molecules orient back and forthWater molecules orient back and forth Liquid water heats due to molecular Liquid water heats due to molecular

“friction”“friction” Ice doesn’t heat due to orientational Ice doesn’t heat due to orientational

stiffnessstiffness 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 foodFood’s liquid water content heats the food

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InterferenceInterference

Identical waves that overlap can interfereIdentical waves that overlap can interfere Interference is when the fields add or cancelInterference is when the fields add or cancel

Adding fields are constructive interferenceAdding fields are constructive interference Canceling fields are destructive interferenceCanceling fields are destructive interference

Reflects cause interference in a microwaveReflects cause interference in a microwave Interference causes uneven cookingInterference causes uneven cooking Good microwaves “stir” waves or move foodGood microwaves “stir” waves or move food

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Question 2Question 2

How does metal respond to How does metal respond to microwaves?microwaves?

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Effects of MicrowavesEffects of Microwaves

Non-Conductors: PolarizationNon-Conductors: Polarization Mobile, polar molecules orient and heatMobile, polar molecules orient and heat Immobile, polar molecules do nothing muchImmobile, polar molecules do nothing much Non-polar molecules do nothing muchNon-polar molecules do nothing much

Conductors: Current flowConductors: Current flow Good, thick conductors reflect microwavesGood, thick conductors reflect microwaves Poor conductors experience resistive heatingPoor conductors experience resistive heating Thin conductors experience resistive heatingThin conductors experience resistive heating

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Introductory Question Introductory Question (revisited)(revisited)

If you put a CD in a microwave If you put a CD in a microwave oven, it willoven, it will

A.A. do nothing.do nothing.

B.B. burn up the microwave oven.burn up the microwave oven.

C.C. burn up the CD.burn up the CD.

Microwave Ovens 13

Question 3Question 3

How does the oven create its How does the oven create its microwaves?microwaves?

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Generating MicrowavesGenerating Microwaves

Magnetron tube has tank circuits in Magnetron tube has tank circuits in itit

Streams of electrons amplify tank Streams of electrons amplify tank oscillationsoscillations

A loop of wire extracts energy from A loop of wire extracts energy from tankstanks

A short ¼-wave antenna emits the A short ¼-wave antenna emits the microwavesmicrowaves

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Summary aboutSummary aboutMicrowave OvensMicrowave Ovens

They cook food because of its water They cook food because of its water contentcontent

Polar water molecules heat in Polar water molecules heat in microwave fieldsmicrowave fields

Thin or sharp metals overheat or Thin or sharp metals overheat or sparkspark

The microwaves are produced by a The microwaves are produced by a magnetronSmagnetronS