Microwave Ovens Microwave Ovens They often cook foods unevenly - PDF document

Microwave Ovens 1 Microwave Ovens 2 Observations About Microwaves Observations About Microwaves � Microwave ovens cook food from inside out Microwave ovens cook food from inside out Microwave Ovens Microwave Ovens � They often cook foods unevenly They often 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 3 Microwave Ovens 4 3 Questions about 3 Questions about Question 1 Question 1 Microwave Ovens Microwave Ovens � Why do microwaves cook food? Why do microwaves cook food? � 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 Microwave Ovens 6 Electromagnetic Spectrum Electromagnetic Spectrum Water Molecules Water Molecules � Microwaves are a class of electromagnetic waves Microwaves are a class of electromagnetic waves � Water molecules are Water molecules are � unusually polar unusually polar � Long Long- -wavelength EM waves: Radio & Microwave wavelength EM waves: Radio & Microwave � Medium Medium- -wavelength: IR, Visible, UV light wavelength: IR, Visible, UV light � An electric field orients An electric field orients water molecules water molecules water molec les water molec les � Sh rt � Short Short-wavelength: X Sh rt wavelength: X-rays & Gamma l n th X l n th X r rays & Gamma- r & G mm r & G mm -rays rays � A fluctuating electric field A fluctuating electric field causes water molecules to causes water molecules to fluctuate in orientation fluctuate in orientation 1

Microwave Ovens 7 Microwave Ovens 8 Microwave Heating Microwave Heating Interference Interference � Microwaves have alternating electric fields Microwaves have alternating electric fields � Identical waves that overlap can interfere Identical waves that overlap can interfere � Water molecules orient back and forth Water molecules orient back and forth � Interference is when the fields add or cancel Interference is when the fields add or cancel � Liquid water heats due to molecular “friction” Liquid water heats due to molecular “friction” � Adding fields are constructive interference Adding fields are constructive interference � Canceling fields are destructive interference C Canceling fields are destructive interference C li li fi ld fi ld d d i i i i f f � Ice doesn’t heat due to Ice doesn’t heat due to orientational orientational stiffness stiffness � Reflections lead to interference in a microwave Reflections lead to interference in a microwave � Steam doesn’t heat due to lack of “friction” Steam doesn’t heat due to lack of “friction” � Interference causes uneven cooking Interference causes uneven cooking � Food’s liquid water content heats the food Food’s liquid water content heats the food � Most ovens “stir” the waves or move the food Most ovens “stir” the waves or move the food Microwave Ovens 9 Microwave Ovens 10 Question 2 Question 2 Effects of Microwaves Effects of Microwaves � How does metal respond to microwaves? How does metal respond to microwaves? � Non Non- -Conductors: Polarization 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 � N n Non-polar molecules do nothing much N n p l r m l polar molecules do nothing much p l r m l l l d n thin m d n thin m h h � 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 � Sharp conductors initiate discharges in the air Sharp conductors initiate discharges in the air Microwave Ovens 11 Microwave Ovens 12 Question 3 Question 3 Generating Microwaves Generating Microwaves � How does the oven create its microwaves? How does the oven create its 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 A short ¼- -wave antenna emits the microwaves wave antenna emits the microwaves 2

Microwave Ovens 13 Summary about Summary about Microwave Ovens 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 3