Microwave Ovens Microwave Ovens do nothing. do nothing. A. A. - PDF document

Microwave Ovens 1 Microwave Ovens 2 Introductory Question Introductory Question If you put a CD in a microwave oven, it will If you put a CD in a microwave oven, it will � Microwave Ovens Microwave Ovens do nothing. do nothing. A. A. burn up the microwave oven. burn up the microwave oven. B. B. burn up the CD. burn up the CD. C. C. Microwave Ovens 3 Microwave Ovens 4 3 Questions about 3 Questions about Observations About Microwaves Observations About Microwaves Microwave Ovens Microwave Ovens � Microwave ovens cook food from inside out Microwave ovens cook food from inside out � Why do microwaves cook food? Why do microwaves cook food? � � They often cook foods unevenly They often cook foods unevenly � How does metal respond to microwaves? How does metal respond to microwaves? � They don’t defrost foods well They don’t defrost foods well � How does the oven create its microwaves? How does the oven create its microwaves? � 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 5 Microwave Ovens 6 Question 1 Question 1 Electromagnetic Spectrum Electromagnetic Spectrum � Why do microwaves cook food? Why do microwaves cook food? � Microwaves are a class of electromagnetic waves Microwaves are a class of electromagnetic waves � � Long Long- -wavelength EM waves: Radio & Microwave wavelength EM waves: Radio & Microwave � Medium Medium- -wavelength: IR, Visible, UV light wavelength: IR, Visible, UV light � Sh rt � Short Sh rt Short-wavelength: X wavelength: X- l n th X l n th X r -rays & Gamma r rays & Gamma- & G mm & G mm r -rays rays 1

Microwave Ovens 7 Microwave Ovens 8 Clicker Question Clicker Question Clicker Question Clicker Question � A US cell phone can communicate using waves A US cell phone can communicate using waves � Wireless internet (Wi Wireless internet (Wi- -Fi) communication is done Fi) communication is done at 850 MHz (35 cm wavelength). How long at 850 MHz (35 cm wavelength). How long at 2.54 GHz (11.8 cm wavelength). How long is at 2.54 GHz (11.8 cm wavelength). How long is should a resonant cell phone antenna be? should a resonant cell phone antenna be? a resonant Wi- a resonant Wi -Fi antenna? Fi antenna? A. 35 cm (14 inches) 35 cm (14 inches) A. 11.8 cm (4.6 inches) 11.8 cm (4.6 inches) A. A. B. 17.5 cm (7 inches) 17.5 cm (7 inches) B. 5.9 cm (2.3 inches) 5.9 cm (2.3 inches) B. B. C. 8.7 cm (3.5 inches) 8.7 cm (3.5 inches) C. 2.9 cm (1.2 inches) 2.9 cm (1.2 inches) C. C. D. 4.4 cm (1.7 inches) 4.4 cm (1.7 inches) D. 1.5 cm (0.6 inches) 1.5 cm (0.6 inches) D. D. Microwave Ovens 9 Microwave Ovens 10 Clicker Question Clicker Question Clicker Question Clicker Question � As the communication wave from a Wi As the communication wave from a Wi- -Fi Fi � As the communication wave from a Wi As the communication wave from a Wi- -Fi Fi transmitter passes an electric charge, what transmitter passes an electric charge, what transmitter passes an electric dipole (a + charge transmitter passes an electric dipole (a + charge happens to that electric charge? happens to that electric charge? attached to an equal – attached to an equal – charge), what happens to charge), what happens to that dipole? that dipole? that dipole? that dipole? A. It accelerates away from the transmitter It accelerates away from the transmitter A. A. It shakes back and forth It shakes back and forth B. It accelerates toward the transmitter It accelerates toward the transmitter A. B. B. It twists back and forth It twists back and forth C. It shakes back and forth It shakes back and forth B. C. C. It accelerates away from the transmitter It accelerates away from the transmitter D. Nothing Nothing C. D. D. Nothing Nothing D. Microwave Ovens 11 Microwave Ovens 12 Water Molecules Water Molecules Microwave Heating Microwave Heating � Water molecules are Water molecules are � Microwaves have alternating electric fields Microwaves have alternating electric fields unusually polar unusually polar � Water molecules orient back and forth Water molecules orient back and forth � An electric field orients An electric field orients � Liquid water heats due to molecular “friction” Liquid water heats due to molecular “friction” water molecules water molecules water molec les water molec les � Ice doesn’t heat due to Ice doesn’t heat due to orientational orientational stiffness stiffness � A fluctuating electric field A fluctuating electric field � Steam doesn’t heat due to lack of “friction” Steam doesn’t heat due to lack of “friction” causes water molecules to causes water molecules to � Food’s liquid water content heats the food Food’s liquid water content heats the food fluctuate in orientation fluctuate in orientation 2

Microwave Ovens 13 Microwave Ovens 14 Clicker Question Clicker Question Question 2 Question 2 � Popcorn kernels feel solid and dry, so how can Popcorn kernels feel solid and dry, so how can � How does metal respond to microwaves? How does metal respond to microwaves? those kernels pop in the microwave oven? those kernels pop in the microwave oven? A. Microwaves heat solid materials quite well Mi Mi Microwaves heat solid materials quite well h h lid lid i l i l i i ll ll A. B. Microwaves heat dry materials quite well Microwaves heat dry materials quite well B. C. Corn kernels contain some trapped liquid water Corn kernels contain some trapped liquid water C. Microwave Ovens 15 Microwave Ovens 16 Effects of Microwaves Effects of Microwaves Clicker Question Clicker Question � Non Non- -Conductors: Polarization Conductors: Polarization � A microwave oven’s cooking chamber has 6 A microwave oven’s cooking chamber has 6 sides. What are those sides made of? sides. What are those sides made of? � 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 p l r m l polar molecules do nothing much l l d n thin m d n thin m h h A. 5 are metal and 1 is plastic mesh 5 are metal and 1 is plastic mesh 5 5 l l d 1 i d 1 i l l i i h h A. � Conductors: Current flow Conductors: Current flow B. 5 are plastic and 1 is plastic mesh 5 are plastic and 1 is plastic mesh B. � Good, thick conductors reflect microwaves Good, thick conductors reflect microwaves C. 5 are metal and 1 is metal mesh 5 are metal and 1 is metal mesh C. � Poor conductors experience resistive heating Poor conductors experience resistive heating D. 5 are plastic and 1 is metal mesh 5 are plastic and 1 is metal mesh D. � 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 17 Microwave Ovens 18 Interference Interference Introductory Question (revisited) Introductory Question (revisited) � Identical waves that overlap can interfere Identical waves that overlap can interfere If you put a CD in a microwave oven, it will If you put a CD in a microwave oven, it will � � 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 do nothing. do nothing. A. A. � 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 burn up the microwave oven. burn up the microwave oven. B. B. � Reflections lead to interference in a microwave Reflections lead to interference in a microwave burn up the CD. burn up the CD. C. C. � Interference causes uneven cooking Interference causes uneven cooking � Most ovens “stir” the waves or move the food Most ovens “stir” the waves or move the food 3