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Faraday Cages and Microwaves Shielding Communications, Cooking Microwave Oven Mysteries Spring 2006 UCSD: Physics 8; 2006 2 Shielding and Faraday Cages ã What keeps microwaves in the microwave? ã Why is cell reception terrible in elevators? ã Why is it safe to be in a car in a lightning storm? ã How can satellite dishes work with just a mesh you can see through? ã All of these relate to the behavior of metal in the presence of an electric field – and electromagnetic radiation c
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  Faraday Cages and Microwaves Shielding Communications, Cooking Microwave Oven Mysteries  Spring 2006 UCSD: Physics 8; 2006 2 Shielding and Faraday Cages ã What keeps microwaves in the microwave? ã Why is cell reception terrible in elevators? ã Why is it safe to be in a car in a lightning storm? ã How can satellite dishes work with just a mesh you can see through? ã  All of these relate to the behavior of metal in the presence of an electric field  – and electromagnetic radiation consists (at least partly) of an oscillating electric field  Spring 2006 UCSD: Physics 8; 2006 3 Electrons on the move ã Electrons are free to move in metals  – this is why they are good conductors ã Instantly respond to electric field, moving accordingly ã Imagine a conducting sphere placed in an electric field: + + + + +                 E-field electrons flow against E-field, leaving net + on top, and   on bottom, where they cluster result is internal electric field that is exactly equal and opposite external field  Spring 2006 UCSD: Physics 8; 2006 4 Internal Electric Field is Zero ã Net result is zero electric field inside conducting sphere ã Same thing happens even if the sphere is hollow:  – the electrons are all pushed to the surface anyhow  – so a metal box is also a perfect shield + + + + +                
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