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Transcript
Electromagnetic Waves Electric Charge Two types: positive and negative Like charges repel, opposites attract Forces come in a matched pair Each charge pushes or pulls on the other Forces have equal magnitudes and opposite directions Forces increase with decreasing separation Charge is quantized Charge is an intrinsic property of matter Electrons are negatively charged (-1.6 x 10-19 coulomb’s each) Protons are positively charged (+1.6 x 10-19 coulomb’s each) Net charge is the sum of an object’s charges Most objects have zero net charge (neutral – equal numbers of + and -) Electric Fields Charges push on each other through empty space Charge one creates an “electric field” This electric field pushes on charge two An electric field is a structure in space that pushes on electric charge The magnitude of the field is proportional to the magnitude of the force on a test charge The direction of the field is the direction of the force on a positive test charge Magnetic Poles Two Types: north and south Like poles repel, opposites attract Forces come in matched pairs Forces increase with decreasing separation Analogous to electric charges EXCEPT: No isolated magnetic poles have ever been found! Net pole on an object is always zero! Most atoms are magnetic, but most materials are not Atomic magnetism is perfectly cancelled Material is indifferent to nearby magnetic poles Some materials do not have full cancellation Ferromagnetic materials respond to magnetic poles Magnetic Fields A magnetic field is a structure in space that pushes on magnetic poles The magnitude of the field is proportional to the magnitude of the force on a test pole The direction of the field is in the direction of the force on a north test pole Electromagnetism (part 1) Magnetic fields Push only on magnetic poles Are produced by magnetic poles Can also be produced by changing electricity! Electric fields Push only on electric charges Are produced by electric charges Can also be produced by changing magnetism Electromagnetism (part 2) Magnetism created by Poles (but isolated poles don’t seem to exist Moving electric charges Changing electric fields Electricity created by Charges Moving magnetic poles Changing magnetic fields Electromagnetic Waves Accelerating charge emits electromagnetic waves Charge produces electric field Current (moving charge) produces magnetic field Changing current (accelerating charge) produces changing magnetic field, which produces changing electric field, which produces changing magnetic field, which produces changing electric field, which … An electromagnetic wave consists only of an electric and magnetic field An electromagnetic wave travels through empty space at the speed of light Structure of Electromagnetic Waves Electric filed is perpendicular to magnetic field Electric field creates magnetic field and vice versa Electric field determines polarization of the wave Electromagnetic Spectrum Long-wavelength electromagnetic waves: Radio and microwave Medium-wavelength: infrared, visible, and ultraviolet light Short-wavelength: X-rays and Gamma-rays