天文学课件之17.pptVIP

Electric Energy New engine for submarine Why? Hall effect (霍尔效应) * * Question: Any other electromotor? Two types: north south Like poles repel, opposites attract Forces consist of a matched pair Forces increase with decreasing separation Analogous to electric charges EXCEPT: No isolated magnetic poles ever found! Net pole on an object is always zero! Magnetic Poles（极） A magnetic field is a structure in space that pushes on magnetic pole The magnitude of the field is proportional to the magnitude of the force on a test pole The direction of the field is the direction of the force on a north test pole Magnetic Fields Electric fields Push only on electric charges Produced by electric charges Can be produced by changing magnetism Magnetic fields Push only on magnetic poles Produced by magnetic poles Can be produced by changing electricity Electromagnetism(电磁学) 1 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 Electromagnetism 2 Current measures the electric charge passing through a region per unit of time Current is measured in coulombs/second or amperes (amps) Electric fields cause currents to flow Currents are magnetic Current Changing magnetic field ? electric field Electric field in conductor ? current Current ? magnetic field Induced magnetic field opposes the original magnetic field change (Lenz’s law) Electromagnetic Induction（电磁感应） Tape Recorder Electromotor Question: What will happens if the diameter of electromotor increase to infinitude (∞)???? Magnetically Levitated Trains(磁悬浮列车) Question: How the magnetically levitated train works? Alternating Current（交流电） Levitation Electrodynamic Levitation Question: What will happen if a magnetically levitated train is filled of gunpowder? Guess the structure! Eddy Current（涡流） Transformer