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Unformatted text preview: Electricity and Magnetism Overview • All matters possess a property called charge , interactions between charges give rise to almost all the phenomena we experienced in our daily life (except for the effects from gravity). Anything you can think of, e.g. contact and frictional forces, chemical reactions, biological activities, light, etc., owe their origin to electromagnetism . • Between two stationary point charges: Coulomb’s law • How about two stationary neutral atoms (or molecules)? Well, atoms really consist of charged constituents (electrons and nuclei). By extending Coulomb’s law using the principle of superposition , the force between two bodies of distributed charge is calculated by taking the sum of the forces between pairs of point charges • Very complicated interaction for atoms or molecules containing many electrons! A way to approximately describe the force is to make use of the concepts of dipoles (and quadrupoles , octupoles , etc., generally called multipoles ) • Assuming that all the charges are at rest: electrostatics . Complicated electrostatic problems can oftentimes be handled by introducing the concept of a potential function . • A uniform stream of moving charges gives rise to a steady current . A current would generate a magnetic field: BiotSavart law. The magnetic field in turn exerts a magnetic force on a moving charge . • A steady current produces a constant magnetic field: magnetostatics . Alternatively, a stationary magnet can also produce a constant magnetic field. • Static electric and magnetic fields appear to be two distinct quantities. Is it true? Consider the following question: • A charge moving to the right with a speed v is pulled by the magnetic field generated by the current in a neutral wire. Therefore, the charge will eventually hit the wire. • Now, if an observer is moving to the right with a speed v , so that the charge appears stationary. The charge should experience no magnetic force because v = 0 , and should therefore maintain the same distance from the wire. • Contradiction? • Therefore, an electric field must exist in the case of the moving observer in order to pull the charge toward the wire • A region with no electric field but having a magnetic field will appear to have an electric field by a moving observer • Electric field and magnetic field are inseparable to each other : electromagnetism Consider another question : • A conducting wire loop is moving to the right through a region of magnetic field. The freely moving charge inside the wire is pushed up by the magnetic force, giving rise to a current through the loop and heat up the resistor • To an observer moving to the right with the same speed as the wire loop, he must reach the same conclusion that the resistor is heated up. Hence there must be an electric field within the wire to drive the current • Therefore : A changing magnetic flux ⇒ emf or A changing magnetic field ⇒ electric field • Next, consider the following situation :...
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 Spring '10
 DrKwong
 Charge, Magnetism, Gravity, Magnetic Field, Electric charge, point charge

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