electric%20fields%201

electric%20fields%201 - Coulomb's Law Charges: A property...

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1 Coulomb’s Law c Charges: b A property of matter. b Positive and Negative charges, the unit of charge is Coulomb. b Like sign charges repel, unlike sign charges attract. b Charges conserve in a system. c Conductor, insulator and ground. c Force between two point charges: Force is a vector. c Charge induction in conductor and in insulator. 12 12 12 r r F ) ) r 2 2 1 0 2 2 1 4 1 r q q r q q k e πε = =
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2 Electric Field c The electric field b Why needed? Electrical forces described by Coulomb’s Law act at a distance. In Physics, interactions take place with either the two parties touching each other (! Inter !action), in the case of mechanical forces, like a car tire touches the ground, or through a medium or force carrier. In the case of electric (and magnetic) forces, the force carrier is a photon (light), but that’s beyond this course. There is a more traditional concept, the field, that provides an explanation. We encountered this concept in the gravitational force, which was explained by the gravitational field, do you still remember? When you are in an airplane flying in the Earth’s gravitational field (the influence range of the Earth’s gravitational force), it is this field that keeps you from going into the outer space. E r
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3 Electric Field c The Definition Since the electric field is used to help explain the electric force, it must be defined using the electric force: test q F E r r • The electric field at a location is generated by source charges, not the test charge. It is there regardless one measures it with a test charge or not. • The electric field is the force a unit positive charge experiences at its location hence electric field is a vector . • The positive test charge is small enough so as not to distort the electric field around it. Source charge Test charge F r E r
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4 Electric Field c The graphic representation of electric field: Field from a positive charge. Field from a positive and two negative charges. Field from a negative charge. Question: why the E field points away from a + charge, but points to a – charge? A: the definition of the E field: the force on a positive unit charge.
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Electric Field Lines c Field lines give us a means of representing the electric field pictorially c The electric field vector is tangent to the electric field line at each point c The line has a direction that is the same as that of the electric field vector c The number of lines per unit area through a surface
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This note was uploaded on 05/06/2008 for the course PHYS 1304 taught by Professor Ye during the Spring '08 term at SMU.

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electric%20fields%201 - Coulomb's Law Charges: A property...

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