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Unformatted text preview: Physics 240 Fall 2007 Lecture #4 Dr. Dave Winn 2405 Randall Lab winn@umich.edu Electric and Gravitational Field Electric field defined by: Same idea used for gravitational field: In both cases, this idea of the field eliminates the need for actionata distance. Electric (gravitational) forces are local interactions between charges (masses) and electric (gravitational) fields This field concept is central to modern physics. test test Q F E K G = test test m F g K G = E Field due to a point charge What is the quantitative electric field due to a positive point charge? Points everywhere away from the charge, falls off with distance r r kQ r r kQQ Q Q F E test test test test 1 2 2 = = = K G E Field due to a dipole Field at each point is the vector sum of the field from the positive charge and the negative charge: + + = E E E G G G E + E Dipole field along axis Calculate field at point P Reducing this with algebra given in the book we find: The quantity q*d is called the dipole moment p of the dipole. 2 2 + = r kq r kq E ) ( 2 2 3 2 qd z k z d z kq E alongaxis = = 3 2 z kp E alongaxis = General methods for calculating fields from charge distributions To find the total field from a distribution of charge: 1. Find a simple way to break the object up into little elements of charge dq (charge/unit length , charge/unit area , charge/unit volume ) 2. Express the field d E from each little piece of charge....
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 Fall '08
 DaveWinn
 Physics

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