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**Unformatted text preview: **Class 4 The Electric Field and the Electric Potential Physics 106 Fall 2011 Press CTRL-L to view as a slide show. Learning Outcomes from Last Time You should be able to describe the following: I the nature of forces I charging by conduction and induction I Coulombs law - the force between point charges I how to use Coulombs law Learning Outcomes After todays class, you should know the following items: I The electric field is defined by E = F / q I The electric field points in the direction of the force on a small positive test charge I The electric field can be described by electric field lines I The electric field vector, ~ E , is tangent to the electric field lines at each point I Field lines are closer together where the field is stronger I Be able to draw the electric fields of simple arrangements of charges. I Know the characteristics of electric fields near conductors I Work standard problems involving the electric field. The Electric Field What is a Field? I A field is a function defined at every point in a region of space. I If a scalar is defined at every point, the field is a "scalar field" I If a vector is defined at every point, the field is "vector field" Gravitational Field I How can you tell there is a gravitational field in this room? I Write down a fact about the gravitational field. I The force on an eraser is F = G M earth meraser R 2 I Can you define the gravitational field of the earth at the position of the eraser? Electric Field I We say that charges produce an electric field in a region of space. I What does this mean? I The electric field is defined in terms of the force experienced by a small test charge in this region. Electric Field I Q produces an electric field I A small test charge, q o , placed in the field, will experience a force Electric Field I The force depends on the charges and the distance between the charges. Electric Field F = k e Qq r 2 F = q k e Q r 2 I q depends on the test charge I k e Q / r 2 depends on the source and r I k e Q / r 2 is called the electric field Electric Field I Mathematically, E = F q = k e Q r 2 for a point charge I SI units are N/C or volts/meter I The electric field is a vector quantity...

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