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September 7, 2011 Physics for Scientists & Engineers 2, Chapter 22 1 Please register your iClicker at http://www.iclicker.com/support/ registeryourclicker/ Include the A when you enter your student ID!

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September 7, 2011 Physics for Scientists & Engineers 2, Chapter 22 2 The Electric Field So far we have thought of the electric force between two stationary charges Suppose one charge were moving, how would the second charge know that the ±rst charge has moved? What if there were other charges, how would one charge know about the extra charges? To deal with these situations, we introduce the concept of an electric ±eld , which is de±ned at any point in space as the net electric force on a charge, divided by that charge e net force on any charge is then ! E r ( ) = ! F ! r ( ) q ! F ! r ( ) = q ! E r ( )
September 7, 2011 Physics for Scientists & Engineers 2, Chapter 22 3 The Electric Field e electric force on a charge is parallel or antiparallel to the electric ±eld at that point e magnitude of the force is F = q E F F positive charge negative charge E E

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September 7, 2011 Physics for Scientists & Engineers 2, Chapter 22 4 The Electric Field If several sources of electric ±elds are present, the electric ±eld is given by the superposition of the electric ±elds from all sources Follows from superposition of forces from mechanics e superposition principle for the total electric ±eld at any point in space due to n electric ±eld sources can be written as e electric ±eld at any point is the vector sum of all the electric ±elds Note that this means x -, y -, and z - components E t r ( ) = E 1 r ( )+ E 2 r + E n r ( )
Field Lines An electric ±eld can change as a function of the spatial coordinate e changing direction and strength of the electric ±eld can be visualized by means of electric ±eld lines Electric ±eld lines graphically represent the net vector force on a unit positive test charge e tangent to the ±eld line give the direction of the ±eld and the density of ±eld lines is proportional to the magnitude of the force Electric ±eld lines can be compared to the streamlines of wind direction September 7, 2011 Physics for Scientists & Engineers 2, Chapter 22 5

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Field Lines To draw an electric ±eld line, we imagine placing a small
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