17.3 CHAPTER 17. ELECTROSTATICS
Parallel plates ESBHJ
One very important example of electric fields which is used extensivelyis the electric field between
two charged parallel plates. In this situation theelectric field is constant. This is used for many
practical purposes and later we will explain howMillikan used it to measure the charge on the
electron.
Field map for oppositelycharged parallel plates
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This means that the force that a test charge would feel at any point between the plates would be
identical in magnitude and direction. The fieldson the edges exhibit fringe effects, i.e. they bulge
outwards. This is because a test charge placed here would feel the effects of charges only on one side
(either left or right depending on which side it is placed). Test charges placed in the middle
experience the effects of charges on both sides so they balance the components in the horizontal
direction. This is clearlynot the case on the edges.
Strength of an electric field
When we started making field maps we drew arrows to indicate the strength of the field and the
direction. When we moved to lines you might have asked “Did we forget about the field strength?”.
We did not. Consider the case for a single positive charge again:
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Notice that as you movefurther away from the charge the field lines become more spread out. Infield
map diagrams, the closer together field lines are, the stronger the field. Therefore, the electric field is
stronger closer to the charge (the electric field lines are closer together)and weaker further fromthe
charge (the electric fieldlines are further apart).