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Electric Field
An electric charge, q, can
exert its force on other charged objects even though they are some
distance away. Every charge has an electric field associated
with it, which exerts an electric force over all charges within
that field. We can represent an electric field graphically by drawing
vectors representing the force that would act upon a positive point
charge placed at that location. That means a positive charge placed
anywhere in an electric field will move in the direction of the
electric field lines, while a negative charge will move in the opposite direction
of the electric field lines. The density of the resulting electric
field lines represents the strength of the electric field at any
particular point.
Calculating Electric Field
The electric field is a vector field: at each point in
space, there is a vector corresponding to the electric field. The
force F experienced by
a particle q in electric field E is:
Combining this equation with Coulomb’s Law, we can also
calculate the magnitude of the electric field created by a charge q at
any point in space. Simply substitute Coulomb’s Law in for 
, and you get:
, and you get:
Drawing Electric Field Lines
SAT II Physics may ask a question about electric fields
that involves the graphical representation of electric field lines.
We saw above how the field lines of a single point charge are represented.
Let’s now take a look at a couple of more complicated cases.
Electric Fields for Multiple Charges
Just like the force due to electric charges, the electric
field created by multiple charges is the sum of the electric fields
of each charge. For example, we can sketch the electric field due
to two charges, one positive and one negative:
Line Charges and Plane Charges
Suppose we had a line of charge, rather than just a point
charge. The electric field strength then decreases linearly with
distance, rather than as the square of the distance. For a plane of
charge, the field is constant with distance.
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