Electrostatics: Forces & Energy
Electric Field
Definition of the Electric Field, 
If a test charge 
experiences
a force 
at
a given location, the electric field at
that location is
Units: N/C
The electric field is the force per charge
at a given location.
Video
of Electric Field
Solving for the force gives us the force on a given charge:
Example 15-10
In a certain region of space, a uniform
electric field has a magnitude of 
and
points in the positive x direction. Find the magnitude and direction of the
force this field exerts on a charge of (a) +2.80 µC
and (b) -9.30 µC.
Electric Field of a Point Charge
Magnitude of the Electric Field Due to a Point Charge
Example 15-11
Find the electric field produced by a 1.0 µC point charge at a distance of (a) 0.75 m and (b) 1.5 m.
Example 15-12
Two charges, 
and 
,
have equal magnitudes q and are placed as shown in the figure. The net electric
field at point P is vertically upward. We conclude that (a) is
positive, is
negative; (b) is
negative, is
positive; or (c) and have
the same sign.
Example 15-13
Two charges, each equal to +2.90 µC, are placed at two corners of a square 0.500 m on a side, as shown. Find the magnitude and direction of the net electric field at a third corner of the square, the point labeled 3 in sketch.
Electric Field Lines
Rules for Drawing Electric
Field Lines
Electric field lines:
1. Point in the direction of the electric
field vector at
every point;
2. Start at positive (+) charges or at infinity;
3. End at negative (-) charges or at infinity;
4. Are more dense where has
a greater magnitude. In particular, the number of lines entering or leaving
a charge is proportional to the magnitude of the charge;
5. Lines of the net field never cross.
The electric field lines due to two positive charges of equal value.
The electric field lines due to a positive and a negative charge of equal value. Also known as a dipole.
A sketch of the electric field of two charges +4Q and -Q. Notice that the field is zero at point P.
This is the same above but now the colors show the strength of the field. Red is the strongest down to blue which is the weakest.
The electric field in a parallel plate capacitor and a close up of the edge effect.
Example 15-14
The electric field between the plates of a parallel-plate capacitor is horizontal, uniform, and has a magnitude E. A small object of mass 0.0250 kg and charge -3.10 µC is suspended by a thread between the plates, as shown in the sketch. The thread makes an angle of 10.5° with the vertical. Find (a) the tension in the thread and (b) the magnitude of the electric field.
Example 15-15
The figure shows a particle with charge +9q at the origin of an x axis and a particle with charge -q at x = L. At which points is the net electric field due to these two charges zero?
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