Chapter
15 & 16
Electrostatics:
Forces & Energy
Electric
Field Lines
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
1514
The
electric field between the plates of a parallelplate 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.
Solution:
Set the net
force in the x direction equal to zero: 

Set the net
force in the y direction equal to zero: 

Solve the
y force equation for T: 

Use the x
force and the T above to solve for E: 

Example
1515
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?
Solution:
For the electric
field to be zero the two fields must be equal to each other: 

Use the electric
field equation: 

Cancel common
terms: 

Solve for
x: 

On
to Electric Potential 