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 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.



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 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?


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