1. Electric Charge, Force and Field

2. Properties of Charge Unlike charges attract, like charges repel
Force between charges varies as an inverse square (same as gravity)
Charge is conserved
Charge is quantized
p+ is 1800× heavier than e-
Benjamin Franklin

3. Electrical Properties
Conductor
Allows easy current
Metals
Insulator
Prevents current flow
Plastics, glass, & air
Semiconductor
Intermediate behavior
Carbon, silicon, & germanium

4. Coulomb’s Law q1 q2 r
F12
F21
e0 - Permittivity of free space

5. Superposition The net force from multiple charges is additive.
Ex. A charge of C is located at the origin and a charge of C is located at (10 m, 0 m). What is the force exerted on a C charge located at (5 m, 5 m)?

6. Electric Field The force per unit charge experience by a charge.
For a point charge the electric field is
Units (N/C)

7. Drawing Electric Fields
Lines begin on positive charges and end on negative charges.
# of lines leaving a charge is proportional to the magnitude of the charge.
Therefore, the strength of the electric field is proportional to the density of the electric field lines.
No two field lines can cross.

8. Field Lines
How would you draw the field lines for the following sets of charges?
Point Charge
Dipole
Like Charges
See 2-D Electrostatics Applet at
Can also look at the 3-D Electrostatic Fields Applet

9. Continuous Charge
Use superposition to add all the infinitesimal charges, dq.
If charge is uniformly distributed, then
Line Charge
Surface Charge
Volume Charge

10. Electric Field on a Wire
Ex. A wire has a length of 2 m and a charge of +20 mC. What is the electric field at a point 4 cm away from the center of the wire? (Assume the charge is uniformly distributed.)

11. Charge in a Field
Charge will accelerate in the direction of the electric field lines.
Ex. What is the acceleration of an electron in a 25 N/C electric field? e-

12. Torque on a Dipole E Remember then q+ q- 2a F
is a unit vector going from the
negative to the positive charge