Thursday, January 21, 1999
Announcements:  Physics Learning Resource Center
                          201 Osmond LAb
Sunday: 8-10 pm
Monday : 2:30-4:30 pm and 6-9 pm
Tuesday: 2:30- 4:30 pm and 6- 10 pm
Wednesday: 2:30 - 4:30 pm and 6- 10 pm
Thursday: 6- 10 pm

202 Instructors will be available on Sunday and Monday Evenings!

Lecture notes:

Coulomb's law tells us what the force is acting between two specific charges

•  If we are interested in knowing the of q₁ on a charge q₃, at a different position, then we have to start from scratch.
•  It is convenient to "map" out the "force per unit charge" due to charge q₁, at all displacement from q₁; This is called  the electric field due to q₁.
• To do this, we " imagine a test charge q_o, that is positive and very small and calculate the electrostatic force between q₁ and q_o as we move q_o around.
•  q_o is very small so that it does not distort what is going on near q₁
• F_1o= F =   1           q1qo
                           4pe_o       r²
• What is the electrostatic force due to q₁ on q₃?
• F₁₃= q₃E
• E is the electric field
• Electric Field due to a positive charge, q₁

• Electric Field due to a negative charge

 

• Electric Field due to a positive and negative charge (Dipole)

• Recall from Chemistry that molecules often organize themselves in dipole moments
• Electric Field due to two positive charges

• The Electric Field is zero at the center between the two positive charges
• The lines are not "real" ; they simply tell us what the orientation of the electric field is.
• Electric Field due to an infinitely large, non-conducting sheet with a uniform distribution of positive charges.

• With an infinitely large, uniformly distributed surface all non perpendicular lines will cancel out.
• What about an infinitely large conducting sheet carrying an excess positive charge?
• Carrying a negative charge?

• The electric field E at a particular point is given by the tangent of the field line
• The magnitude of E is reflected by the density of field lines
• When the lines are close together, E is large
• When the lines are far apart, E is small
• Electric Field lines extend away from positive charge and towards the negative charge
• Field lines do not cross each other.