1. Electricity and Magnetism
Physics 208
Dr. Tatiana Erukhimova
Lecture 4

2. The electric field y x
has the same direction as

3. Michael Faraday
“The best experimentalist in the history of science”

4. Electric field lines
These are fictitious lines we sketch which point in the direction of the electric field.
1) The direction of at any point is tangent to the line of force at that point.
2) The density of lines of force in any region is proportional to the magnitude of in that region
Lines never cross.

7. How to calculate ?
1) Put a “test charge” at some point and do not allow it to move any other charges
2) Calculate the electric force on and divide by to obtain
The force that N charges exert on a test charge :

8. We also calculated the force that a blob of charged material with charge density exerts on a test charge:
We wrote the similar formulas if the charge is smeared out over a surface with surface density , or over a line with line density
In all of these cases, the force ends up proportional to the test charge We might factor it out. This is the electric field!

9. (N point charges)
(Charge continuum)
Given an electric field, we can calculate force exerted on some point charge :

10. Example 1: Electric field of a point charge is directly radially away from or toward the charge.
Example 2: Electric field of a dipole

11. Example 3: Electric field at the center of a charged ring
Think first!
(before you start to do calculations)

12. Example 4: Find the electric field at the center of a semi-circle of radius R, if a charge Q is uniformly spread over the semi-circle. O

13. Have a great day! Hw: All Chapter 2 problems and exercises
Reading: Chapter 2