1. Electricity and Magnetism
Physics 208
Dr. Tatiana Erukhimova
Lecture 6

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

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

4. 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

5. Example 5: Electric field on ring’s axis

6. Exercise 5 page 33
In a famous experiment Millikan measured the size of the electron’s charge by adjusting an field so that the force of gravity pulling down on a small, charged oil drop was cancelled by the electric force pushing up. If the mass of the drop was kg and it contained 10 electronic charges, what size field was necessary to keep the drop in equilibrium?

7. Robert Millikan’s oil-drop experiment (1909)

8. Robert Andrews Millikan 1868-1953
American experimental physicist
1923 Nobel Prize

9. Millikan received a Bachelor’s degree in the classics from Oberlin College in 1891 and his doctorate in physics from Columbia University in 1895 – he was the first to earn a Ph.D. from that department.
"At the close of my sophomore year [...] my Greek professor [...] asked me to teach the course in elementary physics in the preparatory department during the next year. To my reply that I did not know any physics at all, his answer was, 'Anyone who can do well in my Greek can teach physics.' 'All right,' said I, 'you will have to take the consequences, but I will try and see what I can do with it.' I at once purchased an Avery’s Elements of Physics, and spent the greater part of my summer vacation of 1889 at home – trying to master the subject. [...] I doubt if I have ever taught better in my life than in my first course in physics in I was so intensely interested in keeping my knowledge ahead of that of the class that they may have caught some of my own interest and enthusiasm."

10. Exercise 5 page 33
In a famous experiment Millikan measured the size of the electron’s charge by adjusting an field so that the force of gravity pulling down on a small, charged oil drop was cancelled by the electric force pushing up. If the mass of the drop was kg and it contained 10 electronic charges, what size field was necessary to keep the drop in equilibrium?

11. Motion in an electric field
A positively charged object, with mass m, is placed at rest in a constant field. How fast will the object be moving after it has traveled a distance L?

12. Example 2
Consider a constant, vertical electric field somehow created in a limited region of space. An electron enters the region traveling horizontally with speed If the region has a length L, how much will the electron be deflected at the end of the region?

13. Example 3
A particle with mass m and charge q is ejected from the lower of two parallel plates with velocity of magnitude as shown. If a constant electric field exists between the plates, magnitude E, where will the particle return to the lower plate? How large must L be so that the particle doesn’t strike the upper plate? (Neglect gravity.) y E L x

14. P218 Review: Conservative forces
One-dimensional problem:

15. Two-dimensional problem:
does NOT depend on path!
around the closed path is zero!

16. Have a great day! Hw: All Chapter 3 problems and exercises
Reading: Chapter 3