1. Lecture 5: Electric Potential January 22, 2009 Chapter 21: 1-2

2. Economics “Economists have successfully predicted nine out of the last five recessions” Yoram Bauman—University of Washington

3. Quick Review E=0 inside a conductor Field lines are perpendicular to a conducting surface All charge on a conductor is on the surface

4. Cavity in conductor E=0 in cavity if no charge in cavity What if a positive charge in the cavity?

5. At point A or point B will a proton have the higher potential energy? 1.A 2.B +Q A B

6. Potential Energy Gravitational potential energy Uniform gravitational field – Lift a book a distance y vertically, starting at y=0: – Compress a spring a distance x from equilibrium:

7. Electric Potential Energy Uniform electric field + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - -- - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - F elec F US y=0 y=d q

8. Work done by us moving q For a uniform electric field ELECTRIC POTENTIAL Units = J/C=volt

9. Energy Conservation The electric force is conservative, so if only electric forces present (that is, we do not apply any outside forces)