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

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

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

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

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

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:

Electric Potential Energy Uniform electric field F elec F US y=0 y=d q

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

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