Example: Potential Difference of Point Charges Consider a point charge q. What is the potential difference between point x 1 and point x 2 ? q x2x2 x1x1  V depends on starting, ending point:

Potential Difference and Potential of Point Charge Can check all cases: q positive or negative; r 1 > r 2 or r 2 > r 1 ; 1  2 or 2  1 q r2r2 r1r1 HAVE to talk about potential DIFFERENCE; One exception: Potential of point charge WITH RESPECT TO INFINITY

ConcepTest #9: Consider the positive point charge shown in the figure. What is the potential difference going from point A at distance a to point B at distance b ? + A B  a b

ConcepTest #10: Consider four identical point charges – q arranged at the corners of a square of side length d as shown in the figure. What is the electric potential at the center of the square (relative to infinity)? d d

Consider a thick neutral spherical conducting shell with inner radius a and outer radius b. A positive point charge Q is at the center of the shell, as shown. 1) Write down the electric field in the regions i) r < a ii) a < r <b iii) r > b 2) Set the potential at r = ∞ to zero. Determine the electric potential in the regions i) r > b ii) a < r <b a b iii) r < a Example: Spherical Conductor, Field, Potential

Current, Potential Difference, and Electric Field in Conductors (?) Current: motion of charge (“charge flow”). Amount of charge that passes a point (or through a cross sectional area) in some amount of time Current : “conventional current”  motion of positive charge Electron current  motion of negative charge Assume electric field in wire (or circuit element) is constant, then: How can there be electric field in conductor? If electric field in conductor, why don’t charges accelerate?