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Electric Potential Electric forces are conservative. Work done by an electric force is W=-qoEd DU=-W Electric Potential • The convention in our text is that the work done on a system is has a negative sign. • Unfortunately the college board help sheets use the opposite convention. Figure 20-1 Change in Electric Potential Energy Electric Potential Definition of Electric Potential V DU W DV qo qo J joule SI unit volt ,V C coulomb Electric Potential When working with electric potential we measure only changes in electric potential. The zero point can be set arbitrarily. Figure 20-2 Electric Field and Electric Potential Electric Potential Electric field and the rate of change of electric potential. W qo E Ds DV E Ds qo qo DV E Ds Electric Potential Electric potential of point charges. kq V r electric potential energy for a point charge kqqo U qoV r Electric potential Equipotentials are surfaces where the electric potential is the same. Figure 20-6 Equipotentials for a Point Charge Figure 20-7 Equipotential Surfaces for a Uniform Electrical Field Figure 20-8 Equipotential Surfaces for Two Point Charges Solve problems 1-7, 16-19, and 21-24 on pages 672 and 673. Electric fields around conductors Excess electric charges move to the surface of a conductor because that is where they are least affected by the internal charges of the material. Figure 19-18 Charge Distribution on a Conducting Sphere Electric fields around conductors When electric charges are at rest, the electric field within a conductor is zero. This works even if the conductor has an internal cavity. Figure 19-19 Electric Field Near a Conducting Surface Electric fields around conductors Electric field lines enter conductor surfaces at right angles. Figure 19-21 Shielding Works in Only One Direction Figure 19-20 Intense Electric Field Near a Sharp Point Electric fields around conductors Electric field lines are more intense near a sharp point. Charging by induction Figure 19-22ab Charging by Induction Figure 19-22cd Charging by Induction Capacitors A capacitor is a device that has the ability “capacity” to store electric charge and energy. Capacitors Capacitance is defined as Q C Q the charge on the plates V V electric potential across the plates coulomb Unit : farad : F volt Figure 20-13 A Parallel-Plate Capacitor Capacitors Permitivity of free space o o 1 4 k =8.85 1012 C2 k Coulomb's constant 2 N m Capacitors Capacitance of a parallel plate capacitor o A C d d= distance between plates A= area of the plates Capacitors A dielectric material is an insulator that increases the capacitance of a capacitor when placed between the plates. Each material has a dielectric constant k not to be confused with k.(p 665) Figure 20-15 The Effect of a Dielectric on the Electric Field of a Capacitor Capacitors Capacitance of a parallel plate capacitor filled with a dielectric k o A C d d= distance between plates A= area of the plates k dielectric constant Capacitors Energy stored in a capacitor 2 1 1 Q 2 U QV CV 2 2 2C