Electric Potential As with mechanics energy is a

Electric Potential • • As with mechanics, energy is a useful quantity in studying electric forces electric potential or potential • • electric potential energy per unit charge potential difference, Φ • • • only difference in potential energy is meaningful Ve is the electrostatic potential energy relative to some reference point Volt: 1 V = 1 J/C General Physics 2 Electric Potential 1

Analogy to Gravitational PE Gravitation PE depends on mass Electric potential energy depends on charge Φb General Physics 2 Electric Potential Φab Φa 2

Electric Potential of Point Charge • Set V=0 at a distance of infinity General Physics 2 Electric Potential 3

Electric Potential of Multiple Charged Objects • Superposition principle General Physics 2 Electric Potential 4

Electric Potential General Physics 2 Electric Potential 5

Equipotential Lines • • connect points in space at same potential equipotential surface is perpendicular to electric field lines General Physics 2 Electric Potential 6

Electric Field & Equipotential Lines • Draw field lines and equipotential lines for the following charge distributions - General Physics 2 Electric Potential 7

Electric Field & Equipotential Lines • Draw field lines and equipotential lines for the following charge distributions + General Physics 2 Electric Potential 8

Relating Electric Field to Potential • The change in electrical potential for a small displacement in a static electric field General Physics 2 Electric Potential 9

Electrostatic Potential & PE • When dealing with discrete point charges: Ve and Φ are scalars General Physics 2 F and E are vectors Electric Potential 10

Electric Potential Think of analogy with gravitational PE, where object with positive PE will fall if released. Here, p+ has high PE when near + plate and will “fall” toward negative plate when released Think of + charge when wondering if something is high or low potential + charges move from high to low potential

Electric Potential low PE High PE - - Unlike mass, charge can be + or -. Thus, PE depends on the sign of the charge. For example, an e- will have high PE when near the - plate and will “fall” toward the + plate when released - charges move from low to high potential

Capacitance • a capacitor stores electric charge General Physics 2 Capacitors 13

Properties of Capacitors • Capacitance, C • • • Q=CV Farad (F) = Coulomb / Volt C depends on size, shape, relative position of parallel plates (and material that separates them) permittivity of free space = 2/(N m 2) 8. 85 General x 10 -12 C Physics 2 surface area per plate Capacitors separation between plates 14

Electron Volt, a Unit of Energy • electron volt (e. V) • • • Joule is very large for dealing w/electrons, atoms and molecules e. V is energy required to move 1 e- through a potential difference of 1 V 1 e. V = qe V = (1. 6 x 10 -19 C)(1. 0 V) 1 e. V = 1. 6 x 10 -19 J General Physics 2 Electric Potential 16

Sign of Electric PE • • PE = 0 at infinity Did you have to do work to bring charges from infinity? • yes -> charges have positive PE Example: - • - no -> charges have negative PE Example: - General Physics 2 + Electric Potential 17

Concep. Test 1. 2. 3. positive negative zero

Concep. Test

Plug & Chug • How much work does the electric field do in moving a -7. 7 C charge from the ground to a point whose potential is +55 V higher? General Physics 2 Electric Potential 20

Think-Pair-Share • How much kinetic energy will an electron gain (in joules and e. V) if it accelerates through a potential difference of 23, 000 V in a TV picture tube? What is its final velocity? General Physics 2 Electric Potential 21

Electric Potential & Electric Field • For a uniform electric field General Physics 2 Electric Potential 22

Electric Field & Equipotential Lines • Draw field lines and equipotential lines for the following charge distributions - General Physics 2 - Electric Potential 23

Which set has a positive potential energy?

Which set has a negative potential energy?

Plug & Chug • What is the electric potential 15. 0 cm from a 4. 00 x 10 -6 C point charge? General Physics 2 Electric Potential 26

Think-Pair-Share • What is the potential at point A, taking V=0 at a great distance? General Physics 2 Electric Potential 27