General Physics 2Electric Potential1 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 V e is the electrostatic potential energy relative to some reference point Volt: 1 V = 1 J/C

General Physics 2Electric Potential2 Analogy to Gravitational PE ΦbΦb ΦaΦa Φ ab Electric potential energy depends on charge Gravitation PE depends on mass

General Physics 2Electric Potential3 Electric Potential of Point Charge Set V=0 at a distance of infinity

General Physics 2Electric Potential4 Electric Potential of Multiple Charged Objects Superposition principle

General Physics 2Electric Potential5

General Physics 2Electric Potential6 Equipotential Lines connect points in space at same potential equipotential surface is perpendicular to electric field lines

General Physics 2Electric Potential7 Electric Field & Equipotential Lines Draw field lines and equipotential lines for the following charge distributions -

General Physics 2Electric Potential8 Electric Field & Equipotential Lines Draw field lines and equipotential lines for the following charge distributions +

General Physics 2Electric Potential9 Relating Electric Field to Potential The change in electrical potential for a small displacement in a static electric field

General Physics 2Electric Potential10 Electrostatic Potential & PE When dealing with discrete point charges: V e and Φ are scalarsF and E are vectors

Electric Potential + charges move from high to low 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

Electric Potential - charges move from low to high potential 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 -- High PElow PE

General Physics 2Capacitors13 Capacitance a capacitor stores electric charge

General Physics 2Capacitors14 Properties of Capacitors Capacitance, C Q = C V Farad (F) = Coulomb / Volt C depends on size, shape, relative position of parallel plates (and material that separates them) permittivity of free space = 8.85 x C 2 /(N m 2) surface area per plate separation between plates

General Physics 2Electric Potential16 Electron Volt, a Unit of Energy electron volt (eV) Joule is very large for dealing w/electrons, atoms and molecules eV is energy required to move 1 e- through a potential difference of 1V 1 eV = q e V = (1.6 x C)(1.0 V) 1 eV = 1.6 x J

General Physics 2Electric Potential17 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: -- -+

ConcepTest 1. positive 2. negative 3. zero

ConcepTest

General Physics 2Electric Potential20 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 2Electric Potential21 Think-Pair-Share How much kinetic energy will an electron gain (in joules and eV) if it accelerates through a potential difference of 23,000 V in a TV picture tube? What is its final velocity?

General Physics 2Electric Potential22 Electric Potential & Electric Field For a uniform electric field uniform electric field

General Physics 2Electric Potential23 Electric Field & Equipotential Lines Draw field lines and equipotential lines for the following charge distributions --

Which set has a positive potential energy?

Which set has a negative potential energy?

General Physics 2Electric Potential26 Plug & Chug What is the electric potential 15.0 cm from a 4.00 x C point charge?

General Physics 2Electric Potential27 Think-Pair-Share What is the potential at point A, taking V=0 at a great distance?