Electric Potential and Energy

Objectives Define work and relate it to energy Define electric potential difference, and relate it to the work done on an electric charge in an electric field Define capacitance Write equations for work, electric potential, and capacitance Use equations to solve problems

Work is Energy Charge is stored at a “potential” or “voltage” –A battery stores a certain amount of charge at a specific voltage –When the charge runs out, the battery is “dead” – –Unit of work is a joule (J), just like energy Work is equal to a force exerted over a distance Work is done to move a charge in an electric field W = F · d

More Specifically Positive work is stored energy Negative work is released energy Work is only done if there is a component of the electric field in the direction of the motion –If the motion is perpendicular to the electric field lines, no work is done –The electric field is constant in that direction

Work ∆V = Voltage or “potential difference”

Volts Unit is joules per coulomb, or volt (V) Potential difference, or voltage (V)

Example Moving a charge of 3.2 x coulombs between two points in an electric field requires 4.8 x joules of energy. What is the potential difference between these points?

Solution

Example Electric field between two parallel plates V = 60 Vd = 3.0 cm What is the magnitude of the electric field between them? Textbook, Pg. 491

Example An elementary charge is moved through a potential difference of 1 volt Calculate the amount of work done on the charge

Solution

Electrical Energy At A an electron has 100 eV of electric potential energy At B the electron has 100 eV of kinetic energy e A B 100 V

Capacitance Capacitance depends on the structure of the device A capacitor with a given amount of charge, will have a specific potential difference

Capacitance Unit of capacitance is the farad (F) One farad equals one coulomb per volt WARNING Capacitors can be lethal For a parallel plate capacitor

Summary Electric potential difference –Work done per unit charge to move a charged body in an electric field –Unit is joule per coulomb, or “volt” Capacitance –Ratio of an object’s stored charge to its potential difference –Measured in farads

Equations E = Electric Field F = Force q = Charge W = Work = Electric Potential Energy V = Potential Difference = Electric Potential C = Capacitance

Practice Problems Pg. 493 #5 – 8