2Review What is potential energy? Energy at rest or energy due to position
3What is a conservative force? A conservative force is a force that does work in such a way that the work done depends only upon the initial and final positions.
4Frictional force is dissipative. The Coulomb force is conservative.
5What is meant by the conservation of energy? Energy cannot be created or destroyed, only transformed.
6Gravitational force is conservative. Gravitational potential energy depends upon mass, gravity, and position.
7Introduction New concepts Electric potential energy Electric potential Voltage between two pointsElectric potential differenceCapacitance
8Work Done on a Charge Electrostatic force is conservative Work may be done by a conservative force
9The equation for work done on a charge: Since F = qEW = (F)d = (qE)d
10Electric Potential Electric potential (V) is a scalar quantity It is defined as the energy per unit charge (J/C) at a given pointThe unit for electric potential is the volt (1 volt = 1 Joule/Coulomb)
11Potential DifferenceDefinition of the potential difference (DV) between points A and BDV = VB - VA = DPE/q = -E.dThe units are Volts or Joules /Coulomb
13Electrical PE and KEA positive charge gains electrical PE when it moves against the field.A positive charge gains KE when it moves in the same direction as the field.166, 16.2
14A negative charge gains KE when it moves against the field. A negative charge gains electrical PE when it moves in the same direction as the field.A negative charge gains KE when it moves against the field.167
15Potential Difference And Electric Potential A 12-volt automobile batteryMaintains a potential difference across its terminalsThe positive terminal is at a higher potential. (+ 12 volts)The negative terminal is at a lower potential (0 volts) and is connected to the car frame.Charge moves around the circuit.16.3
16Zero PotentialA point of zero potential is usually defined by grounding some point in the circuit.
17A point at infinity may be considered to be at zero potential in relation to a positive charge.
18Electric Potential at a Point The electric potential, at a particular location, created by a point charge can be found by using:V = ke(q/r)
19Electric PotentialThe electric potential depends upon only two things:
20The electric potential depends upon only two things: ChargeLocation
21The electric potential exists at every point surrounding a given charge. A second charge is not needed.
22Electric potential is a scalar quantity. If there are two or more charges, the potentials add algebraically at a given point.no vectors to worry about
23Electric PEElectric potential energy is the energy that a charge has due to its position in the electric field produced by another charge. It is measured in Joules.133
24The electric potential (PE) energy of a pair of charges can be found by using: PE = q2V1 = q2(keq1/r)PE = ke (q1q2/r)16.7
25The electric potential energy is positive if the charges are identical and negative if they are different.
26EquipotentialsNo work is required to move a charge between two points that are at the same potential.W = -q(VB – VA)
27All points on the surface of a charged conductor which is in electrostatic equilibrium are at the same potential.
28The electric potential is constant everywhere on the surface of a charged conductor in equilibrium.
29The electric potential is constant everywhere inside a conductor and equal to its value at the surface.
30The Electron-VoltThe electron-volt is the energy of an electron after it has been accelerated across a potential difference of 1 volt.It is a very small unit of energy.1 eV = 1.6 x J
31Equipotential Surfaces What is an equipotential surface?An equipotential surface is one where all points are at the same potential.169
32No work is required to move a charge at a constant speed along an equipotential surface. 134
33The electric field at every point of an equipotential surface is perpendicular to the surface. 16.7
34Equipotential Lines Equipotential lines Two-dimensional views Equipotential lines are always perpendicular to electric field lines.16.10, 170, 16.6
35Applications Electrostatic precipitator Electrostatic air cleaners Power company smokestacksElectrostatic air cleanersFurnace filtersSmoke eaters at party halls
36How does a copy machine work? CorotronSelenium drumHeated pressure rollers165
39Capacitors What is a capacitor? A capacitor is made up of two parallel metal plates with a surface area (A) separated by a dielectric with thickness (d)171
40How it works: 68 Each plate is connected to one side of a battery Charge flows until the plates have the same potential difference as the batteryOne is positive while the other is negative (+ Q and –Q)68
41The Definition Of Capacitance What is capacitance?Capacitance is the ratio of the magnitude of the charge on either conducting surface to the potential difference between the two conducting surfaces.C = Q/DVTherefore: Q = DV.C
42What is the unit of capacitance? Farad (F)Named after Michael Faraday1 farad = 1 Coulomb/voltMicrofarads (mF) and picofarads (pF) are more commonly used
43Capacitance Applications Tuning stations on radio and television receiversStoring charge in electronic flash unitsComputer keyboards
44The Parallel Plate Capacitor Factors affecting capacitance:Area (A)Separation (d)Permittivity of free space (eo)C = eoA/d (in air)Also: ke = 1/4peo