Copyright © 2009 Pearson Education, Inc. Chapter 23 Electric Potential

Copyright © 2009 Pearson Education, Inc. Electric Potential Energy and Potential Difference Relation between Electric Potential and Electric Field Electric Potential Due to Point Charges Potential Due to Any Charge Distribution Equipotential Surfaces Electric Dipole Potential Units of Chapter 23

Copyright © 2009 Pearson Education, Inc. E Determined from V Electrostatic Potential Energy; the Electron Volt Units of Chapter 23

Copyright © 2009 Pearson Education, Inc. The electrostatic force is conservative – potential energy can be defined. Change in electric potential energy is negative of work done by electric force: 23-1 Electrostatic Potential Energy and Potential Difference

electron proton Electron Proton + - 1) proton 2) electron 3) both feel the same acceleration 4) neither – there is no acceleration 5) they feel the same magnitude acceleration but opposite direction A proton and an electron are in a constant electric field created by oppositely charged plates. You release the proton from the positive side and the electron from the negative side. Which has the larger acceleration? ConcepTest 23.1Electric Potential Energy ConcepTest 23.1 Electric Potential Energy

electron proton Electron Proton + - 1) proton 2) electron 3) both feel the same acceleration 4) neither – there is no acceleration 5) they feel the same magnitude acceleration but opposite direction F = maelectron is much less massiveelectron experiences the larger acceleration Since F = ma and the electron is much less massive than the proton, the electron experiences the larger acceleration. A proton and an electron are in a constant electric field created by oppositely charged plates. You release the proton from the positive side and the electron from the negative side. Which has the larger acceleration? ConcepTest 23.1Electric Potential Energy ConcepTest 23.1 Electric Potential Energy

Copyright © 2009 Pearson Education, Inc. Electric potential is defined as potential energy per unit charge: Unit of electric potential: the volt ( V ): 1 V = 1 J / C Electrostatic Potential Energy and Potential Difference

Copyright © 2009 Pearson Education, Inc. Only changes in potential can be measured, allowing free assignment of V = 0: 23-1 Electrostatic Potential Energy and Potential Difference

Copyright © 2009 Pearson Education, Inc. Analogy between gravitational and electrical potential energy: 23-1 Electrostatic Potential Energy and Potential Difference

Copyright © 2009 Pearson Education, Inc Electrostatic Potential Energy and Potential Difference Conceptual Example 23-1: A negative charge. Suppose a negative charge, such as an electron, is placed near the negative plate at point b, as shown here. If the electron is free to move, will its electric potential energy increase or decrease? How will the electric potential change?

Copyright © 2009 Pearson Education, Inc Electrostatic Potential Energy and Potential Difference Electrical sources such as batteries and generators supply a constant potential difference. Here are some typical potential differences, both natural and manufactured:

Copyright © 2009 Pearson Education, Inc Electrostatic Potential Energy and Potential Difference Example 23-2: Electron in CRT. Suppose an electron in a cathode ray tube is accelerated from rest through a potential difference V b – V a = V ba = V. (a) What is the change in electric potential energy of the electron? (b) What is the speed of the electron (m = 9.1 × kg) as a result of this acceleration?

Which group of charges took more work to bring together from a very large initial distance apart? +1 dd d +2 d Both took the same amount of work. ConcepTest 23.2Work and Potential Energy ConcepTest 23.2 Work and Potential Energy

total PE The work needed to assemble a collection of charges is the same as the total PE of those charges: Which group of charges took more work to bring together from a very large initial distance apart? +1 dd d +2 d Both took the same amount of work. For case 1: only 1 pair For case 2: there are 3 pairs added over all pairs ConcepTest 23.2Work and Potential Energy ConcepTest 23.2 Work and Potential Energy

Copyright © 2009 Pearson Education, Inc Relation between Electric Potential and Electric Field The general relationship between a conservative force and potential energy: Substituting the potential difference and the electric field:

Copyright © 2009 Pearson Education, Inc Relation between Electric Potential and Electric Field The simplest case is a uniform field:

Copyright © 2009 Pearson Education, Inc Relation between Electric Potential and Electric Field Example 23-3: Electric field obtained from voltage. Two parallel plates are charged to produce a potential difference of 50 V. If the separation between the plates is m, calculate the magnitude of the electric field in the space between the plates.

Copyright © 2009 Pearson Education, Inc Relation between Electric Potential and Electric Field Example 23-4: Charged conducting sphere. Determine the potential at a distance r from the center of a uniformly charged conducting sphere of radius r 0 for (a) r > r 0, (b) r = r 0, (c) r < r 0. The total charge on the sphere is Q.

Copyright © 2009 Pearson Education, Inc Relation between Electric Potential and Electric Field The previous example gives the electric potential as a function of distance from the surface of a charged conducting sphere, which is plotted here, and compared with the electric field:

Copyright © 2009 Pearson Education, Inc Electric Potential Due to Point Charges To find the electric potential due to a point charge, we integrate the field along a field line:

Copyright © 2009 Pearson Education, Inc Electric Potential Due to Point Charges Setting the potential to zero at r = ∞ gives the general form of the potential due to a point charge:

Copyright © 2009 Pearson Education, Inc Electric Potential Due to Point Charges Example: Work required to bring two positive charges close together. What minimum work must be done by an external force to bring a proton q = 1.60× C from a great distance away (take r = ∞) to a point 1.60× m from another proton?

Copyright © 2009 Pearson Education, Inc Electric Potential Due to Point Charges Example: Work required to bring two positive charges close together. What minimum work must be done by an external force to bring a proton q = 1.60× C from a great distance away (take r = ∞) to a point 1.60× m from another proton? W = ke 2 /r = (9×10 9 ) (1.6× ) 2 / (1× ) = 2.3× J = 2.3× J

1) V > 0 2) V = 0 3) V < 0 A B What is the electric potential at point B? ConcepTest 23.3Electric Potential ConcepTest 23.3 Electric Potential

zero Since Q 2 and Q 1 are equidistant from point B, and since they have equal and opposite charges, the total potential is zero. 1) V > 0 2) V = 0 3) V < 0 A B What is the electric potential at point B? ConcepTest 23.3Electric Potential ConcepTest 23.3 Electric Potential Follow-up: What is the potential at the origin of the x y axes?

Copyright © 2009 Pearson Education, Inc Electric Potential Due to Point Charges Example 23-7: Potential above two charges. Calculate the electric potential (a) at point A in the figure due to the two charges shown.

Copyright © 2009 Pearson Education, Inc Potential Due to Any Charge Distribution The potential due to an arbitrary charge distribution can be expressed as a sum or integral (if the distribution is continuous): or

Copyright © 2009 Pearson Education, Inc Potential Due to Any Charge Distribution Example 23-8: Potential due to a ring of charge. A thin circular ring of radius R has a uniformly distributed charge Q. Determine the electric potential at a point P on the axis of the ring a distance x from its center.

Copyright © 2009 Pearson Education, Inc Potential Due to Any Charge Distribution Example 23-9: Potential due to a charged disk. A thin flat disk, of radius R 0, has a uniformly distributed charge Q. Determine the potential at a point P on the axis of the disk, a distance x from its center.

Which of these configurations gives V = 0 at all points on the x axis? 4) all of the above 5) none of the above 1) x +2  C -2  C +1  C -1C-1C 2) x +2  C -1  C +1  C -2  C 3) x +2  C -1  C -2  C +1  C ConcepTest 23.5Equipotential Surfaces ConcepTest 23.5 Equipotential Surfaces

Only in case (1), where opposite charges lie directly across the x axis from each other, do the potentials from the two charges above the x axis cancel the ones below the x axis. Which of these configurations gives V = 0 at all points on the x axis? 4) all of the above 5) none of the above 1) x +2  C -2  C +1  C -1  C 2) x +2  C -1  C +1  C -2  C 3) x +2  C -1  C -2  C +1  C ConcepTest 23.5Equipotential Surfaces ConcepTest 23.5 Equipotential Surfaces

Copyright © 2009 Pearson Education, Inc. An equipotential is a line or surface over which the potential is constant. Electric field lines are perpendicular to equipotentials. The surface of a conductor is an equipotential (E // =0 → ∂V/∂s // = 0) 23-5 Equipotential Surfaces

Copyright © 2009 Pearson Education, Inc Equipotential Surfaces Equipotential surfaces are always perpendicular to field lines; they are always closed surfaces (unlike field lines, which begin and end on charges).

Copyright © 2009 Pearson Education, Inc Equipotential Surfaces A gravitational analogy to equipotential surfaces is the topographical map – the lines connect points of equal gravitational potential (altitude).

Copyright © 2009 Pearson Education, Inc. The potential due to an electric dipole is just the sum of the potentials due to each charge, and can be calculated exactly. For distances large compared to the charge separation: 23-6 Electric Dipole Potential

Copyright © 2009 Pearson Education, Inc E Determined from V If we know the field, we can determine the potential by integrating. Inverting this process, if we know the potential, we can find the field by differentiating: This is a vector differential equation; here it is in component form:

Copyright © 2009 Pearson Education, Inc E Determined from V Example 23-11: E for ring and disk. Use electric potential to determine the electric field at point P on the axis of (a) a circular ring of charge and (b) a uniformly charged disk.

Copyright © 2009 Pearson Education, Inc Electrostatic Potential Energy; the Electron Volt The potential energy of a charge in an electric potential is U = qV. To find the electric potential energy of two charges, imagine bringing each in from infinitely far away. The first one takes no work, as there is no field. To bring in the second one, we must do work due to the field of the first one; this means the potential energy of the pair is:

Copyright © 2009 Pearson Education, Inc. One electron volt (eV) is the energy gained by an electron moving through a potential difference of one volt: 1 eV = 1.6 × J. The electron volt is often a much more convenient unit than the joule for measuring the energy of individual particles Electrostatic Potential Energy; the Electron Volt

Copyright © 2009 Pearson Education, Inc Electrostatic Potential Energy; the Electron Volt Example 23-12: Disassembling a hydrogen atom. Calculate the work needed to “disassemble” a hydrogen atom. Assume that the proton and electron are initially separated by a distance equal to the “average” radius of the hydrogen atom in its ground state, × m, and that they end up an infinite distance apart from each other.

Copyright © 2009 Pearson Education, Inc. Back to the two protons W = ke 2 /r = (9×10 9 ) (1.6× ) 2 / (1× ) = 2.3× J = (2.3× J)/(1.6× J/eV) = 1.4×10 6 eV = 1.4 MeV eV convenient for Atomic/Molecular Physics MeV convenient for Nuclear Physics GeV convenient of High Energy Physics A = Angstrom = m for AMO Physics & Chem fm = Fermi = femtometer = m for NP & HEP o

Which requires the most work, to move a positive charge from P to points 1, 2, 3 or 4 ? All points are the same distance from P. 1) P  1 2) P  2 3) P  3 4) P  4 5) all require the same amount of work P ConcepTest 23.7Work and Electric Potential ConcepTest 23.7 Work and Electric Potential

path #1 against hard to do For path #1, you have to push the positive charge against the E field, which is hard to do. By contrast, path #4 is the easiest, since the field does all the work. Which requires the most work, to move a positive charge from P to points 1, 2, 3 or 4 ? All points are the same distance from P. 1) P  1 2) P  2 3) P  3 4) P  4 5) all require the same amount of work P ConcepTest 23.7Work and Electric Potential ConcepTest 23.7 Work and Electric Potential

Copyright © 2009 Pearson Education, Inc. Summary of Chapter 23 Electric potential is potential energy per unit charge: Potential difference between two points: Potential of a point charge:

Copyright © 2009 Pearson Education, Inc. Equipotential: line or surface along which potential is the same. Electric dipole potential is proportional to 1/r 2. To find the field from the potential: Summary of Chapter 23