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Physics II- Chapter 20

by: ShayD

Physics II- Chapter 20 1012

GPA 3.74

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About this Document

These notes combine both the notes from class and a detailed summary of chapter 20. This covers topics like electric potential, electric potential energy, energy conservation, electric potential of...
Basic Physics II
David Hornes
Class Notes
electric potential, electric potential energy, energy conservation, electric potential of point charges, equipotential surfaces, capacitors/dielectrics
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This 6 page Class Notes was uploaded by ShayD on Friday February 5, 2016. The Class Notes belongs to 1012 at University of Missouri - St. Louis taught by David Hornes in Spring 2016. Since its upload, it has received 52 views. For similar materials see Basic Physics II in Physics 2 at University of Missouri - St. Louis.


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Date Created: 02/05/16
Dudaie 1 Physics II Chapter 20­ Electric Potential and Electric Potential Energy  1. Electric Potential Energy and the Electric Potential a. Electrical and gravitational forces have many similarities (both  conservative forces) i. As a result there is an electric potential energy (U), similar to  the gravitational potential energy; since U is equivalent to ­W  done. 1. Associated with electrical force a. ΔU = ­W = qEd ii. As far as electrical potential (V) is the electrical potential  energy per charge  1. ΔV=ΔU/q a. Since potential energy is measured in joules and  charge is measured in coulombs so we use volts 1. 1 V= 1 J/C b. Another common unit of energy is electron volts  ­19 (eV)­­> 1 eV= 1.6 x 10  J b. Rate of change of electric potential i. There is a connection between electric field and electric  potential 1. ΔV = ­EΔs  or E = ­ΔV/Δs c. SUMMERY:  i. electric field depends on the rate of change of the electric  potential with position→ to equate it to gravitational analogy­  Dudaie 2 potential V as the height of the hill and electric field E as the  slope of the hill 1. electric potential decreases as one moves in the direction  of the electric field CONCEPT CHECK:  If in a certain position of space the electric potential V is constant what the electric  field… is positive/negative/zero? Answer: Zero, since electric field is related to the rate of the electric potential.  Constant V= 0 change in the electric field 2. Energy Conservation  a. Electrical force is also conservative so the total energy of an electric  charge must be conserved i. The sums kinetic and electrical potential energies must be the  sane at any two points 1. K AU = A +U B B ii. We can express electric potential energy in terms of electric  potential: 1. U=qV iii. ***Active Example 20­2*** b. In general: i. Positive charges accelerates in the direction of decreasing  electric potential  ii. Negative charges accelerates in the direction of increasing  electric potential  Dudaie 3 3. The Electric Potential of Point Charges  a. We can think of electric potential on point charges like we’re holding  a point charge stationary form its origin i. If the charges on the 2 particles are the same, the repulsive  forces will call the point charge to accelerate away from the  origin ii. If the charges are the same the 2 particles are opposite the  attractive forces will make the point charge accelerate towards  the origin    1. Electrical potential energy depends inversely on their  separation  a. Electric potential for a point charge kq i. V= r b. Electric potential energy for point charges  separated by r i. U=qV= kq0q   r **The electric potential energy of two charges that are separated by an infinite  distance is zero  iii. V depends on the sign of the charge in question  1. If the charge at the origin is positive, a positive charge  will move away from origin, as if sliding downhill on the  electric potential while if the charge at origin is  negative a positive test charge will move towards the  origin Dudaie 4 b. Superposition of electric potential i. Like many physical quantities the electric potential obey one  simple superposition principle 1. The total potential due to two more charges is equal to  the sum of the potentials due to each charge separately a. Since electric potential is a scalar, it’s just as  simple as adding numbers of various signs  4. Equipotential Surfaces and the Electric Field a. Electric fields  i. Electric field is always perpendicular to the equipotential  surfaces, the points in the direction of decreasing electric  potential  b. Ideal Conductors i. Ideal conductors are equipotential surfaces; every point on or within an ideal conductor is at the same potential Dudaie 5 5. Capacitors and Dielectrics  a. The capacitor gets its name from the fact that it has the capacity to  store both electric energy and charge i. The capacitor is nothing more the 2 conductors (plates)  separated be a distance, when a charge is connected to the  battery the plates become charged – each one will carry a  different charge 1.  the greater the Q the greater the capacitance  Q a. C= V b. Exercise 20­3 b. Parallel­Plate Capacitor i. Capacitance of a parallel­plate  1. C=ε A/d c. Dielectrics  i. One way to increase the capacitance of a capacitor is to insert  an insulating material (dielectrics) 1. With a dielectric in place a capacitor can store more  energy for the same volume  Dudaie 6 C= kεA a. d   k is a give called the dielectric  constant d. Dielectric breakdown i. If the electric field is  large enough, it can literally  tear atoms apart, allowing it to conduct electricity  the maximum field can stand before breakdown (dielectric Strength) 6. Electrical Energy Storage  a. The electrical energy stored in a capacitor can be expressed as  follows: 2 1 1 2 Q i. U= Q2= CV =2 2C b. Electric energy density of an electric field  2 i. U E electric energy density= ½ ε E0


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