p2020chap02 - PHYS-2020: General Physics II Course Lecture...

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Unformatted text preview: PHYS-2020: General Physics II Course Lecture Notes Section II Dr. Donald G. Luttermoser East Tennessee State University Edition 3.3 Abstract These class notes are designed for use of the instructor and students of the course PHYS-2020: General Physics II taught by Dr. Donald Luttermoser at East Tennessee State University. These notes make reference to the College Physics, 9th Edition (2012) textbook by Serway and Vuille. II. Electrical Energy & Capacitance A. Potential Difference & Electric Potential. 1. Like gravity, the electrostatic force is conservative . a) From General Physics I , the work done from points A to B is defined by W AB = Fd , (II-1) where F is the force supplied to a moving object and d is the distance between points A and B ( i.e. , the distance that the particle travels). b) If we have a uniform electric field that supplies the work on a positively charged particle, then F = qE (see Eq. I-3) and the work supplied by the E-field is given by W AB = qEd . (II-2) c) As such, as the positively charged particle moves along the E-field, its velocity increases, which increases its kinetic energy (KE). d) Since the electric force is conservative, as the charge parti- cle gains KE, it loses an equal amount of potential energy (PE). e) From the work-energy theorem, W = KE, we then see that we can also write W AB =- PE from the statement above = hence, the work done on an object by the elec- tric force is independent of path. f) Finally, if the E-field is uniform, we can write this con- servative form of the work-energy theorem as PE =- W AB =- qEd . (II-3) II1 II2 PHYS-2020: General Physics II 2. The potential difference V between points A and B is defined as the change in potential energy (final minus initial values) of a charge q moved from A to B divided by the charge: V V B- V A = PE q . (II-4) Note that it is standard practice to express V as just V AB , or even more simply as V . a) Potential difference is not the same as potential energy = it is a potential energy per unit charge . b) Electric potential ( V ) is a scalar quantity. c) Electric potential is measured in volts V (do not confuse the unit V [volts] with the variable V [potential]): 1 V = 1 J/C (Joule/Coulomb). (II-5) d) Plugging Eq. (II-4) into Eq. (II-3) gives V =- Ed (positive charge in E-field) V = + Ed (negative charge in E-field) . (II-6) i) A positive charge gains electrical potential energy when it is moved in a direction opposite the electric field. ii) A negative charge loses electrical potential energy when it is moved in a direction opposite the electric field. e) As a result of Eq. (II-6), we see that the electric field can be measured with two separate sets of units: [ E ] = 1 N/C = 1 V/m . Donald G. Luttermoser, ETSU II3 Example II1. A 4.00-kg block carrying a charge Q = 50.0 C is connected to a spring for which k = 100 N/m. The block lies on a frictionless horizontal track, and the system is immersed in a uniform...
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p2020chap02 - PHYS-2020: General Physics II Course Lecture...

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