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Unformatted text preview: Physics 1112 Spring 2010 University of Georgia Instructor: HBSch¨uttler Formula Sheet for In-Class Exam #3 Reading and thoroughly familiarizing yourself with this formula sheet is an important part of, but it is not a substitute for, proper exam preparation. The latter requires, among other things, that you have re-worked all assigned homework problem sets (PS) and the in-class quizzes, studied the posted PS solutions, and worked and studied the assigned conceptual practice (CP) problems, as well as (optionally) some practice test (PT) problems, as posted on the LON-CAPA homework and on the PHYS1112 examples and homework web pages. You should consult the syllabus, and in particular review the Class Schedule on the last syllabus page (posted on the PYS1112 course web site), to find out which topics you should cover in preparing for this exam. Capacitors, Capacitance, Electric Field Energy (1) Definition of Capacitance : For two oppositely charged metallic objects a and b , with- Q stored on a and Q stored on b , their electric potential difference V ≡ V b- V a is proportional to the charge Q . The capacitance of the two metallic objects is then defined as: C ≡ Q V , hence Q = CV or V = Q C (2) Voltage and Capacitance of a Planar Capacitor : For two oppositely charged, parallel planar metallic plates, each of opposing surface area A , closely spaced with distance d , the voltage V and capacitance C are | V | = Ed = | Q | d κ o A ; C ≡ | Q | | V | = κ o A d ≡ κ C o where κ is the dielectric constant of the dielectric (insulating) material between the plates and κ = 1 for vacuum or air, and C o ≡ o A/d is the capacitance without dielectric. (3) Electric Field Energy Storage in a Capacitor: The energy U E required to build up a charge Q and a voltage V = Q/C in a capacitor is stored as electric field energy between the capacitor plates and it is given by U E = 1 2 C Q 2 = 1 2 CV 2 (4) Electric Field Energy Density: Energy per volume, u E , stored in an electric field is given in terms of the field strength E u E = o 2 E 2 1 Physics 1112 Spring 2010 University of Georgia Instructor: HBSch¨uttler Current, Resistance, Ohm’s Law, Electric Power Dissipation (1) Definition of Current: For a charge Δ Q flowing through a wire or, more generally, some cross-sectional area of a conducting object, over a time interval Δ t , the current I is I ≡ Δ Q Δ t (2) Ohm’s Law: The current I flowing through an ”ohmic” conductor ( e.g. , metallic wire) and the applied voltage drop V ≡ V a- V b across the conductor are proportional: V = RI ∝ I with R ≡ V I = constant , i.e. , the conductor’s resistance R is independent of I or V . Here V a and V b denote the electric potential at current’s point of entry a and current’s point of exit b into/from the conductor, respectively....
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This note was uploaded on 04/23/2010 for the course PHYS 1112L taught by Professor Staff during the Spring '10 term at UGA.
- Spring '10