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Pitt PHYS 0175 - Potential Energy

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PHYS 0175 1st Edition Lecture 11Outline of Last Lecture II. Gauss’ Law vs Coulomb’s LawIII. Shell theoremsIV. Insulating stringV. Work done by electric forceVI. Conductor with cavityVII. Electric potential energyVIII.Potential energy of two point chargesIX. Point charges and parallel platesOutline of Current Lecture X. Electric Potential Energy Between two Point ChargesXI. Potential Energy of a charge in a fieldXII. Potential Energy in a uniform fieldXIII. Charged balloon between parallel wallsXIV. Electric Potential Energy of a set of point chargesXV. Electric potential XVI.Change of electric Potential near a point chargeCurrent LectureII. Electric Potential Energy Between two Point Chargesa. Kqq0/rb. Units in Joulec. Reference point is rinfinity U=0d. U is result of a mutual interactionIII. Potential Energy of a charge in a fielda. If positivei. Move in direction of E1. Field does positive work on charge2. Potential U decreases(easier to go in same direction as E)a. Analogous to falling massii. Move against direction of E1. Charge + move in direction opposite/against E2. Negative work3. U increases(lower to higher)a. Analogous to lifting object4. W by electro=-delta U=-W appliedb. If negativei. Move in direction of E1. Does negative work on q2. U increases3. This is bc E points from positive towards negative plate, but since now q is negative, it is harder to bring two like charges together, increasing potential energya. System wants lowest U(PE) as possibleii. Move against direction of E1. If –q move in direction opposite E, field does +W2. U decreases3. Because easier for –q to go towards +plate away from field linesIV. Potential Energy in a uniform fielda. W=F*d==q=*E*d=-delta Ub. Need to integrateV. Charged balloon between parallel wallsa. Can apply conservation of energyb. Electrostatic force is conservativeVI. Electric Potential Energy of a set of point chargesa. can assemble them one by oneb. or use algorithmc. or halve the total of all the U(double count included) VII. Electric potential a. V=U/q0b. Independent of test q0c. Analogous to electric field(independent of test charge, only on source)d. Scalar quantitye. 1 J/C=1 Voltf. U=qVg. Potential vs Ui. Charges can acquire U if V is presentii. Just like how charges acquire force with field presentVIII.Change of electric Potential near a point chargea. Positive Point qi. Towards increases(opp electric field-negative work)ii. Away decreases(same dir as field lines, easier)iii. Accelerates from region higher to lower Vb. Negative point qi. Towards decreases(same dir electric field lines-point toward neg)ii. Away increases(oppose electric field lines)iii. Accelerates from region of lower to higher V1. Because where V is lower, there is a greater presence of a negative charge(because field lines point toward negative charges), and since like charges repel, it will accelerate away with a repulsive


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