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# Lecture%2020A0 - Physics 1B Lecture 20A Electric Flux rong...

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Physics 1B Lecture 20A

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Electric Flux rong statement from a onceptuoal question in quiz 1: he electric flux through a urface is proportional to the osine of the angle between the lectric field and the surface. In reality, the electric flux is proportional to the cosine of the angle between the actual orientation of the surface and its optimal (orthogonal) orientation Or to the cosine of the angle between the electric field and a normal to the surface. A r E r θ cos EA A E Φ E = = r r
Electric Potential Energy If I were to lift a mass up off of the floor and put it over my head, what have I done to that mass? I have increased the gravitational potential energy of the mass. If I let go of the mass it has the potential to move towards the floor. In order to increase the energy of the mass, I had to perform work on the mass. Since the gravitational force is a conservative force, the work I put into the mass is equivalent to the increase in its gravitational potential energy. s F W PE r r Δ = = Δ

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Electric Potential Energy If I were to move a negatively charged sphere away from a positively charged wall, what have I done to that sphere? I have increased the electric potential energy of the sphere. If I let go of the sphere it has the potential to move towards the wall. In order to increase the energy of the sphere, I had to perform work on the sphere. Since the electric force is a conservative force, the work I put into the sphere is equivalent to the increase in its electrical potential energy. s E q s F s F W U e r r r r r r Δ = Δ = Δ = = Δ 0
Electric Potential Energy Recall that gravitational potential energy, PE grav , for point masses (or spherically symmetric masses) was: where zero potential energy was defined as having a separation distance of infinity. Similarly, the electric potential energy, PE elec , for a system of two point charges, q and q 0 , (or spherically symmetric charge distributions) is: r q q k U PE e elec 0 =

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Electric Potential Energy As with all potential energy, it is far more useful to look at changes in electric potential energy as opposed to absolute electric potential energy. For a system of two point charges, the change in potential energy between points A and B is given by: Note the lack of absolute value signs. The sign of the charge must be taken into account. A e B e A B AB r q q k r q q k U U U 0 0 = = Δ = Δ A B e AB r r q q k U 1 1 0
Example A proton is placed 3.0cm to the right of a +1.0nC charged sphere. It is then brought in to a distance of 1.0cm from the sphere. What is the change in potential energy of the proton? Answer

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## This note was uploaded on 01/30/2012 for the course PHYSICS 1B 1B taught by Professor Grosmain during the Winter '10 term at UCSD.

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Lecture%2020A0 - Physics 1B Lecture 20A Electric Flux rong...

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