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2BPreLec

# 2BPreLec - Physics 1B Lecture 20A Electric Potential Energy...

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Physics 1B Lecture 20A "Energy is eternal delight.” --William Blake 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. 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. Electric Potential Energy The electric potential energy, U e , for point charges (or spherically symmetric charge distributions) is: where zero potential energy was defined as having a separation distance of infinity. U e = k e q 1 q 2 r

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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 point charge, 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. U e,A B = U e,B U e,A = k e q 1 q 2 r B k E q 1 q 2 r A U e,A B = k e q 1 q 2 1 r B 1 r A Electric Potential Energy 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.
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