# E5 - Phy sics 7B WS E5(rev 3.0 Page 1 E-5 Electric...

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Physics 7B WS E5 (rev. 3.0) Page 1 E-5. Electric Potential Summary An electric potential energy requires two charges. An electric potential is established by a single charge Questions for discussion 1 . True or false: A point charge placed in an external electric field accelerates toward the direction of decreasing electric potential . 2 . A point mass m = 0.05 kg, with charge Q = +3 C, is sitting in an external potential V i = 17 V. How much work will it take to move the point charge to a point with higher potential V f = 22 V? 3 . The diagram below shows some equipotential lines associated with a group of three fixed point charges. (The point charges are not shown. The letters A, B, and C in the figure are not point charges. They refer simply to locations in the diagram.) a) How much work would it take to move a +1 Coulomb test charge from point A to point B? b) How much work would it take to move a +1 Coulomb test charge from point A to point C? c) How much work would it take to move a +1 Coulomb test charge from point B to point C? Why?

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Physics 7B WS E5 (rev. 3.0) Page 2 d) Where in the diagram is the electric field the strongest? How do you know? e) Draw the electric field lines for this configuration of charges. Explain how you generated your picture. 4 . Recall that in a conductor, the electric field within a conducting material must be zero. What does this imply about the electric potential within the material? (Hint: How are the electric field and the electric potential related mathematically?
Physics 7B WS E5 (rev. 3.0) Page 3 Problems 1 . The source of the sun’s energy is a process called nuclear fusion. In this process, protons collide and create larger particles. As a result, huge amounts of energy are liberated. The problem, however, is that for fusion to occur, the protons have to “collide”—that is, they have to pass near to each other—say, within a proton diameter, 10 -15 m. a) Suppose that one proton is fixed in place. If another proton starts out from very far away, then how fast must it be going initially, if it is to approach to within 10 -15 m of the fixed proton? The question now arises, Are protons inside the sun actually moving that fast? b)

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## This note was uploaded on 11/19/2010 for the course LECTURE 1 taught by Professor Yildiz during the Fall '10 term at Berkeley.

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E5 - Phy sics 7B WS E5(rev 3.0 Page 1 E-5 Electric...

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