720_Physics ProblemsTechnical Physics

# 720_Physics ProblemsTechnical Physics - 60 Electric...

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60 Electric Potential * P25.28 (a) In an empty universe, the 20-nC charge can be placed at its location with no energy investment. At a distance of 4 cm, it creates a potential V kq r e 1 1 99 8 9 91 0 2 01 0 004 450 == ×⋅ × = . . . Nm C C m kV 22 ej e j . To place the 10-nC charge there we must put in energy Uq V 12 2 1 93 5 10 10 4 5 10 4 50 10 × × = × −− C V J .. . Next, to bring up the –20-nC charge requires energy U U qV q V V 23 13 3 2 3 1 3 2 1 55 20 10 8 99 10 10 10 20 10 4 50 10 4 50 10 += + = + =− × × × + × F H G I K J × × bg N m C C 0.04 m C 0.08 m J J . The total energy of the three charges is UUU 12 23 13 5 450 10 ++= −× . J . (b) The three fixed charges create this potential at the location where the fourth is released: VV V V V =++= × × + + × × F H G I K J 123 9 9 3 899 10 20 10 003 10 10 20 10 005 300 10 . . Cm V Energy of the system of four charged objects is conserved as the fourth charge flies away: 1 2 1 2 0 40 10 3 00 10 1 2 200 10 0 2 1 20 10 21 0 346 10 1 3 2 4 13 4 mv qV mv qV v v if + F H G
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## This note was uploaded on 12/14/2011 for the course PHY 203 taught by Professor Staff during the Fall '11 term at Indiana State University .

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