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Summary 785 When a positive test charge q 0 is moved between points A and B in an electric field E, the change in the potential energy of the charge–field system is (25.1) The electric potential V # U/q 0 is a scalar quantity and has the units of J/C, where 1 J/C " 1 V. The potential difference $V between points A and B in an electric field E is defined as (25.3) The potential difference between two points A and B in a uniform electric field E, where s is a vector that points from A to B and is parallel to E is (25.6) where . An equipotential surface is one on which all points are at the same electric poten- tial. Equipotential surfaces are perpendicular to electric field lines. If we define V # 0 at r A # * , the electric potential due to a point charge at any distance r from the charge is (25.11) We can obtain the electric potential associated with a group of point charges by The potential energy associated with a pair of point charges separated by a distance r 12 is (25.13) This energy represents the work done by an external agent when the charges are 12 . We obtain the potential energy of a distribution of point charges by summing terms like Equation 25.13 over If we know the electric potential as a function of coordinates x, y, z, we can obtain the components of the electric field by taking the negative derivative of the electric (25.16) The electric potential due to a continuous charge distribution is (25.20) Every point on the surface of a charged conductor in electrostatic equilibrium is at the same electric potential. The potential is constant everywhere inside the conductor V # k e
!
dq r E x # ! dV dx U # k e
q
1 q
2 r
12 V # k e
q d # # s # $ V # !Ed $ V " $ U q 0 # ! ! B A
E"d
s $ U # !q 0
! B A
E"d
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