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SECTION 25.4 • Obtaining the Value of the Electric Field from the Electric Potential 773 If the charge distribution creating an electric field has spherical symmetry such that the volume charge density depends only on the radial distance r, then the electric field E " ds # E r dr, and we can express dV in the form dV # ! E r dr. Therefore, (25.17) For example, the electric potential of a point charge is V # k e q/r. Because V is a func- tion of r only, the potential function has spherical symmetry. Applying Equation 25.17, r # k e q/r 2 , a familiar result. Note that the potential changes only in the radial direction, not in any direction r ) is a function only of r. Again, this is consistent with the idea that equipotential surfaces are perpendicular to field lines. In this case the equipotential surfaces are a family of spheres concentric with the spherically The equipotential surfaces for an electric dipole are sketched in Figure 25.13c. V(r) is given in terms of the Cartesian coordinates, the electric field components E x , E y , and E z can readily be found from V(x, y, z) as the partial derivatives 3 (25.18) For example, if V # 3x 2 y ) y 2 ) yz, then , V , x # , , x
(3x
2 y ) y 2 ) y
z) # , , x
(3x
2 y) # 3y
d dx
(x
2 ) # 6x
y E x # ! , V , x
E y # ! , V , y
E z # ! , V , z E
r # ! dV d
r 3 In vector notation, E is often written in Cartesian coordinate systems as where - is called the gradient operator. E # !-V # ! ' iˆ , , x ) jˆ , , y ) kˆ , , z ( V Quick Quiz 25.8 In a certain region of space, the electric potential is zero everywhere along the x axis. From this we can conclude that the x component of the Quick Quiz 25.9 In a certain region of space, the electric field is zero. From this we can conclude that the electric potential in this region is (a) zero (b) constant Example 25.4 The Electric Potential Due to a Dipole An electric dipole consists of two charges of equal magni- (A) Calculate the electric potential at point P. Solution For point P in Figure 25.14, 2k e
qa x 2 ! a 2 V # k e &
q
i r i # k e
' q x ! a ! q x ) a ( # a a q P x x y –q Figure 25.14 (Example 25.4) An electric dipole located on the x axis. Finding the electric field from the potential |