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S E C T I O N 2 3 . 5 • Electric Field of a Continuous Charge Distribution 721 total electric field at some point must be replaced by vector integrals. Divide • Symmetry: with both distributions of point charges and continuous charge dis- Example 23.7 The Electric Field Due to a Charged Rod is particularly simple in this case. The total field at P due to all 3 where the limits on the integral extend from one end of the e and 3 can be removed from the integral to yield where we have used the fact that the total charge Q ! 3!. What If? Suppose we move to a point P very far away from the rod. What is the nature of the electric field at such a point? Answer If P is far from the rod (a -- !), then ! in the " k e Q /a 2 . This is just the form you would expect for a point charge. Therefore, at large values of a/!, the ) often is a good method for checking a mathe- matical expression. a/! : 4 k e
Q a(! # a) ! k e
3
& 1 " 1 ! # a ' ! E ! k e 3 % !# a a
dx
2 ! k e 3 ( " 1 x ) a !# a E ! % !# a a k e 3 dx
2 A rod of length ! has a uniform positive charge per unit Solution Let us assume that the rod is lying along the The field d E at P due to this segment is in the negative x direction (because the source of the field carries a positive Because every other element also produces a field in the neg- dE ! k e
dq
2 ! k e
3 dx x
2 3 It is important that you understand how to carry out integrations such as this. First, express the charge element dq in terms of the other variables in the integral. (In this example, there is one vari- x y ! a P x dx dq =
dx E 3 Figure 23.17 (Example 23.7) The electric field at P due to a uniformly charged rod lying along the x axis. The magnitude of the field at P due to the segment of charge dq is k e dq/x 2 . The total field at P is the vector sum over all segments of the rod. This field has an x component dE x ! dE cos + along the x axis and a component dE ⊥ perpendicular to the x axis. As we see in Figure 23.18b, however, the resultant field at P d
E ! k e
dq
2 A ring of radius a carries a uniformly distributed positive Solution The magnitude of the electric field at P due to Example 23.8 The Electric Field of a Uniform Ring of Charge |