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If the surface under consideration is not perpendicular to the field, the flux through it must be less than that given by Equation 24.1. We can understand this by (24.2) From this result, we see that the flux through a surface of fixed area A has a maximum We assumed a uniform electric field in the preceding discussion. In more general situations, the electric field may vary over a surface. Therefore, our definition of flux A i whose magnitude represents the area of the ith element of the surface and whose direction is defined to be perpendicu- E i at the location of this element makes an angle ) i with the vector + A i . The electric flux +! E through this element is where we have used the definition of the scalar product (or dot product; see Chapter A ! B # AB cos )). By summing the contributions of all elements, we obtain the total flux through the surface. If we let the area of each element approach 1 (24.3) Equation 24.3 is a surface integral, which means it must be evaluated over the surface E depends both on the field pattern and on the surface. ! E # lim + A i : 0
!
E i " +
A i # " surface E"d
A +! E # E i
+ A i cos ) i # E i "+
A i ! E # E
A* # E
A cos ) SECTION 24.1 • Electric Flux 741 Figure 24.2 Field lines representing a uniform electric field penetrating an area A that is at an angle ) to the field. Because the number of lines that go through the area A* is the same as the number that go through A, the flux through A* is equal to the flux through A and is given by ! E # EA cos ) . A θ θ A ′ = A cos E Normal θ Figure 24.3 A small element of surface area +A i . The electric field makes an angle ) i with the vector + A i , defined as being normal to the surface element, and the flux through the element is equal to E i + A i cos ) i . ∆A i E i θ i 1 Drawings with field lines have their inaccuracies because a limited number of field lines are typically drawn in a diagram. Consequently, a small area element drawn on a diagram (depending on Definition of electric flux |