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S E C T I O N 2 3 . 4 • The Electric Field 717 direction as the field. If q is negative, the force and the field are in opposite directions. F g ! m g (Eq. 5.6). The vector E has the SI units of newtons per coulomb (N/C). The direction of E, as shown in Figure 23.11, is the direction of the force a positive test charge experiences an electric field exists at a point if a test charge at that point experiences an electric force. Once the magnitude and direc- When using Equation 23.7, we must assume that the test charge q 0 is small enough that it does not disturb the charge distribution responsible for the electric field. If a 0 is placed near a uniformly charged metallic sphere, as in Figure 23.12a, the charge on the metallic sphere, which produces the electric field, 0 , -- q 0 ), as in Figure 23.12b, the charge on the metallic sphere is redistributed and the ratio of the e , /q 0 , . F e /q 0 ). That is, because of this redistribu- tion of charge on the metallic sphere, the electric field it sets up is different from the 0 . To determine the direction of an electric field, consider a point charge q as a source charge. This charge creates an electric field at all points in space surround- 0 is placed at point P, a distance r from the source charge, as in Figure 23.13a. We imagine using the test charge to determine the direction of the where rˆ is a unit vector directed from q toward q 0 . This force in Figure 23.13a is directed away from the source charge q. Because the electric field at P, the position E ! F e /q 0 , we find that at P, the electric field created by q is (23.9) If the source charge q is positive, Figure 23.13b shows the situation with the test charge E ! k e
q r
2
rˆ
F e ! k e
0 r
2
rˆ Source E (N/C) Fluorescent lighting tube 10 Atmosphere (fair weather) 100 Balloon rubbed on hair 1 000 Atmosphere (under thundercloud) 10 000 Photocopier 100 000 Spark in air - 3 000 000 Near electron in hydrogen atom 5 & 10 11 Typical Electric Field Values Table 23.2 (a) (b) q 0 + q ′ 0 >>q 0 + – – – – – – – – – – – – – – – – – – – – – – – – Figure 23.12 (a) For a small enough test charge q 0 , the charge distribution on the sphere is undis- turbed. (b) When the test charge q 0 , is greater, the charge distribu- tion on the sphere is disturbed as the result of the proximity of q 0 , . Active Figure 23.13 A test charge q 0 at point P is a distance r from a point charge q. (a) If q is positive, then the force on the test charge is directed away from q. (b) For the positive source charge, the electric field at P points radially outward from q. (c) If q is negative, then the force on the test charge is directed toward q. (d) For the negative source charge, the electric field at P points radially inward toward q. At the Active Figures link at http://www.pse6.com, you can move point P to any position in two-dimensional space and observe the electric field due to q. (b) E q r P rˆ + (a) F q q 0 r P rˆ + – (c) F q q 0 P rˆ – (d) E q P rˆ r |