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S E C T I O N 2 3 . 3 • Coulomb’s Law 713 Quick Quiz 23.5 Object A has a charge of #2 *C, and object B has a charge of # 6 *C. Which statement is true about the electric forces on the F AB ! " 3 F BA (b) F AB ! " F BA (c) 3 F AB ! " F BA (d) F AB ! 3 F BA (e) F AB ! F BA (f) 3 F AB ! F BA Example 23.2 Find the Resultant Force q 2 ! " 2.0 *C, and a ! 0.10 m. Find the resultant force exerted on q 3 . Solution First, note the direction of the individual forces 1 and q 2 on q 3 . The force F 23 exerted by q 2 on q 3 is attractive because q 2 and q 3 have opposite signs. The force F 13 exerted by q 1 on q 3 is repulsive because both charges are positive. The magnitude of F 23 is In the coordinate system shown in Figure 23.8, the attractive F 23 is to the left (in the negative x direction). ! 9.0 N ! (8.99 & 10 9 N'm 2 /C 2 ) (2.0 & 10 " 6 C)(5.0 & 10 " 6 C) (0.10 m) 2 F 23 ! k e
! q
2 !! q
3 ! a 2 Consider three point charges located at the corners of a right 1 ! q 3 ! 5.0 *C, When dealing with Coulomb’s law, you must remember that force is a vector quan- tity and must be treated accordingly. The law expressed in vector form for the electric 1 on a second charge q 2 , written F 12 , is (23.6) where rˆ is a unit vector directed from q 1 toward q 2 , as shown in Figure 23.7a. Because the electric force obeys Newton’s third law, the electric force exerted by q 2 on q 1 is equal in magnitude to the force exerted by q 1 on q 2 and in the opposite direction; that is, F 21 ! " F 12 . Finally, from Equation 23.6, we see that if q 1 and q 2 have the same sign, as in Figure 23.7a, the product q 1 q 2 is positive. If q 1 and q 2 are of opposite sign, as shown in Figure 23.7b, the product q 1 q 2 is negative. These signs describe the relative direction of the force but not the absolute direction. A negative product indi- 1 q 2 —whether the force on an individual charge is in the positive or nega- tive direction on a coordinate axis depends on the location of the other charge. For 1 q 2 is positive, but F 12 points in the # x direction and F 21 points in the " x direction. F 12 ! k e
q
1 q
2 r
2 ˆ r – + r (a) F 21 F 12 q 1 q 2 (b) F 21 F 12 q 1 q 2 rˆ + + Active Figure 23.7 Two point charges separated by a distance r exert a force on each other that is given by Coulomb’s law. The force F 21 exerted by q 2 on q 1 is equal in magnitude and opposite in direction to the force F 12 exerted by q 1 on q 2 . (a) When the charges are of the same sign, the force is repulsive. (b) When the charges are of opposite signs, the force is attractive. At the Active Figures link at http://www.pse6.com, you can move the charges to any position in two-dimensional space and observe the electric forces on them. F 13 q 3 q 1 q 2 a a y x – + + F 23 2a √ Figure 23.8 (Example 23.2) The force exerted by q 1 on q 3 is F 13 . The force exerted by q 2 on q 3 is F 23 . The resultant force F 3 exerted on q 3 is the vector sum F 13 # F 23 . Vector form of Coulomb’s law When more than two charges are present, the force between any pair of them is given by Equation 23.6. Therefore, the resultant force on any one of them equals the vector F 1 ! F 21 # F 31 # F 41 |