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Unformatted text preview: Chapter 19 11. Picture the Problem: Two charges of unequal magnitude exert an electrostatic force on each other. Strategy: Use Coulombs Law (equation 19-5) to find the magnitude of the force between the two charges. Solution: 1. (a) Apply equation 19-5 directly: 2. (b) The magnitude of the electrostatic force depends upon the product of the charges of both particles, so the negative charge experiences a force magnitude that is the same as that experienced by the positive charge. Insight: The forces experienced by the two charges must also be the same magnitude and opposite in direction in order to be consistent with Newtons Third Law. 15. Picture the Problem: Three charges are arranged as indicated in the figure and exert electrostatic forces on each other. Strategy: Let the x-axis be along the line of the three charges with the positive direction pointing to the right. Use Coulombs Law (equation 19-5) and the superposition of forces to find the net electrostatic force (magnitude and direction) on q 2 . The force from q 1 will be attractive and to the left, and the force from q 3 will be attractive and to the right. Solution: 1. (a) Write Coulombs law using vector notation: 2. Substitute the charge magnitudes given in the figure 3. The net electrostatic force on q 2 is 4. (b) If the distance d were tripled, the magnitude would be cut to a ninth and the direction would be unchanged. Insight: The force is toward the right because q 3 is larger in magnitude and the distances are the same. We wrote the answer as 0.20 kN to emphasize there are only two significant figures, which wouldnt be clear if we wrote 200 N. 21. Picture the Problem: Four charges are situated at the corners of a square as shown in the diagram at the right. Strategy: The force on charge q 3 is a vector sum of the forces from the other three charges. Let q 3 be at the origin and q 2 be on the negative x- axis. Use Coulombs Law (equation 19-5) to find the vector sum of the three forces, from which we can find the magnitude and direction of the net electrostatic force on q 3 ....
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