Chapter 21, Electric Charge, and electric Field

21-5 Coulomb’s Law The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. Figure 21-15. The direction of the static electric force one point charge exerts on another is always along the line joining the two charges, and depends on whether the charges have the same sign as in (a) and (b), or opposite signs (c).

21-5 Coulomb’s Law Unit of charge: coulomb, C. The proportionality constant in Coulomb’s law is then: k = 8.99 x 109 N·m2/C2. Charges produced by rubbing are typically around a microcoulomb: 1 μC = 10-6 C.

Question 21-5 Coulomb’s Law What is the correct Forces Diagram for charge 3 where F31 is the force on Q3 due to Q1 and F32 is the force on Q3 due to Q2? Question Q1 Q2 Q3 + - + F31 F31 a) b) c) d) 3 3 F32 F32 F31 F31 3 3 F32 F32

An  particle with a charge +2e and a mass of 4mp is on a collision course with a proton with a charge +e and mass mp. The only force acting is the electrostatic force. Which of the following statements is true about the magnitudes of the forces on the two particles? Question A) F= Fp B) 2F= Fp C) 4F= Fp D) F= 2Fp E) F= 4Fp  p Charge +2e +e Mass 4mp mp

An  particle with a charge +2e and a mass of 4mp is on a collision course with a proton with a charge +e and mass mp. The only force acting is the electrostatic force. Which of the following statements is true about the accelerations of the two particles? Question A) a= ap B) 2a= ap C) 4a= ap D) a= 2ap E) a= 4ap  p Charge +2e +e Mass 4mp mp

Problem 10  

Problem 11 11. (II) Two positive point charges are a fixed distance apart. The sum of their charges is QT .What charge must each have in order to (a) maximize the electric force between them, and (b) minimize it?

Problem 12 12.(II) Particles of charge +75 μC, +48μC, and -85μC are placed in a line (Fig. 21–52). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two.

21-5 Coulomb’s Law Example 21-3: Electric force using vector components. Calculate the net electrostatic force on charge Q3 shown in the figure due to the charges Q1 and Q2. Figure 21-18. Determining the forces for Example 21–3. (a) The directions of the individual forces are as shown because F32 is repulsive (the force on Q3 is in the direction away from Q2 because Q3 and Q2 are both positive) whereas F31 is attractive (Q3 and Q1 have opposite signs), so F31 points toward Q1. (b) Adding F32 to F31 to obtain the net force. Solution: The forces, components, and signs are as shown in the figure. Result: The magnitude of the force is 290 N, at an angle of 65° to the x axis.

Problem 20 20. (III) Two small positively charged spheres hang from cords of equal length l as shown in Fig. 21–55 and make small angles θ1 and θ2 with the vertical. (a) If Q1=Q, Q2=2Q, and m1=m2=m determine the ratio θ1/ θ2 (b) If Q1=Q, Q2=2Q, and m1=m and m2=2m, determine the ratio θ1/ θ2 (c) Estimate the distance between the spheres for each case.