Question

Z In an attempt to observe and determine a value for a small electric force, a student set up an experiment as shown in Figure 5.9.A and B ngth 73 cm are two equally charged spheres. A is fixed in position, but B is free to swing away on the end of an insulating thread a Draw a free-body diagram of sphere B. b Calculate the horizontal force repelling B away from A. c Calculate the charge on the spheres :20° mass 0.12g

Answer 1

FBD rightarrow {Q = 20 degree L = 73 cm m = 0.12 times 10^- 3 kg} (b) force on B by A rightarrow F = kg^2/r^2 rightarrow F = kg^2/(L sin theta)^2 (1) also, from sine rule, T/sin(90) = F/sin(180-theta) = mg/sin(90 + theta) rightarrow F/sin(180-20) = 0.12 times 10^- 3 times 9.81/sin (90 + 20) F = 4.28 times 10^-4 N (c) charge on spheres rightarrow from eq (1) F = k g^2/L^2 sin^2 theta rightarrow 4.28 times 10^-4 = 9 times 10^9 q^2/0.73^2 times sin^2 20 rightarrow g = 5.45 times 10^- 18 c