Four charges are in a line. Charge A has a charge of 8.9 uC, B has -4.9 uC, C has 2.5 uC, and D has 43.9 nC. Each charge is 5 mm away from the charges next to it. What is the net force on charge B?

Question

The u stands for micro

Answer 1

First, draw a picture. I've got to assume that the charges are listed in order from right to left, so you have

q_A ------------q_B------------q_C------------q_D

^{x x x}

where x=5cm

Now, calculate the individual forces between B and the other charges, using

F=kq₁q₂/r²

r_AB=r_BC=5 mm; r_BD=10 mm; q's as listed in the problem. [note that F_BD will be very small, due to its small charge and large distance compared to A and C.]

q_B is negative, and the other charges are all positive. So all forces will be attractive (pointed towards B). This means that F_AB will be to the left (negative), and F_BC and F_BD will be to the right (positive).

Finally, add all the forces, and that's your final answer.