Question

Charges of +2 µC and +3 µC are 4 mm from each other. Raina’s group draws four diagrams trying to represent the electrical force between the charges.

4 pairs of charged particles. Each pair is labeled W, X, Y and Z respectively. A dot representing a charge of a positive 2 microcoulombs is on the left with a vector pointing left. A dot representing a charge on the right is labeled +3 microcoulombs with a vector pointing right. The pair is labeled W. A dot representing a charge of a positive 2 microcoulombs is on the left with a vector pointing right. A dot representing a charge on the right is labeled +3 microcoulombs with a longer vector pointing right. The pair is labeled X. A dot representing a charge of a positive 2 microcoulombs is on the left with a vector pointing right. A dot representing a charge on the right is labeled +3 microcoulombs with a vector pointing left. The pair is labeled Y. A dot representing a charge of a positive 2 microcoulombs is on the left with a vector pointing right. A dot representing a charge on the right is labeled +3 microcoulombs with a smaller vector pointing left. The pair is labeled Z.
Which diagram is correct?

W
X
Y
Z

Answer 1

W.

Both charges are positive so they repel: the 2 µC charge feels a force left (away from the 3 µC) and the 3 µC feels a force right (away from the 2 µC). By Newton’s third law the forces have equal magnitude, which is the arrangement shown in diagram W. (If you want the magnitude: F = k q1 q2 / r^2 ≈ 8.99×10^9·(2×10^-6)(3×10^-6)/(4×10^-3)^2 ≈ 3.37×10^3 N.)