A physics team investigates the interaction between two identical negative point…
MATHEMATICS · Multi-part · ISC · Physics · Electrostatics · Coulomb's law and superposition of forces
Problem
A physics team investigates the interaction between two identical negative point charges fixed in space. Using a force probe, the team measures the magnitude of the electric force F exerted by one charge on the other at various separations r. The data are summarized in the table below.
Separation r (m) Measured Force F (N)
0.20 3.6
0.30 2.4
0.40 1.8
0.60 1.2
0.80 0.9
(a) Using the data, construct a force-versus-separation graph on axes you will provide in your answer booklet. Plot the five data points and connect them with straight-line segments.
(b) Estimate the electric work done by the force of one charge on the other as the separation increases from 0.20 m to 0.80 m. Clearly indicate on your graph the regions you use in your estimation and show how you obtain a numerical value (in joules).
(c) Calculate the change in electric potential energy of the system over the same interval using an appropriate analytic expression. Compare the magnitude of this value with your graphical estimate from part (b) and account for any discrepancy.
(d) Suppose an external agent moves the charges from 0.80 m to 0.30 m instead. Predict whether that external agent must do positive or negative work and justify your prediction using both the data and energy concepts.