PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
Two identical pith balls, each of mass 0.50g, are suspended from a common point by massless insulating threads of length 30cm so that they hang side-by-side and just touch. The following sequence is carried out:
• The first pith ball (Ball A) is given a charge of +6.0μC by contact with a charged rod while Ball B remains neutral.
• The two balls are then allowed to touch each other momentarily and are released so they can swing apart.
Assume air resistance is negligible and the only forces after release are tension, weight, and electrostatic force.
(a) Determine the net charge on each ball immediately after they touch. Show your reasoning based on charge conservation.
(b) After separation, the threads make equal angles θ with the vertical and the balls remain at rest in this equilibrium position. Derive an algebraic expression that relates θ to the magnitude of the charge on each ball.
(c) Using the charges found in part (a), calculate the numerical value of θ. Express your answer in degrees.
(d) A student claims that if Ball A had instead started with −6.0μC, the equilibrium angle would be the same magnitude. Evaluate this claim and justify your answer using the principles of electrostatic interaction.