A laboratory capacitor has circular plates of radius separated by
PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A laboratory capacitor has circular plates of radius 7.0cm separated by 2.0mm. The capacitor is charged until the magnitude of charge on each plate is 5.0μC, after which it is isolated from the charging source so that the charge remains fixed.
The plates are then pulled apart, increasing the separation to 4.0mm while keeping them parallel and facing each other. Throughout the process the plates remain uniformly charged, and fringing is negligible.
(a) Determine the potential difference between the plates before they are separated.
(b) Determine the potential difference and the electric field magnitude between the plates after they are separated to the larger distance.
(c) Calculate the electrical energy stored in the capacitor (i) before and (ii) after the separation. Hence find the amount of work required to pull the plates apart.
(d) Immediately after the plates have been moved to the 4.0mm separation, a proton is released from rest at the negatively charged plate and allowed to accelerate toward the positively charged plate. Neglecting gravitational effects and assuming the field remains uniform, determine the speed of the proton just before it reaches the opposite plate.