READING ASSIGNMENT The following reading assignment covers two topics we will no

Question

READING ASSIGNMENT The following reading assignment covers two topics we will not have time to discuss in lecture. Finish the reading assignments first, post any questions on Piazza, then solve the online quiz and problems below 1. Section 25-4 on energy stored in the electric field (you may leave out the diversion on airborne dust) 1 Questions related to the reading assignment (8 pts) (a) In your own words and no more than three sentences(!): What does the relation Cp2 refer to and mean? b) How much energy is spent to transfer 10 pC to a 1 nF capacitor? (c) In your own words and no more than three sentences(): What is the meaning of the expression u -oE2 and what are its units? d) The plates of a charged parallel-plate capacitor are moved farther apart. Does u change if a voltage source remains attached to the capacitor? What if the the voltage source is removed before increasing the plate distance?

Explanation / Answer

a) the relation 0.5*C*V^2 represents energy stored in capacitor

b) U = Q^2/(2*C) = (10*10^-12)^2/(2*1*10^-9) = 5*10^-14 J

c) u = 0.5*eo*E^2 represents energy density

units are J/m^3

d) if source is attached to the capacitor and plates are moved farther apart

then

E decreases and hence u decreases

if voltage source is removed then

u remains constant

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