Homework SourseA parallelplate capacitor is constructed from two 13 cm 13
A parallel-plate capacitor is constructed from two 13 cm × 13 cm electrodes spaced 1.1 mm apart. The capacitor plates are charged to ± 20 nC , then disconnected from the battery.
Part A How much energy is stored in the capacitor?
Part B Insulating handles are used to pull the capacitor plates apart until the spacing is 2.4 mm . Now how much energy is stored in the capacitor?
Part C Energy must be conserved. How do you account for the difference between A and B?
| a) The battery supplied additional energy. | |
| b) The electric field supplied additional energy. | |
| c) Kinetic energy of the charges was transformed into potential energy. | |
| d) Work was done to pull the plates apart. |
Solution
C = A*apsalan/d = (0.13^2*8.85*10^-12)/(1.1*10^-3) = 1.359*10^-10 F
a)
U = Q^2/2C = ((20*10^-9)^2)/(2*1.359*10^-10) = 0.0000014716 = 1.47*10^-6 J
b)
C2 = A*apsalan/d = (0.13^2*8.85*10^-12)/(2.4*10^-3) = 0.623*10^-10 F
c)
U2 = ((20*10^-9)^2)/(2*0.623*10^-10) = 0.00000321 J
difference = U2 - U1 = 0.00000321 - 0.0000014716 = 0.000001738 J OR 1.738*10^-6 J
