How much work is done on the slab as it is inserted? pulled: Is the slab pulled

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How much work is done on the slab as it is inserted?

pulled: Is the slab pulled in or must it be pushed in? (You have only ONE try for this answer!)

You are correct.
Your receipt no. is 153-6793 Previous Tries How much work is done on the slab as it is inserted? Work could be positive or negative. pulled: Is the slab pulled in or must it be pushed in? (You have only ONE try for this answer!) You are correct. Your receipt no. is 153-8217 hrwc26_37.gif (c26p74) A slab of copper of thickness b = 1.406 mm is thrust into a parallel-plate capacitor of C = 3.00½10-11 F of gap d = 9.0 mm, as shown in the figure; it is centered exactly halfway between the plates. If a charge q = 8.00½10-6 C is maintained on the plates, what is the ratio of the stored energy before to that after the slab is inserted? 1.19 You are correct. Your receipt no. is 153-6793

Explanation / Answer

The conductor reduces the plate separation and in effect makes it two capacitors in series

U initial = 1/2*Q^2/C = 1/2*(6.00x10^-6)^2/4.00x10^-11 = 0.450J

After the slab is inserted the distance for each capacitor is (6.00 - 0.9231)/2 mm = 2.5385 mm

So the new capacitors C1 & C2 are equal to 4.00x10^-11*6.00/2.5385) = 9.455x10^-11F

Since the charge is maintained the energy stored in each new capacitor is

1/2*Q^2/C1 = 1/2*(6.00x10^-6)^2/9.455x10^-11 = 0.190J

So the total energy after the slab = 2*0.190 = 0.380

and the ratio = 0.45/0.38 = 1.18

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