# A 3500-pF air-gap capacitor question

1. Apr 8, 2015

### skepticwulf

A 3500-pF air-gap capacitor is connected to a 32-V
battery. If a piece of mica is placed between the plates,
how much charge will flow from the battery?

2. Relevant equations
C = K€A/d

3. The attempt at a solution
Q1=C1V1
Q2=C2V2

V1=V2=V
C2=7C1(as only diference between C1 and C2 is the mica and it's K is 7 so it's 7 times C1)
Result: Q2=7.84 x 10^-7Coloumb

**But solution manual say it's 6.7 x 10^-7Columb BECAUSE it calculates the new K by subtracting 1 from 7 (1 is the K of air) ???!!! Why subtract??? totally lost :(

2. Apr 8, 2015

### ehild

The question is how much charge is added to the already existing charge to the capacitor, which is Q2-Q1.

3. Apr 8, 2015

### skepticwulf

Ahh so by saying "how much charge will flow from the battery?" it means "how much ADDITIONAL/EXTRA charge will flow from the battery?" .. OK I've got got it now but the that part if the question is a bit misty, don't you think?

Thank you!