# Capacitor question.

Discussion in 'Homework Help' started by Jimbolimbo, Oct 6, 2014.

1. ### Jimbolimbo Thread Starter New Member

Oct 6, 2014
1
0
Greetings everybody, I have a small assignment in phsyics, in which I cannot grasp the answer.
Translated it says something like this: the voltage between the plates of a mica capacitor is 300 V, if the mica material were suddenly to disappear, what would the voltage be (im guessing it is replaced by air) ?
The capacitance would increase 7x, but is the capacitance C not a constant in a capacitor- meaning the more voltage you apply, the more charge your capacitor can hold. The voltage would stay the same ? And when you let it fully charge, would the capacitor not be an open loop ? The answer in the book says it is 1800 volts.
What to make of this ?

2. ### MrChips Moderator

Oct 2, 2009
12,657
3,461
Before you can answer the question you have to have basic knowledge of the physics of capacitors.

1) What is the capacitance value of a parallel plate capacitor? How does the capacitance depend on the dielectric constant?

2) What is the relative permittivity of mica compared with air or vacuum?

3) What is the formula that relates charge, voltage and capacitance?

4) What happens to the charge when the capacitance is altered?

The bottom line is the relative permittivity of mica:air is 6.
Hence the capacitance decreases by a factor of 6. The charge is constant. Hence the voltage increases by a factor of 6. The answer is 1800V.