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A parallel-plate capacitor with only air between the plates is charged by connecting it to a battery. The capacitor is then disconnected from the battery, without any of the charge leaving the plates.

A) A voltmeter reads V_1 when placed across the capacitor. When a dielectric is inserted between the plates, completely filling the space, the voltmeter reads V_2. What is the dielectric constant of this material?

I know the answer to this: it is V_1/V_2 (as the definition of K).

B) What will the voltmeter read if the dielectric is now pulled partway out so it fills only one-third of the space between the plates?

I don't understand how to answer this one. I have tried going into the definition of capacitance with and without a dielectric, and came up with V_1/(k/3), but that is not the right answer. I don't understand how to figure this out...any and all help is much appreciated. Thank you