Problem with model answer to exam

[madah12]

Homework Statement

http://physics.ku.edu.kw/LectureNotes/TheoryCourses/PHYS102/ExamPapers/y1011-2-md2.pdf

so i just did this exam and they gave us model solutions and I don't agree with conceptual question number 1 the answer should be 2 because it isn't connected to battery therefore voltage should change
and so should electric field
E=(sigma)/k*e_0 , charge is constant k is equal to 2 so it should be halfed
i don't see why they conclude its connected to a battery? it says fully charged so who would connect a fully charged capacitor to a battery also it doesn't say it is connected

Homework Equations

The Attempt at a Solution

 

[SammyS]

The energy stored in a capacitor is given by $$U_{Stored}=\frac{1}{2}\frac{Q^2}{C}\,.$$

If the charge on the capacitor is constant and some change is made to the capacitor which doubles the stored energy, that implies that the capacitance is halved. To do this by inserting a dielectric can only be done if a dielectric material of K replaces one of 2K . (That would double the electric field.) However, that goes against goes against the gist of the problem, which seems to imply (without stating so explicitly) that the capacitor initially has no dielectric.

 

[Quinzio]

This problem seems very confusing.

 

[madah12]

ok i talked to my instructor and he says we were supposed to infer that it was connected to battery because they said energy increases what do you think?

 

[Delphi51]

Well it is on the tricky side, but you can deduce that the V is maintained from the energy increase, so I guess it is okay.

Interesting comparison of the two situations here:
http://uk.answers.yahoo.com/question/index?qid=20090815093952AAaJiPp

 

[madah12]

yeah i guess well I usually hate it if doctors go well this is so obvious if you can't do this then you don't understand anything.