SAT physics question

Test Preparation SAT Subject Test Preparation
1

<p>Which of the following would increase the capacitance of a parallel-plate capacitor?</p>

<p>(A) Using smaller plates
(B) Replacing the dielectric material between the plates with one that has a smaller dielectric constant
(C) Decreasing the voltage between the plates
(D) Increasing the voltage between the plates
(E) Moving the plates closer together</p>

<p>The Answer is E, which I understand from C=E_0*A/d, but why is choice C wrong? Q=VC-->C=Q/V so if I decrease the voltage, I'd increase the capacitance...right?</p>

2

<p>No, the charge on the plates would go down so as to keep the capacitance constant. Similarly in answer D the charge on the plates would go up.</p>

3

<p>Ah I see, and this is because E field decreases right? E field decreases—>Q decreases</p>

4

<p>V= (kQ)/r</p>

<p>Since you are keeping the distance r constant and decreasing voltage, Q must decrease by the same factor. Therefore by C=Q/V, C wouldn’t change.</p>