Parallel Plates Capacitors (Conceptual)

[anastasiaw]

Homework Statement

The sketch below is a side view of two capacitors consisting of parallel plates in air. The capacitor plates are equal in area but the plate separation differs as shown. Individual capacitors are specified with two letters, for example RS is a single capacitor. The charge on plate R is represented by QR. The capacitors are charged so that the potential (voltage) at A, VA, initially equals 11 volts. For each of the statements choose the proper response.

http://img524.imageshack.us/img524/2959/plotmr2.png

Homework Equations

U = (1/2)CV^2 = (1/2)QV

The Attempt at a Solution

1) The energy stored in capacitor TU is ... the energy
stored in capacitor RS.
LESS THAN: As the charge separation between two plates
increases, so does the energy stored between them. Since
there is a greater charge separation in the plates R and S,
the energy stored in capacitor RS will be greater.

2) If the plate separation for capacitor RS decreases, the
charge on QT will ... .
decrease?: Distance between R and S decreases, so the
voltage between them also decreases. So the voltage between
T and U increases? That means the charge built up on QT
will decrease.

3) QT is ... QR.
GREATER THAN: Since T is closer to its other plate than
U is, QT will be greater than QU. Less charge separation
means a greater charge will build up on the plate.

4) If the plate separation for capacitor TU decreases, the
energy stored in RS will ... .
increase?: Distance between T and U decreases, voltage
across TU decreases, voltage across RS increases, energy
stored increases?

5) The voltage across capacitor RS is ... that across
capacitor TU.
GREATER THAN: The voltage and the charge separation are
directly proportional, so the greater the charge separation
the greater the voltage.

6) The electric field between plates T and U is ... that
between plates R and S.
GREATER THAN: The distance between the two plates is
inversely proportional to the electric field between them.

7) QR + QS is ... zero.
EQUAL TO: The charges on opposite plates have equal and
opposite charges.

 

[anathema]

So the sum of QR and QS will cancel out
and result in zero charge.

8) The voltage at B is ... the voltage at A.
EQUAL TO: Since the capacitors are connected in parallel,
the voltage at all points will be equal. Therefore, the
voltage at B will be equal to the voltage at A.

 

[baba89]

This means that the total charge on these
two plates will cancel out and result in a net charge of zero.