A Leyden Jar, consists of a piece of foil wrapped
around the outside of a glass jar, and another piece o
foil lining the inside of a glass jar. The energy stor
in a charged Leyden Jar is actually stored:
A) On the metal foil inside the jar.
B) On the metal foil outside the jar.
C) On both the inside and outside foil.
D) Inside the jar itself.
f
ed
E) In the glass between the foils.
The electric field stores the energy, and the field
resides in the glass. In addition the polarization
of the molecules (which reduces the field) also stores energy.
Consider a simple capacitor made of a pair of conducting plates in close
proximity. Supposed the plates are charged + and -, and then discharged
producing a spark. Next the plates are charged again, exactly as they
previously were, only this time after they are charged they are pulled further
apart. This time the spark that is produced will be:
A) Bigger than the first spark.
B) The same as the first spark.
C) Smaller than the first spark.
D) There won’t be any spark.
Pulling the plates apart requires a force working against the
electrostatic (attractive) force. This force over the distance that the
plates are pulled apart means that you did work, which means the
energy went somewhere. It went into the E-field.
You can also look at it as: the charge stays the same, the
capacitance is reduced, so the voltage goes up. Stored energy is U=
½CV2=½Q2/C , so as C is reduced, U goes up.
The capacitor of a Vandergraaf's generator consists of just a large metal
sphere. If the size of the sphere is doubled, the capacitance:
A) Remains the same.
B) Halves.
C) Doubles.
D) Quadruples.
E) Don’t know.
For a sphere, 0
0
4
4
QQ
RV
VCR
π
ε
πε
=⇒==
