Download A parallel plate capacitor is made up ofa pair of circular metal plates

Lesson 9
(1)A parallel plate capacitor is made up of a pair of circular metal plates of
radius 25cm separated by a distance of 1.0cm. The plates are connected
to the terminals of a voltage source that delivers 160V. Find
(a) the capacitance
(b) the stored charge,
(c) the stored energy,
(d) the electric field between the plates.
(2) What is the highest voltage that can be safely carried by the capacitor of
the previous problem if the breakdown electric field of air is 3 MV1m?
(3) After charging a parallel plate capacitor and disconnecting the voltage
source, the distance between the plates is reduced.
1.
2.
3.
Does the voltage increase or decrease?
Does the energy stored increase or decrease?
Is mechanical work required to move the plates?
(4) Repeat problem (3) if the distance is reduced while the plates are still
connected to the voltage source.
(5) After charging, the plates of the capacitor of Problem 1 are
disconnected from the voltage source. The separation of the plates is
now reduced to O.7cm.
Find the new energy stored.
(6) Find the electrostatic energy of a thin spherical shell of radius R
carrying a total charge Q.
(7) A co-axial cable consists of an inner cylindrical shell of radius a and an
outer shell of radius b, both oflength L. The inner shell carries a line
charge density A. Use Gauss law to determine the electric field at a
point a distance r from the axis where acr-cb, assuming Lcan be
considered infinite. By integration, find the potential difference
between the shells. Hence find an expression for the capacitance of the
cable.
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