Which statements are true for two oppositely charged, isolated parallel plates:

Question

Which statements are true for two oppositely charged, isolated parallel plates: C=capacitance, U=stored energy (Q and -Q = charge on the plates). Note: Isolated plates can not lose their charge.

a)When the distance is doubled, U increases.

b)Inserting a dielectric increases Q.

c)Inserting a dielectric decreases U.

d)Inserting a dielectric increases C.

e)When the distance is doubled, C increases.

f)Increasing the distance increases the Electric field.

g)When the distance is halved, Q stays the same.

Explanation / Answer

capacitance C = e0*A/d

energy U = (1/2)*C*v^2 = (1/2)*Q^2/C

(a)

When the distance is doubled

C' = C/2

U' = (1/2)*Q^2/C' = 2*U

U' > U

increases

TRUE

==============

(b)

Inserting a dielectric

as the plates are isolated Q remians same

FALSE

-------------

(C)

Inserting a dielectric

C' = kC

U' = (1/2)*Q^2/C' = U/k

U' < U

TRUE

--------------

(d)

Inserting a dielectric

C' = kC

C' > C

TRUE

-----------

(e)

C' = eo*A/(2d) = C/2

C' < C

FALSE

------------

f)

E = Q/Ae0

E does does not depend on d

FALSE

-----------

g)

TRUE

a c d g area TRUE

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