# Capacitance when dielectric is introduced

1. Jul 14, 2011

### logearav

1. The problem statement, all variables and given/known data
a dielectric of dielectric constant 3 fills three fourth of the space between the plates of a parallel plate capacitor. What percentage of the energy is stored in the dielectric.

2. Relevant equations

U=CV2/2
= epsilon0AV2/d.2

3. The attempt at a solution
When dielectric is introduced the above equation changes to epsilon0AV2/(d-t+t/epsilonr)
I substituted 3d/4 for t since dielectric fills three fourth of space between the plates of capacitor and 3 for relative permeability.
When simplified i got the answer epsilon0AV2/d
which is twice the value of U that is 2U
In the book the answer is given as 50% which i don't understand. How this 50% comes? Thanks in advance, revered members
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 14, 2011

### cupid.callin

how did you take V to be constant?
is it given that capacitor is connected to a battery?

3. Jul 14, 2011

### logearav

no sir. its not given. please guide me to solve this problem

4. Jul 14, 2011

### cupid.callin

is that the exact question given to you?
because i think something else should also be given.
ans also, the language of question is new to me

5. Jul 14, 2011

### logearav

yes sir i have the typed the exact question. no other details are given

6. Jul 14, 2011

### cupid.callin

there is only 1 way i got 50% energy increase due to introduction of dielectric.
first let capacitor is isolated (safe to assume)
let it has plate area A and separation d.
Energy, U = Q^2/2C

find energy when there is no dielectric, in terms of epsilon,A,d not in C (pretty easy :) )

then find net capacitance when dielectric is inserted.

now find energy,
*note: now Q will remain same as capacitor is separated.

you'll see that increase in energy is 1/2 of initial energy ... which is stored due to (or in other words, stored inside) capacitor.

7. Jul 20, 2011

### logearav

sir, can i proceed in this way as given in my attachment? Of course i dont know how 50% comes as given in the book? thanks in advance,sir

#### Attached Files:

• ###### capacitance.gif
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Last edited: Jul 20, 2011