# The electric field in the dielectric is wasted?

1. Apr 3, 2014

### baby_1

Hello
as you see the problem and solution (see the attachment)the input electric filed is 2K v/m but in the glass the electric filed decrease approximately to 705.9 v/m.i want to know
1- The electric field in the dielectric is wasted?
2-why electric field decrease?

Thanks

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2. Apr 3, 2014

### baby_1

and another question
permittivity can be changed as electric field booster.it means we can set the permittivity of the second environment that electric filed increase(like an electric filed amplifier)?if yes how it does?

3. Apr 3, 2014

### rude man

Because at the boundary between the air and the glass, polarization surface charges are set up. These charges set up an opposing E field to the external E field so the sum of the two E fields is less than the external E field alone.

4. Apr 3, 2014

### baby_1

Thanks dear rude man
"and another question
permittivity can be changed as electric field booster.it means we can set the permittivity of the second environment that electric filed increase(like an electric filed amplifier)?if yes how it does? "

is it possible the permittivity can increase electric filed?is possible that polarization surface charges setup an agree electric filed that increase E filed in the second environment?

5. Apr 3, 2014

### lightgrav

using a dielectric (in a capacitor gap) means that you can store κ times as much charge on the plates
(which would otherwise fill the gap with tremendously intense E-field) with the same voltage supply.
Because the charge in each potential carries Energy : (½QaVa + ½QbVb) , the stored Energy is also increased by factor κ .

At the molecular level, electric dipoles arise with their (+) end in the E-field's direction,
(so their internal E will always tend to cancel the external E-field).
Molecules that have permanent electric dipole try to rotate so as to reduce their PE in the external E-field ... this rotation always will reduce the local Electric field (to the extent that it occurs; solids severely limit rotation)

We can not force (torque?) molecular dipoles to orient themselves along the external E-field, the way magnets do naturally.

6. Apr 3, 2014

### rude man

True. Assume you have a slab of dielectric between two parallel plates. The plates are charged to some level Q. Then it takes positive work to remove the slab from between the plates. The new E field where the dielectric was is now larger than the E field when the dielectric was in place. This assumes Q did not change.