Dielectric constant of water?

Question

Why Dielectric constant of water is greater than silicon?

Dielectric constant of water is also high than air.Why?

Answer 1

Water is a polar molecule--it has a nonzero dipole moment even in the absence of an electric field. Elemental silicon is covalently bonded and the crystal has inversion symmetry--therefore it has no net dipole moment in zero field.

Molecules in liquid water will line up their dipole moments parallel to an applied field. To polarize the silicon, first you have to generate a dipole moment--the moment is proportional to the applied field (for weak enough fields).

The polarizability of an atom is roughly determined by the energy gap between the atomic states you have to mix together to make a charge distribution which points in a specific direction. The larger the gap, the smaller the polarization. For a gap on the order of an electron-volt, using V=E*d over the size of an atom, this means a pretty substantial field is required to polarize an atom to the degree that a more ionic molecule is polarized on its own.

To get a sense of the relative scale of things, let's call that (large) field "1V/angstrom." The polarization of the silicon is proportional to E<<1 whereas the polarization of the water is on the order of 1.

Answer 2

Because the molecules of water are free to line their little electrostatic moments up with the applied field. Silicon atoms, since they're locked into a crystal lattice, are not so free. As a result, a water dielectric becomes more polarized under a weaker applied E field and its' dielectric constant is higher.

And yes, the world of materials characteristics was made that way specifically to f*ck with your head ☺


Doug

Edit: C'mon Cirric. Ya gotta believe he's talking about water with the usual 1 part in 10^(-7) hydronium and hydroxyl concentrations ☺

Answer 3

At lower frequencies, the highly polar water molecules can rotate their dipole moments to align with the applied elec. field. This results in a dielectric constant of roughly 80, which is huge.

But at some sufficiently high frequency, the water molecule is too heavy to rotate rapidly enough and its dielectric const. drops off dramatically. I think that freq. is in the range of 100 MHz or 1 GHz or higher, but I'm not sure.

Answer 4

It is different for all of the people, I have some friends who don't like milk, but I myself prefer milk. If you are on the diet, then have oatmeal with water, because there will be less calories and fats. If you aren't then just eat it with milk, it tastes better and it's healthy too :)

Answer 5

Hi. I don't think it is. Water's dielectric strength varies with its ion content.

Special comment for Doug. I hate it when I make the wrong assumption. Thanks for the reminder!