effect of electric field on water


by BlueIntegral
Tags: effect, electric, field, water
BlueIntegral
BlueIntegral is offline
#1
Jan26-10, 10:22 PM
P: 5
I have been reading about dipoles in a uniform electric field and I was wondering: if you had some water molecules in a uniform electric field, could you get them to line up? Would this polarize the water? Also, would this be helpful for growing crystals at all (for liquids other than water of course that have a dipole)? If all the molecules do line up, then it's kind of like a lattice right?
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torquil
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#2
Jan29-10, 05:21 AM
P: 640
Check this out (about polarized water drops and lightning):

http://www.school-for-champions.com/..._lightning.htm

I don't know how rigid this structure is, but I guess in principle it could be classified as a lattice. I don't see why the dipoles wouldn't line up under a strong enough electric field. I wouldn't know about the rigidity. Maybe you could try to calculate it classically, for idealized dipoles? E.g. resistance towards pressure in different directions, and shear forces.

Maybe someone has done experiments on this, and observed "ice" under conditions where it would otherwise not appear. It's an interesting idea.

Torquil
BlueIntegral
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#3
Jan31-10, 12:19 AM
P: 5
I've done a little digging and found this article that seems to be relevant. I'm going to ask my professor on monday what he thinks too.

http://www.physorg.com/news110191847.html

I also found this, but it is very technical. From what I understood, it seems like a large electric field can have an effect on ice formation. If you can understand it better than I can, let me know if that's what it's saying.
http://www1.lsbu.ac.uk/water/magnetic.html

torquil
torquil is offline
#4
Jan31-10, 03:23 AM
P: 640

effect of electric field on water


First of all, I'm amazed that someone came up with this:

"Thus, a high-voltage electric field (333 kV m-1) has been shown to raise the water activity in bread dough, so ensuring a more efficient hydration of the gluten"

However, I doubt that there electric fields effects will be exploited by any bakers...

This sounds reasonable:
"lower fields (10^5 V m-1) encouraging ice formation in supercooled water"

This is probably an intermediate field strength. Strong enough to have an effect, but not strong enough to destroy the ordinary ice lattice. I'm assuming that they are cooling the water while a constant field is being applied, since the sudden application of a field across supercooled water leading to quicker ice formation would be a less interesting effect, I think (since it probably creates a random disturbance from which the lattice will start to form). This field streng is not extremely high, so it's not unreachable ouside the lab.

"High fields affect hydrogen bonding in an anisotropic manner, hydrogen bonds being strengthened along the field but weakened orthogonal to the field".

So this is as expected. The strong electric fields creates some kind of nonisotropic structure. I wonder if someone have measured fluid properties, i.e. non-isotropicity of stress tensor used in the Navier-Stokes equation, caused by the electric field.

The one about the water bridge was interesting as well. I guess in a way the rigidity of the water bridge would be explainable by using such a modified stress-tensor in the Navier-Stokes equation, if that equation can be used at all in this case.

If the water is that much polarized from the electric field that it manages to climb up and create such a bridge, then it should also climb up and pour of of the glass in which is collects, in an attemt to reach the cathode source of the field, since the field there is just as strong, right?

Torquil
Bob S
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#5
Feb1-10, 03:38 PM
P: 4,664
Water has a very high dc dielectric constant (relative permittivity ε = 80), which should lead to some interesting characteristics in an electric field.

The stored energy in a parallel plate capacitor is W = ˝CV2 = ˝εε0AV2/d

where ε0 is the permittivity of free space, A = plate area, and d the separation.

If we have a parallel plate capacitor roughly half submerged in ultrapure water, such that the electric field is horizontal along x, the plate width is y, separation d (along x), and the plate height is h. Plate area A = yh. If the depth of the capacitor plate in the water is z, then the total capacitance is

C = ε0y[εz + (h-z)]/d

When the voltage is turned on, the vertical force pulling the water up between the plates is

Fz = ∂W/∂z = ˝ε0y[ε-1]V2/d

This upward force is balanced by the downward gravitational force Fg on the water raised up a distance z-z0 between the plates, where z0 is the equilibrium height of the water in the capacitor gap with the voltage off. Fg = yd(z-z0)ρg, where ρ is the density of water.

Setting Fz = Fg and rearranging, we get (note: y cancels out, so y= ~ 5 cm is adequate):

z-z0 = ˝ε0(ε-1)V2/d2ρg

Using ε0= 8.85 x 10-12 Farads/m, d=0.003 m, V = 1000 volts, ε=80, ρ=1000 Kg/m3, g = 9.81 m/s2,

we get z-z0 = 0.004 m (=4 mm)

The separation d is sufficient to permit the water level in gap between the plates reach equilibrium (Adding some photographic Photo Flo to the water might be useful), and the electric field is about 3300 volts per cm (not enough to spark). Increasing V by a factor of 2 will increase z-z0 by a factor of 4.

So an electric field has an effect on water.

Bob S


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