2017-11-20 08:43:29 UTC
"Dielectrics in capacitors"
When the dielectric is liquid, e.g. water, additional effects are observed:
1. If the capacitor is only partially immersed, water rises:
"A plane capacitor with rectangular plates is fixed in a vertical position. [...] The capacitor is charged and disconnected from the battery. [...] The lower part of the capacitor is now brought into contact with a dielectric liquid:
When the plates contact the liquid's surface, a force in the upward direction is exerted on the dielectric liquid. The total charge on each plate remains constant..." [END OF QUOTATION] http://electron6.phys.utk.edu/PhysicsProblems/E&M/2-Dielectrics/capacitors_with_dielectrics.html
2. PERPETUAL (limited only by the deterioration of the system, e.g. slow discharge of the capacitor) FLOWS are seen between the plates:
"Liquid Dielectric Capacitor"
In an electric field, water develops a specific local pressure that pushes in all directions and results in perpetual (limited only by the deterioration of the system) flows in the bulk of the liquid. This pressure is NON-CONSERVATIVE. This means that, if suitably harnessed, the pressure will do work AT THE EXPENSE OF AMBIENT HEAT (in violation of the second law of thermodynamics). Here is the molecular mechanism:
If it were not for the indicated (with an arrow) dipole, other dipoles in the picture are perfectly polarized as if there were no thermal motion. Of course, this is an oversimplification – thermal motion is a factor which constantly disturbs the polarization order. The crucial point is that, as can be inferred from the picture, any thermal disturbance contributes to the creation of a local pressure. Consider the indicated dipole. It has just received a strong thermal stroke and undergone rotation. As a result, it pushes adjacent dipoles electrostatically. One can say, somewhat figuratively, that the indicated dipole has absorbed heat and now, by pushing adjacent dipoles, is trying to convert it into work.
The sum of all such disturbances is macroscopically expressed as multiple flows within the liquid and an overall pressure that counteracts the original electrostatic attraction between the plates:
"However, in experiments in which a capacitor is submerged in a dielectric liquid the force per unit area exerted by one plate on another is observed to decrease... [...] This apparent paradox can be explained by taking into account the DIFFERENCE IN LIQUID PRESSURE in the field filled space between the plates and the field free region outside the capacitor." http://farside.ph.utexas.edu/teaching/jk1/lectures/node46.html
Wolfgang K. H. Panofsky, Melba Phillips, Classical Electricity and Magnetism, pp.115-116: "Thus the decrease in force that is experienced between two charges when they are immersed in a dielectric liquid can be understood only by considering the effect of the PRESSURE OF THE LIQUID ON THE CHARGES themselves." http://www.amazon.com/Classical-Electricity-Magnetism-Second-Physics/dp/0486439240?tag=viglink21401-20
Tai Chow, Introduction to Electromagnetic Theory: A Modern Perspective, p. 267: "The strictly electric forces between charges on the conductors are not influenced by the presence of the dielectric medium. The medium is polarized, however, and the interaction of the electric field with the polarized medium results in an INCREASED FLUID PRESSURE ON THE CONDUCTORS that reduces the net forces acting on them." http://www.amazon.com/Introduction-To-Electromagnetic-Theory-Perspective/dp/0763738271
Here is an even more spectacular illustration of perpetual (limited only by the deterioration of the system) motion of water in an electric field:
"The Formation of the Floating Water Bridge including electric breakdowns"
The system is obviously able to produce work - e.g. by rotating a waterwheel - and this work will be done at the expense of ambient heat, in violation of the second law of thermodynamics.