What are the range of values for Electric Susceptibility? In case of air, I know it is zero(0).
My doubt is w.r.t. polarizability of air. If air has poor polarizability, then how does displacement current flow in vacuum/air. What is displaced in vacuum/air for the flow of Displacement current?
How can air be called as a dielectric even though it has a poor polarizability?
Rate of change of Electric Displacement field (D), which deals with charge migration, dipole reorientation is responsible for displacement current. But since air has poor polarizability, in what manner does displacement current flows through vacuum?
Dear Sir,
if you have not a metamaterial, the you have a positive value over Zero.
Yours Sincerely
F. Gräbner
Dear Suhas,
I see many sub-questions in your question, so I try to answer to a part of it.
The air changes a lot its dielectric strength with the humidity. Dry air has a dielectric strenght of 3 kV /mm, so it is quite good dielectric.
Just think to the MVolts of the electric lightings due to charges stored in the clouds (one electrode), and normally well separated from air to the ground (the other electrode).
The poor polarizability of the air comes when the air is very humid, so the insulating capacity of air falls and we have the electric discharge
Then how does displacement current flow under humid conditions? What is displaced so that this current flows by time varying E and H fields (Maxwell-Faraday, Maxwell-Ampere Laws, since no bound charges can be displaced in humid conditions)?
How does communication by Satellite, Mobile, Wi-Fi, Bluetooth et al. occur when no charges are displaced under humid conditions?
Displacement current does not require the movement of charges in a dielectric. There is a displacement current density in vacuum. It is epsilon nought times the rate of change of the electric field strength. This is where the "1" comes from when converting from electric susceptibility to relative permittivity (which relates displacement current to rate of change of electric field times the permittivity of free space). The presence of other dielectric materials that have moving polarization charges increases the displacement current, but there is displacement current even when there is no material there at all. The electric susceptibility of vacuum is zero. For air it is not quite zero, but mostly we ignore the difference. Don't think of displacement current density as the movement of charges, but as proportional to the rate of change of electric field strength.
There is no need for anything to be displaced for there to be displacement current - it only needs the electric field to be changing. This is true in vacuum, in air or in anything. In anything except vacuum something does get displaced, but doesn't account for all the displacement current.
Thank you sir Malcolm White, sir Frank Gräbner and sir Alberto Moro for your replies!
I understand that nothing can be displaced in vacuum unlike dielectrics. Then why do we call it as DISPLACEMENT CURRENT?
The name was invented by Maxwell - see this link for instance, and he did believe in an aether in vacuum that we now 'know' does not exist, from the Michelson Morely experiment to try to find how fast it was moving, which was one of the key experiments that started relativity. See the links below. The one on Maxwell involves div and curl, so if you don't have a grasp of these you may need to find another similar site where the equations are laid out in full.
http://farside.ph.utexas.edu/teaching/em/lectures/node46.html
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/mmhist.html
My curiosity asks, won't there be a displacement current due to polarization when the air has a lot of water vapour in it?
Also, what will be the direction of both the displacement currents produced by polarization and time varying Electric Fields?
Yes it will have - see the link. There is a displacement current due to polarisation in air, too. You could find this out yourself - I only had to google 'susceptibility of water vapour' to get this as the second or third hit. You won't understand this unless you do it yourself and work out the maths and make the different websites agree, or find out why they don't. You don't have to buy the whole article - reading the abstract referred to below is enough to answer your question.
http://journals.aps.org/pr/abstract/10.1103/PhysRev.48.538
Displacement current is a mathematical variable to solve the issue in Ampere's law for time varying signal. As of now, excluding near fields, they don't have clear meaning. The closed near field components, irrespective of material, carry energy equivalent of charges which seems like current. I still believe them as current, as a Maxwellian!!
Hi Kannadassan Sir,
How are you?
If displacement current doesn't have any meaning in the far field, how does an EM wave propagate in the far field?
Isn't displacement current that produces time varying magnetic flux (by Maxwell-Ampere Law) and in turn a time varying electric flux (by Faraday's Law) responsible for EM wave/radiation in the far field?
Sir Malcolm White and Sir Kannadassan, as Polarization reduces the net Electric field within a material due to formation of dipoles, won't the net displacement current reduce too?
ID = Itime varying electric flux - Idisplacement due to polarization.
Electric Susceptibility =ε0ε. ε0 - the dielectric susceptibility of the vacuum, the value depends on the system of units . For vacuum ε =1 (not 0 !!!). For any environment ε > 1. This follows from the laws of thermodynamics.
What can I conclude about air? Is it an insulator (cannot be polarized) or a dielectric (can be polarized)?
Sir M. L. Shur,
I was saying that air has 0 polarizability, not 0 permittivity. Obviously, air has relative permittivity of 1 (1.00054 to be exact) and vacuum has relative permittivity equal to 1.
Thank you for you reply sir!
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