If a material had a relative permittivity less than 1 the charge stored would be less than with air/vacuum. the polarisation would have to be reversed so that the dielectric accumulated surface charge of the same polarity as the adjacent plate.
This separation of charge it isn't going to happen naturally in a material. it would require an external energy source to 'force' it's mobile charges or polar molecules to do this.
For this reason there isn't a material that inherently has a relative permittivity that is less than 1. However, covering a metal plate with a material of relative permittivity 0 on one side. Then place a charge on the metal plate. The system as a whole will accelerate towards the metal side since there is no flux on the covered side and so the charge is accelerated in the direction of the uncovered side. This is a violation of the conservation of energy principle, so it is impossible. Extending the argument to relative permittivity simply less than one instead of 0, there will be less flux on the covered side, so again the charge is accelerated in the direction of the uncovered side. So this must also not be possible. But we do have materials with relative permittivity greater than one, and I could use the same argument in that case. Why can I not, or why is it invalid to extend the argument for 0 permittivity to finite permittivity?
Dear Mohammadmahdi,
I do not think I understand your argument(s) and therefore I just make a couple of general comments :
1. In general, the dielectric permitivity is NOT a constant, but time/frequency dependent response function.
2. In causal and linear systems, the real(non-dissipative) and imaginary(dissipative) parts of the dielectric response function are related through Kramers-Kronig integral relations and for frequencies just beyond any resonant absorption band (Infra Red, Vis, Uv, X-rays, ..), the real part of the dielectric permitivity increases from -infinity( no life time or other types of broadening) to a constant positive value. At highest frequencies this constant value reaches value 1.0 from below and from there on, the material is completely transparent.
So, it all depends on the frequency you are looking at.
The argument with various metal plates, covered by dielectrics of Epsrel=0 and so on is in my yes un-physical (such dielectric does not exist, you are covering the metal plate with nothing (vacuum) !), but here I am apparently missing something (see Behman's comment above).
With best regards
Petr
Dear Mohammadmahdi,
As Petr mentioned above permittivity of a macroscopic medium
depends on frequency. It is well known that permittivity of noble metals
is negative at optical frequencies (plasmons). The same is true for
gaseous atmospheric plasma at MHz frequenies. Also permittivity of superconcductors is negative at microwave frequencies (IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 2, APRIL 2013. p. 1501011).
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