In electrostatic fields relative permittivity of any material $ \epsilon_r > 1 $, meaning that electrostatic interaction within material is always weaker that in vacuum.
If E changes (electomagnetic fields) then many abstraction are used to describe it, for example complex permittivity and displacement current. I do not understand these notions very deep (say, I don't understand how real part of permittivity can be negative, what exactly real and imaginary parts mean physically).
But physically my question is simple:
- Force of interaction between a charged particle and electromagnetic field can be greater within any medium, than it is within vacuum?
This question is because I cannot imagine that in electrostatic field force of electrostatic interaction within medium is greater than it is in vacuum ($ \epsilon_r > 1 $). And I do not see that it shall be different in electromagnetic fields as well.