On the following link were a discussion about energy in capacitor moving parallel to its field: Where's the energy in a boosted capacitor?
My question is what happen if capacitor is moving perpendicular to its field (=parallel to its plates)? There is also magnetic field and energy is stored both in magnetic and electric field and again is not increased by gamma factor from energy in capacitor that does not move. In this work i found (The theory of electromagnetic field motion) it is said that EM energy density must be calculated differently if source of field is moving. Is it correct? I don't like that explanation, I like more with force that holds capacitor separated.