I am having a bit of trouble with these two problems, I am new to electromagnetism, would anyone be ab le to shed some light on them? thanks. Given that air becomes ionised at an electric field strength of 3x10^6 Vm^-1 , find the greatest charge per metre that can be placed on a straight wire of 1cm diameter in air. and Two dielectric materials of relative permittivity 3 and 7 respectively are placed in contact with another and subjected to an electric field at right angles to the boundary between them. If the electric field strength in the dielectric of relative permittivity 3 is 300Vm^-1, what is the electric field strength in the other dielectric?
For the first one you need to find the electric field of an infinite straight wire as a function of charge per meter. http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/elecyl.html#c1 For the second one you just need to know the boundary conditions for electric field. They're related by the permittivity.