If I fill a plastic tube with copper powder, what difference in conductivity will it be with a solid metal wire of same diameter.
Copper powder will be less conductive than wire. Copper wire is produced by drawing single ingots of copper into long strands, so there is very little other than atomic scale defects, to disrupt the conduction of electrons. Copper powder, unless you produce the powder and fill it into the tube (and seal it) in a high vacuum or inert atmosphere, will have a thin layer of copper oxide around each particle which will reduce the conductivity. And unless you compress the powder, you have to consider the size and shape of the particles; spherical particles will have very small amounts of surface actually in contact with nearby particles. To have about the same conductivity as wire, you'd have to compress and heat (sinter) it in a reducing atmosphere, but then it's not powder anymore, it's just a wire produced through powder metallurgy instead of drawing.
The powder will probably conduct less because you dont know the surface area contacting adjacent particles. If the powder was compressed, it would probably conduct better, but not as well as solid copper
