I was discussing Ohm's Law with my teacher and asked about what would happen if the resistance of a circuit was zero, to which he replied that the wire would melt. This got me wondering about plasma/ionised gas and whether or not it would have zero/negative resistance.
The conductivity of plasmas is very high, though not infinite.
In a metal wire the applied voltage accelerates the conduction electrons, but the electrons collide with and scatter off the atoms that make up the crystal lattice of the metal. This transfers energy from the electrons to the metal and the metal heats up as a result. The energy lost as heat is what leads to resistance.
In a plasma applying a voltage causes the electrons to accelerate, but because a plasma has a much lower density than a metal the scattering and consequent energy loss of those electrons is very low. There will still be some scattering because electrons will scatter off the positive ions and any unionised gas molecules and indeed each other. So although the resistance is low it won't be zero.