In a conductive material, the positive charges are locked into a solid matrix, while the negative charges are not. The negative electrons of a conductor are still orbiting their protons. They orbit in amongst all the protons of the material with wandering paths. They take a particular energy level, in somewhat the same way that the electrons of an individual atom can only orbit at a certain energy level. A block of conductive material is something like a gigantic atom, it has many atomic nucleii all spread out, but it has just one electron cloud with a particular energy level.
Different materials have different energy levels.
Junctions between different conductors is the key to the useful effects displayed by LEDs, solar cells, and thermocouple devices. It's like a waterfall. If the electrons are forced to flow from the higher energy to the lower energy, each electron gives off a pulse of vibration as it falls down the step. In metals, the jump in energy is small, and the vibration is a heat vibration rather than a photon of light. So, whenever an electric current goes down an energy step as it travels from one type of material to another, it gives off energy.