depends on what the source voltage is. Also depends on if the white wire is actually being used as a neutral and not hooked up as a hot wire. Electrons don't care what color the insulation is on a wire.
If you are talking about a typical and properly wired electrical circuit in a house, the black is the hot wire, the white is the neutral and the bare copper wire is the ground.
The current in such a circuit is alternating. The white wire will have the same voltage as the black wire if the circuit is currently powering a load (lamp, etc.), but not at the same time. (The current will change directions in each wire according to the cycles of the current source.)
If a second white wire is used (necessary for a 3-way switch to operate), the end of the white wire is required to be painted black.