Why is the velocity of a voltage wave in a pure copper wire so high? A pressure wave in air travels at the speed of sound. However a voltage wave in a copper conductor travels at a significant fraction of light speed. Why the difference of many orders of magnitude?
 A: This answer will probably seem lame to you -- for the real answer, take a course in electrodynamics.  In the one I took, in my third year in an electrical engineering program, this question was answered about three or four weeks into the course, at least in the context of a parallel-conductor transmission line.
The instructor wrote Maxwell's equations in the upper left corner of the board, and 50 minutes later we had an answer in the bottom left corner of the board.
The short and unsatisfying answer is that at those time scales a conductor does not so much conduct electrons as it guides the electromagnetic field, and that electromagnetic field naturally travels at the speed of light.
A: The speed of sound depends on the bulk properties of the medium that is carrying the sound.  Generally the speed is:
$$v=\sqrt{\frac{\text{elastic properties}}{\text{inertial properties}}}$$
For a solid, the "elastic property" is the Young's modulus for the substance, and the "inertial property" is its density.
But the transmission of an electric signal doesn't depend on the bulk properties of the conductor, since the signal is carried in the electromagnetic field. The conductor acts as a waveguide, steering the flow of energy carried in the electromagnetic field. The energy is in the em field that surrounds the conductor.  The speed of the waves depends only on the electromagnetic properties, but for good conductors separated by good insulators, the speed is always going to be close to the speed of light.
The speed of sound in a solid will be much slower, since it depends on the Young's modulus and density of the matter that make up the conductor. The speed of electricity doesn't.
So while the speed of sound will be on the order of several thousand km/h, the speed of electricity will be close to the speed of light, c.
