|Transormers work by induction. When the magnetic field through a
coil changes in time, it has the same effect as an electric field
looping around the coil. (This old discovery turns out to be a
consequence of Special Relativity.) The current through the primary
coil makes a magnetic field. It changes when the current changes. That
induces a voltage on the secondary coil. |
Of course if the primary current doesn't change (dc current) this whole process doesn't happen.
You can make something like a dc transformer out of superconducting coils, using special quantum effects. However, if operated dc it does not serve to transmit power from one coil to the other.
In practice, dc-dc power transmission typically involves conversion to ac, use of ordinary transformers, then rectification back to dc. We use ac power lines because this process is cumbersome, and was even more cumbersome when the electrical power system was first being developed.
(published on 10/22/2007)