| Rotating electrical machines – generators and motors – devices that transform mechanical power into | | |
| Electrical power from a central power station can be transmitted and subdivided very | | |
| The operation of electrical machines is explained by | | |
| The first principle is that an electrical current causes a magnetic field which | | |
| The second is that an electrical current in a magnetic field (produced by some other currents) experiences a force perpendicular to both the direction of | | |
| The third is that an electrical conductor, such as a copper wire, moving in a magnetic field has | | |
| The fourth principle is that a changing magnetic field causes a voltage in any | | |
