As mentioned last chapter, the physical dimension of an electronic structure is compared with the signal wavelength to determine its electrical size. When a field is varying at low frequency, its wavelength is so big that it can be considered infinite compared to the physical size of the circuit. For example, 60Hz electric power from a wall outlet has a wavelength of 5 million meters, about the same order of distance from the eastern to western coast of the North American continent. This is a tremendous dimension compared to almost any manmade electric circuitry; therefore many of the equations we discussed earlier can be greatly simplified at low frequency. To be less dramatic, the wavelength of an audio signal broadcast by an FM radio station at 100 MHz is about 3 meters, which is still very big compared with the size of a regular circuit part, usually less than 10cm long, found on a Radio Shack shelf.