In fact, we do not need to go far to find applications which could not be treated using, for example, Faraday’s law. The most obvious is transmission of power as, for example, in radio or television. All applications related to transmission of power (radar, communication, radio, etc.) were conspicuously missing in the previous chapters, but there is an even more important (and related) aspect of the electromagnetic field which was not mentioned until now. Take, for example, induction of voltage in a loop. Faraday’s law gives an accurate statement of how the induction occurs and the magnitude of the induced emf. Now let’s say that two loops are located a short distance from each other and one loop induces an electromotive force in the second. If we were to separate the loops a very long distance from each other and measure the induced voltage in the second loop, the magnitude will be very small. The question is, however, this: Is there any lag in time between switching on the current in the first loop and detection of the induced voltage in the second loop because of the distance between the loops? Faraday’s law says nothing about that and neither do any of the postulates used previously. Intuitively, we know there must be a time lag since nothing can occur instantaneously. In this regard, consider the following: On January 22, 2003, NASA received the last transmission from the Pioneer 10 space probe. At that time, Pioneer 10 was 5 weeks shy of its 31st year of space flight and was over 12.2 billion km from the Earth.1 At that distance, the transmission took approximately 11 h 18 min to reach the Earth. This is hardly instantaneous. In fact, if we divide distance by time, we find that the information has
field? The answer to the latter is emphatically yes.