Abstract
We have mentioned that light is an electromagnetic wave with a frequency that lies in a particular range: from about 4 × 1014-Hz to about 7 × 1014-Hz. But what is an electromagnetic wave? To answer this question we will need to study electricity and magnetism. The principles of this branch of physics are the basis of operation of the various electronic instruments used in sound reproduction, such as radio transmitters and receivers, amplifiers, microphones, and speakers. There is an interesting further relevance: The atom often serves as a primary source of light and is the receiver of light in our eyes. How the atom performs these functions, as well as how the atom is held together, depend upon the laws of electricity and magnetism.
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Notes
- 1.
The north magnetic pole is diametrically opposite, in the southern hemisphere, close to New Zealand. During this century it has moved on average 10 km per year. See the Wikipedia article (12-26-2010): http://en.wikipedia.org/wiki/Magneticdeclination. Also: http://obsfur.geophysik.uni-muenchen.de/mag/news/e_nmpole.htm.
- 2.
The reader who is extremely probing of this process will note that even for the second observer, the electrons in the wire are moving in association with the induced current. As a consequence, the second observer (as well as the first observer) predicts that there will be an additional force—a magnetic force for both observers. This force on the electrons turns out to be directed to the left. As a result, an external force must be applied on the wire in order to prevent it from acquiring a motion to the left.
- 3.
This is an application of thePrinciple of Relativity . We experience one of its consequences when we sit in a subway train and watch a second train moving relative to us while having poor visibility of any other objects such as a train station. We wonder whether it is our train or the second train that is moving with respect to the tracks. For another common example of this principle, imagine yourself in a car stopped at a red traffic light on an upgrade. You see beside yourself a second car that is slowly moving forward. You then check whether your brake pedal is securely pressed because you worry whether it is in fact your car that is slowly moving backward. In this situation, until you discern the state of motion of the road or some other objects beside the road relative to yourself, you are finding it difficult to decide which of the two cars is actually moving with respect to the road. Now imagine yourself in a spaceship in outer space. You look out the window and see a second spaceship moving past you. Which of the two spaceships is moving, you might ask. Such a question has no answer. You can say that the second spaceship is moving with respect to yours or vice versa. Or, you might decide to investigate the state of motion of both spaceships with respect to the earth and find that both are moving with respect to the earth! It is clear that the state of motion, that is, the velocity of an object is a relative one; it depends upon the observer.
- 4.
Einstein’ Theory of Special Relativity does predict a dependence of the current on the state of motion of the observer. The dependence is small when velocities are much less than the speed of light in vacuum (3 × 108 m/s).
- 5.
The term radiation is used to refer to beams of tenuous material. Examples besides electromagnetic waves are beams of protons or electrons.
- 6.
Figure 5.74 of Hertz’s apparatus was generously provided by John Jenkins, who directs the Spark Museum. For more details about the museum, see its website at: http://www.sparkmuseum.com.
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Gunther, L. (2019). Electricity, Magnetism, and Electromagnetic Waves. In: The Physics of Music and Color. Springer, Cham. https://doi.org/10.1007/978-3-030-19219-8_5
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