Abstract
For an observer at rest, electric charges at rest produce only an electric field, but charges in motion produce a magnetic field in addition. This relation between magnetic and electric fields follows from the Rowland experiment. A still closer connection between the two kinds of fields is revealed by induction phenomena. This chapter presents the experimental facts relating to induction in a vacuum (i.e. – for all practical purposes – in air). Chapters 6 and 7 will then deal with their evaluation and explanation.
We start with an inhomogeneous magnetic field of arbitrary origin, e.g. the field from the short current-carrying coil (the field coil FC) in Fig. 5.1. A second coil J is placed in this magnetic field; it will be called the induction coil from now on. Its ends are connected to a voltmeter with a short response time. With this setup, we carry out a series of experiments which can be organized into three groups:
1. We leave the induction coil at rest in the magnetic field and change the strength of the field by varying the current through the field coil (rheostat R and switch).
2. We change the position of the induction coil and the field coil relative to one another by sliding or rotating one of them.
3. Instead of the induction coil J as drawn, we use a ring-shaped coil made of flexible insulated wire. We deform this ring-shaped induction coil in the magnetic field, i.e. we vary its cross-sectional area by moving some parts of its windings relative to other parts.
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Notes
- 1.
The relativity principle mentioned here can be expressed in terms of the following two equivalent statements: 1. “It is impossible to determine by experiment whether one is at rest or in a state of uniform motion”. 2. “When two experiments are carried out under the same conditions in two frames of reference which are moving relative to each other at a constant velocity, both experiments will lead to the same conclusions”. In order to make the relativity principle clear in terms of the setup for an experiment of group 2 as shown in Fig. 5.1, the one frame of reference S is chosen so that the field coil FC is at rest in it. The induction coil J is then moving in that frame at the velocity u, e.g. to the right. In order to measure the induced voltage impulse, the galvanometer is located in frame S (Fig. 5.4). In the other frame of reference S’, the induction coil Jis at rest and the field coil is moving with an equal but opposite velocity, e.g. to the left; then the measurement is carried out with a galvanometer which is at rest in S’. In both frames of reference, the observers measure a voltage impulse when the coils are well separated from each other. Their descriptions are different: The observer in S says that the induction coil is moving through the magnetic field; the other observer, in S’, says that the magnetic field within the fixed induction coil is changing. The relativity principle now postulates, in agreement with experiment, that both observers measure an identical voltage impulse in spite of their different descriptions of the process.
- 2.
Compare the small-print paragraph near the end of Sect. 5.3 for remarks about the sign.
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Lüders, K., Pohl, R.O. (2018). Induction Phenomena. In: Lüders, K., Pohl, R. (eds) Pohl's Introduction to Physics. Springer, Cham. https://doi.org/10.1007/978-3-319-50269-4_5
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DOI: https://doi.org/10.1007/978-3-319-50269-4_5
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