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Electromotive Force Produced by Motion

Author: E.E. Kimberly

If a rod of conducting material be caused to move in a magnetic field, as shown in Fig. 1-3, there will be a disturbance in the balance of protons and electrons within the atoms of the rod. Some electrons will migrate to the near end of the rod, leaving an excess of protons on the far end. This disturbance in balance is said to be caused by an electromotive force (emf) resulting from the motion of the conductor across the magnetic field. The electromotive force is said to produce a difference of potential between the two ends of the rod. In practical units, electromotive force is measured in volts. When the conductor is cutting a magnetic field or "magnetic flux" at the rate of 108 lines of force per second, 1 volt is being generated.

Fig. 1-3. Generation of Electromotive Force in a Conductor

Expressed as a formula, the foregoing statement may be written as follows:



e = instantaneous generated emf;
N = number of conductors in series (1 in Fig. 1-3);
ϕ = number of lines of magnetic flux;
t = time, in seconds.

This equation is called Faraday's Equation in honor of the discoverer of these relationships. The phenomenon is known as electromagnetic induction and is the most common means used in the generation of voltages for commercial use.

If there be a difference in potential between the ends of a rod, as in Fig. 1-3, and the ends be joined by a metal wire, the electrons will flow from the near end of the rod through the wire to the far end of the rod and restore the electron-proton balance. Inasmuch as the excess electrons on the near end of the rod are attracted to the excess positive protons on the far end of the rod, the far end is said to be at positive potential or "positive."

If the direction of motion in Fig. 1-3 be reversed, the direction of the emf generated in the rod will be reversed also. Or, if the direction of motion remain as before and the polarity of the magnet be reversed, the direction of the emf generated in the rod will be reversed. The magnitude of the emf generated in a conductor depends directly on the rate at which that conductor cuts magnetic lines of force. Therefore, the emf is a direct function of the velocity of the conductor and of the strength of the magnetic field through which it moves.

Last Update: 2010-10-06