8.3 Inducing Voltage

Here's the third installment of Module 8, Induction, in Navy Basic Electricity and Electronics series from the 1970s. The first two units were:

8.1 Electromagnetism
8.2 Inductors and Flux Density

1. From an earlier module, we learned that three factors were necessary to induce an EMF in a conductor:

• a magnetic field
• a conductor
• relative motion between conductor and field

This leads us to Faraday's Law: "The EMF induced or generated in a conductor is directly proportional to the rate at which a conductor is cutting the magnetic lines of flux."

2. As current starts through a conductor, it generates a magnetic field, so two of the three conditions for generating an EMF are satisfied. Motion does actually take place because the lines of electric flux start at the center of the conductor and move to its outer part. As the flux moves on each half toward the outer part of the conductor, the lines are moving in opposite directions, satisfying the third condition until current is flowing through the whole wire.

In the meantime, the expansion of the current to fill the wire creates a "counter EMF" or CEMF. For a moment, the resisting EMF reduces the current briefly. The momentary CEMF is almost equal to the source voltage.

When the current is turned off, the same phenomenon occurs. The field collapses, causing relative motion. This time the generated EMF wants to keep moving in the same direction as the current was flowing.

3. If there is a conductor in another circuit, near the first one, a current will be generated in it by the first circuit. The direction of the induced current will be in the opposite direction. This is called Lenz's Law: "The direction of an induced EMF tends to set up a current whose magnetic field is the opposite of the original current."

A DC circuit will only generate current in the second circuit when it is powering up or down. An AC circuit, on the other hand, because it's value is always alternating, will constantly generate current in the second circuit.