Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

 

The concept of Faraday's Law is that any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

 

 

At upper left in the illustration, two coils are penetrated by a changing magnetic field. Magnetic flux F is defined by F=BA where B is the magnetic field or average magnetic field and A is the area perpendicular to the magnetic field. Note that for a given rate of change of the flux through the coil, the voltage generated is proportional to the number of turns N which the flux penetrates. This example is relevant to the operation of transformers, where the magnetic flux typically follows an iron core from the primary coil to the secondary coil and generates a secondary voltage proportional to the number of turns in the secondary coil.

 

Proceeding clockwise, the second example shows the voltage generated when a coil is moved into a magnetic field. This is sometimes called "motional emf", and is proportional to the speed with which the coil is moved into the magnetic field. That speed can be expressed in terms of the rate of change of the area which is in the magnetic field.

 

The next example is the standard AC generator geometry where a coil of wire is rotated in a magnetic field. The rotation changes the perpendicular area of the coil with respect to the magnetic field and generates a voltage proportional to the instantaneous rate of change of the magnetic flux. For a constant rotational speed, the voltage generated is sinusoidal.

The final example shows that voltage can be generated by moving a magnet toward or away from a coil of wire. With the area constant, the changing magnetic field causes a voltage to the generated. The direction or "sense" of the voltage generated is such that any resulting current produces a magnetic field opposing the change in magnetic field which created it. This is the meaning of the minus sign in Faraday's Law, and it is called Lenz's law.

 

 

 

 

120 volts at 60 Hz is applied to the coil. Since it has an iron core, a large alternating magnetic field is produced.

The magnetic field alternates 60 times per second, being produced by an AC, iron core coil. The changing magnetic field induces a voltage in the coil which is sufficient to light the bulb if it is close enough.

When you bring the bulb close enough for it to produce light, you are demonstrating the action of a transformer, which uses the changing magnetic field produced by the current in one coil to induce a voltage into a second coil. This is an example of Faraday's law.

 

 

How do you obtain 40,000 volts across a sparkplug in an automobile when you have only 12 volts DC to start with? The essential task of firing the sparkplugs to ignite a gasolene-air mixture is carried out by a process which employs Faraday's law.

 

 

 

 

The primary winding of the ignition coil is wound with a small number of turns and has a small resistance. Applying the battery to this coil causes a sizable DC current to flow. The secondary coil has a much larger number of turns and therefore acts as a step-up transformer. But instead of operating on AC voltages, this coil is designed to produce a large voltage spike when the current in the primary coil is interrupted. Since the induced secondary voltage is proportional to the rate of change of the magnetic field through it, opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday's Law. The large voltage causes a spark across the gap of the sparkplug to ignite the fuel mixture. For many years, this interruption of the primary current was accomplished by mechanically opening a contact called the "points" in a synchronized sequence to send high voltage pulses through a rotary switch called the "distributer" to the sparkplugs. One of the drawbacks of this process was that the interruption of current in the primary coil generated an inductive back-voltage in that coil which tended to cause sparking across the points. The system was improved by placing a sizable capacitor across the contacts so that the voltage surge tended to charge the capacitor rather than cause destructive sparking across the contacts. Using the old name for capacitors, this particular capacitor was called the "condenser".

 

More modern ignition systems use a transistor switch instead of the points to interrupt the primary current.

 

 

 

 

The transistor switches are contained in a solid-state Ignition Control Module. Modern coil designs produce voltage pulses up in the neighborhood of 40,000 volts from the interruption of the 12 volt power supplied by the battery.