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An inductor is just a coil of wire. It takes advantage of the electromagnetic effect to perform its work.
Figure Simple Inductor
When electric current is passed through a wire, an electromagnetic field is generated around the wire. The next figure shows the concept in a simplified way.
Figure Magnetic Field Around a Current Carrying Wire
With wire wrapped in a coil such that wires are very close together, the magnetic fields combine
Figure Magnetic Fields Combine
Here's what it looks like in a full coil
Figure Electromagnetic Fields in a Coil
Of course, most coils are made to be longer and skinnier. In addition, the strength of the field can be increased considerably by putting a piece of iron in the core of the coil.
Figure Electromagnet with Iron Core
The strength (inductance) of an inductor is affected by four things
- the number of times the wire is wrapped around the core—more turns means higher inductance
- the type of material used for the core—iron gives higher inductance than plastic
- the thickness of the coil—more layers gives higher inductance
- the length of the coil—longer gives more inductance
Inductance is measured in henries. The symbol for an inductor is shown below.
Figure Symbol for Inductance
Inductors have interesting properties. When current begins to flow, the electromagnetic field begins to build around the wire and the coil. Until he field is completely built, the inductor resists the flow of current. So there is a time lag from when the inductor is energized until current is flowing fully through the circuit. The higher the inductance of the coil, the longer this takes. After the field is at full strength, the inductor has no more effect on the current and it flows normally. When the current is turned off, the electromagnetic field does not disappear instantly. Instead, it takes some time for it to collapse from full strength to zero strength. During this time current continues to flow. As the field collapses, the current gets less and less until it reaches zero. Inductors are sometimes used as filters to smooth out ripples in electric current to make it look more like DC battery current.
You've seen that an electric current flowing in a coil induces an electromagnetic field around the coil. The opposite is also true. If you move a coil through a magnetic field, an electric current will be induced in the coil. This property is exploited to make transformers. A transformer consists of two coils in close proximity. One is supplied with electric current and it generates an electromagnetic field. The other coil is surrounded by the electromagnetic field and an electric current is induced. This only happens while the strength of the magnetic field is changing in the first coil. Transformers only work with alternating current. AC changes direction 60 times per second. The electromagnetic field builds to full strength, collapses to zero, and builds to full strength in the opposite direction, again collapsing to zero for each cycle.
Figure Symbol for a Transformer
Transformers are used primarily to convert one AC voltage to another. Transformers used to lower the AC voltage are called step-down and transformers used to raise the AC voltage are called step-up. The effect is controlled by the number of windings in the transformer. More input windings than output windings will reduce the output voltage. Fewer input windings than output windings will increase the output voltage.
A common use is to convert 120 Volts AC house current to 12, 9, 6, or 3 Volts to power electronics equipment. A 12 volt output transformer will have 10 input windings for each output winding (120:12 = 10:1). Of course, the transformer is only part of the power supply. It produces alternating current. The power supply uses diodes to convert the AC to direct current (DC) and inductors and capacitors to filter the electricity and make it smooth DC.
Very large transformers are used on power transmission lines to step the voltage up and down.
For More Information
For more information on these topics, check out these web pages
- Interactive Java Tutorial Magnetic Field Lines
- Interactive Java Tutorial Attraction and Repulsion by Magnetic Poles
- How Electromagnets Work
- Interactive Java Tutorial Faraday's Magnetic Field Induction Experiment
- Interactive Java Transformer Tutorial
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