# Chapter 21: Electromagnetic Induction

Difficulty Level: At Grade Created by: CK-12

If you have ever looked at above ground distribution electric power lines, you have surely seen barrel shaped devices attached to various power poles.  These barrel shaped devices are called transformers and their purpose is to change the form of the electric power.  All metal transmission wires have some amount of resistance to the passage of electric current.  The result of forcing electric current through the resistance is that some of the electric power is converted to heat and the energy lost to the surroundings.  The amount of energy lost is proportional to the square of the electric current.

The electric power transmitted over these lines can be calculated in watts and is equal to the voltage times the amperage of the current.  If you transmitted 100 amps with a voltage of 50,000 volts, the power transmitted would be 5,000,000 watts.  This same amount of power could be transmitted by using an electric current of 5 amps and a voltage of 1,000,000 volts.

Since the energy lost to heat is related to the square of the amps, using the 5 amp current instead of the 100 amp current would make a tremendous difference in the energy lost.  Therefore, it is clearly wise to increase the voltage of the current and reduce the amperage before sending the current over the long transmission lines.  When the electric current reaches houses and factories and other places where the electricity will be used, the appliances cannot use a current of 5 amps with a voltage of 1,000,000 volts.  Therefore, the electric current must be transformed to some form of electric current that households can use.

Therefore, the most efficient system for the transmission of electric power is to step up transform the current to a higher voltage before long distance transmission and then step down transform the current to a lower voltage when the electric current is to be used.  Those transformations are the purpose of the transformers on the power lines.

In this unit, you will learn about transformers and other applications of electromagnetic induction.

Chapter Outline

### Chapter Summary

Electrical resistance within a wire is dependent not only on the material of the wire, but on the length of the wire and the amount of current flowing through it.  By converting electrical power to high voltage and low current, power companies can efficiently transfer energy over great distances with little loss.  The power must be converted back at the destination for use in homes and businesses.

Well-built transformers can function with virtually no loss as the transformation is the result of the expansion and contraction of fields around a core, no resistor is necessary.