http://www.ck12.org Chapter 25. Electromagnetism
components of an electric transformer inFigure25.13.
FIGURE 25.13
An electric transformer connects two cir-
cuits with an iron core that becomes an
electromagnet.As you can see inFigure25.13, a transformer consists of two wire coils wrapped around an iron core. When
alternating primary current passes through coil P, it magnetizes the iron core. Because the current is alternating, the
magnetic field of the iron core keeps reversing. This changing magnetic field induces alternating current in coil S,
which is part of another circuit. InFigure25.13, coil P and coil S have the same number of turns of wire. In this
case, the voltages of the primary and secondary currents are the same. However, when the two coils have different
numbers of turns, the voltage of the secondary current is different than the voltage of the primary current. Both cases
are illustrated inFigure25.14.
- When coil S has more turns of wire than coil P, the voltage in the secondary current is greater than the voltage
in the primary current. This type of transformer is called a step-up transformer. - When coil S has fewer turns of wire than coil P, the voltage in the secondary current is less than the voltage in
the primary current. This type of transformer is called a step-down transformer.
For an animation of a transformer, go to this URL: http://www.youtube.com/watch?v=VucsoEhB0NA.
Electrifying the Home
Power plant generators produce high-voltage electric current. Many power plants also use step-up transformers to
increase the voltage of the current even more (seeFigure25.15). By increasing the voltage, the amount of current
is decreased, so less power is lost as the electricity travels through the power lines. However, the voltage in power
lines is too high to be safe for home circuits. The voltage in power lines may be as great as 750,000 volts, whereas
most home circuits are 240 or 120 volts. One or more step-down transformers decrease the voltage of current before
it enters the home. Other step-down transformers within the home lower the voltage of some of the home’s circuits.
For an overview of electric power generation, transmission, and distribution in the U.S., go to this URL: http://w
ww.youtube.com/watch?v=2eU3BgrmzkY.