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[[File:Transformer-polemount-singlephase-closeup.jpg|thumb|right|A transformer hung on a pole to serve a few houses in a neighborhood.]]A '''transformer''' is a device that transfers [[electrical energy]] from one [[circuit]] to another, usually at different voltages.

[[File:Transformer-polemount-singlephase-closeup.jpg|thumb|right|A transformer hung on a pole to serve a few houses in a neighborhood.]]A '''transformer''' is a device that transfers [[electrical energy]] from one [[circuit]] to another, usually at different voltages.

Early electrical systems were developed using [[direct current]].  However, the heat loss limited the ability to send electric power a long distance from the generating station.  This caused the industry to adopt [[alternating current]].  With alternating current, electric power is generated at a low voltage.  A "step up transformer"  then raises the voltage to a much higher level (for example 220,000 volts).  Power can travel down a transmission line at a high voltage with less energy being converted to heat.<ref>Joule heating equal i²R.  If the voltage doubles, the current is cut in half to transmit the same power.  This will cut the heat loss by 1/4.</ref>  When the electric power arrives at a local substation a "step down transformer" will lower the voltage.  Finally, at a point near an individual customer's house, a final transformer will lower the voltage to 120 volts (in the United States).

Early electrical systems were developed using [[direct current]].  However, the heat loss limited the ability to send electric power a long distance from the generating station.  This caused the industry to adopt [[alternating current]].  With alternating current, electric power is generated at a low voltage.  A "step up transformer"  then raises the voltage to a much higher level (for example 220,000 volts).  Power can travel down a transmission line at a high voltage with less energy being converted to heat.<ref>Joule heating equal i²R.  If the voltage doubles, the current is cut in half to transmit the same power.  This will cut the heat loss by 1/4.</ref>  When the electric power arrives at a local substation a "step down transformer" will lower the voltage.  Finally, at a point near an individual customer's house, a final transformer will lower the voltage to 120 volts (in the United States) or 230 volts (in Europe).

A transformer works because the two circuits are wound around a common magnetized metal core. As the current alternates on the first circuit, it creates a varying [[magnetic field]].  That field, in turn, induces a current in the coils of the second circuit.  The ratio of the voltage on the two circuits is determined by the ratio of the number of times coils on the two circuits are wrapped around the metal core that serves as a magnet.

A transformer works because the two circuits are wound around a common magnetized metal core. As the current alternates on the first circuit, it creates a varying [[magnetic field]].  That field, in turn, induces a current in the coils of the second circuit.  The ratio of the voltage on the two circuits is determined by the ratio of the number of times coils on the two circuits are wrapped around the metal core that serves as a magnet.