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Transformers are often used in electrical circuits to change the voltage of electricity flowing through a circuit.
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Image courtesy of Wikipedia (modified by Windows to the Universe staff).

# Voltage Transformers in Electric Circuits

Transformers (sometimes called "voltage transformers") are devices used in electrical circuits to change the voltage of electricity flowing in the circuit. Transformers can be used either to increase the voltage (called "stepping up") or decrease the voltage ("step down").

The principle of electromagnetic induction is what makes transformers work. When a current flows through a wire, it makes a magnetic field around the wire. Also, if a wire is in a magnetic field that is changing, a current will flow in the wire. In a transformer, a wire brings a current in one side. That current makes a magnetic field, which in turn produces a current in a wire on the other side of the transformer. The second current flows out of the transformer.

Both wires in a transformer are actually wrapped in a coil around an iron core. The iron core is immersed in an insulating oil bath which does not conduct electricity well. The coils of wire are not physically connected. One wire has more turns in its coil than the other wire. The different numbers of coils in the two wires causes the voltage and current in each wire to be different from the other wire. By designing the transformer with just the right number of coils in each wire, electrical engineers can control exactly how much the transformer changes the voltage from the incoming to the outgoing wire.

Transformers only work with AC (alternating current) circuits. Since the AC current on the "incoming" wire is constantly changing, the magnetic field it creates changes too. The changing magnetic field is what forces current to flow in the "outgoing" wire.

Transformers are passive devices - they don't add power. A high voltage and low current exits the transformer carrying almost the same amount of power along the transmission lines that the incoming low voltage and high current did. Transformers are very efficient. Under normal conditions they transmit about 99% of the power that enters them (about 1% of the power is lost as heat).

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