A simple transformer consists of a coil of wire fed by a voltage source such as a generator. The coil is wound around one side of an iron frame. This is the primary coil. The other side of the iron frame is wound with another coil, the secondary, that feeds electricity to a separate circuit. As alternating current from the generator flows in the primary coil, the magnetic field in that coil strengthens, weakens, and then reverses direction as the alternating current changes. The iron core intensifies these magnetic-field changes. As the magnetic field in the secondary coil changes with time, electric charges in the secondary are deflected and current is produced. The alternating voltage produced in the secondary depends on the relative number of turns in the secondary compared to the primary coil. Let's say that V1 is the voltage across the primary, V2 the voltage across the secondary, N1 the number of turns of coil in the primary, and N2 the number of turns of coil in the secondary If the secondary coil, for example, is wound with 100 times as many turns as the primary, the voltage across the secondary will be 100 times larger than that across the primary. A transformer used in this way is said to be a step-up transformer. If there are fewer turns on the secondary, it is a step-down transformer. It might seem that a step-up transformer gives more energy than it uses: after all, the larger secondary voltage means a larger energy change per charge moved in the field. But that voltage increase comes at the expense of a reduced current. If the transformer were 100 percent efficient, then
If I1 and I2 are the currents in the primary and secondary coils, then
Electric power is transmitted most efficiently at high voltage and low current. These conditions result in a minimum loss of energy as heat. On the other hand, low voltages are desirable in generating stations and homes for reasons of safety and insulation. For alternating currents, the job of changing voltage from one value to another can be done easily and cheaply by a transformer. Basically a transformer consists of two coils of wire that are insulated from each other. The coils are wrapped around the same laminated iron core. The coil to which power is supplied is called the primary. The coil from which power is delivered is the secondary. Alternating current flowing in the primary induces an alternating flux in the core. The continual building and collapsing of this flux induces a voltage in the secondary. Thus, a transformer transfers electric energy by magnetic induction. Voltages of the two currents are related directly to the ratio of turns of wire in the two coils. If E1 and E2 are the voltages and N1 and N2 the number of turns of wire in the coils, the relationship is:
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