Introduction
In dc electrical circuits, the relationship among current, voltage, resistance, and power is simple. The same is true for ac circuits as long as those circuits do not store or release any energy during the course of each current cycle. When energy is stored and released during each cycle, an ac circuit is said to contain reactance . This can be caused by inductance, capacitance , or both.
Inductance
Inductance is opposition to ac by temporarily storing some of the electrical energy as a magnetic field. Components that do this are called inductors . Inductors often, but not always, consist of wire coils.
The Property Of Inductance
Suppose that you have a wire 1 million (10 ^{6} ) km long. What happens if you make this wire into a huge loop and connect its ends to the terminals of a battery? A current flows through the loop, and this produces a magnetic field. The field is small at first because current flows in only part of the loop. The magnetic flux increases over a period of a few seconds as the motion of charge carriers (mainly electrons) makes its way around the loop. A certain amount of energy is stored in this magnetic field. The ability of the loop to store energy in this way is the property of inductance, symbolized in equations by an italicized uppercase letter L .
Practical Inductors
In practice, you cannot make wire loops anywhere near 10 ^{6} km in circumference. But lengths of wire can be coiled up. When this is done, the magnetic flux is increased many times for a given length of wire compared with the flux produced by a singleturn loop.
For any coil, the magnetic flux density is multiplied when a ferromagnetic core is placed within the coil of wire. You may remember this from the study of magnetism. The increase in flux density has the effect of multiplying the inductance of a coil so that it is many times greater with a ferromagnetic core than with an air core. The inductance also depends on the number of turns in the coil, the diameter of the coil, and the overall shape of the coil.
In general, inductance of a coil is directly proportional to the number of turns of wire. Inductance is directly proportional to the diameter of the coil. The length of a coil, given a certain number of turns and a certain diameter, has an effect such that the longer the coil is made, the lower the inductance becomes.
The Unit Of Inductance
When a source of dc is connected across an inductor, it takes awhile for the current flow to establish itself throughout the inductor. The current changes at a rate that depends on the inductance. The greater the inductance, the slower is the rate of change of current for a given dc voltage. The unit of inductance is an expression of the ratio between the rate of current change and the voltage across an inductor. An inductance of 1 henry (1 H) represents a potential difference of 1 volt (1 V) across an inductor within which the current is increasing or decreasing at 1 ampere per second (1 A/s).
The henry is an extremely large unit of inductance. Rarely will you see an inductor this large, although some powersupply filter chokes have inductances up to several henrys. Usually inductances are expressed in millihenrys (mH), microhenrys (μH), or nanohenrys (nH). You should know your prefix multipliers fairly well by now, but in case you’ve forgotten, 1 mH = 0.001 H = 10 ^{−3} H, 1 μH = 0.001 mH = 0.000001 H = 10 ^{−6} H, and 1 nH = 0.001 μH = 0.000000001 H = 10 ^{−9} H.
Small coils with few turns of wire have small inductances, in which the current changes quickly and the voltages are small. Huge coils with ferromagnetic cores and many turns of wire have large inductances, in which the current changes slowly and the voltages are large.

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