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Alternating Currents

When a conductor is moved back and forth in a magnetic field, the flow of current in the conductor will change direction as often as the physical motion of the conductor changes direction. Several electricity-generating devices operate on this principle, and the oscillating current produced is called alternating current. Alternating current has several valuable characteristics, as compared to direct current, and is generally used as a source of electric power, both for industrial installations and in the home. The most important practical characteristic of alternating current is that the voltage or the current may be changed to almost any value desired by means of a simple electromagnetic device called a transformer. When an alternating current passes through a coil of wire, the magnetic field about the coil first expands and then collapses, then expands with its direction reversed, and again collapses. If another conductor, such as a coil of wire, is placed in this field, but not in direct electric connection with the coil, the changes of the field induce an alternating current in the second conductor. If the second conductor is a coil with a larger number of turns than the first, the voltage induced in the second coil will be larger than the voltage in the first, because the field is acting on a greater number of individual conductors. Conversely, if the number of turns in the second coil is smaller, the secondary, or induced, voltage will be smaller than the primary voltage.

The action of a transformer makes possible the economical transmission of current over long distances in electric power systems (see Electricity Supply). If 200,000 watts of power is supplied to a power line, it may be equally well supplied by a potential of 200,000 volts and a current of 1 ampere or by a potential of 2,000 volts and a current of 100 amperes, because power is equal to the product of voltage and current. However, the power lost in the line through heating is equal to the square of the current times the resistance. Thus, if the resistance of the line is 10 ohms, the loss on the 200,000-volt line will be 10 watts, whereas the loss on the 2,000-volt line will be 100,000 watts, or half the available power.

The magnetic field surrounding a coil in an AC circuit is constantly changing, and constantly impedes the flow of current in the circuit because of the phenomenon of inductance mentioned above. The relationship between the voltage impressed on an ideal coil (that is, a coil having no resistance) and the current flowing in it is such that the current is zero when the voltage is at a maximum, and the current is at a maximum when the voltage is zero. Furthermore, the changing magnetic field induces a potential difference in the coil, called a back emf, that is equal in magnitude and opposite in direction to the impressed potential difference. So the net potential difference across an ideal coil is always zero, as it must necessarily be in any circuit element with zero resistance.

If a capacitor (or condenser), a charge-storage device, is placed in an AC circuit, the current is proportional to its capacitance and to the rate of change of the voltage across the capacitor. Therefore, twice as much current will flow through a 2-farad capacitor as through a 1-farad capacitor. In an ideal capacitor the voltage is exactly out of phase with the current. No current flows when the voltage is at its maximum because then the rate of change of voltage is zero. The current is at its maximum when the voltage is zero, because then the rate of change of voltage is maximal. Current may be regarded as flowing through a capacitor even if there is no direct electrical connection between its plates; the voltage on one plate induces an opposite charge on the other, so, when electrons flow into one plate, an equal number always flow out of the other. From the point of view of the external circuit, it is precisely as if electrons had flowed straight through the capacitor.

It follows from the above effects that if an alternating voltage were applied to an ideal inductance or capacitance, no power would be expended over a complete cycle. In all practical cases, however, AC circuits contain resistance as well as inductance and capacitance, and power is actually expended. The amount of power depends on the relative amounts of the three quantities present in the circuits.