Alternating current

An alternating electric current is a current with a time-varying direction and force. Those currents, which vary only in magnitude, are called pulsating. In industry and everyday life, an alternating sinusoidal current is most often used .

Conversion of a direct current into an alternating electric one can be performed as follows. We place a coil of wire in a uniform constant magnetic field. With uniform rotation of this turn around the axis, the magnetic flux will change continuously in both magnitude and direction. Consequently, according to the law of electromagnetic induction, a variable in the direction and magnitude of the electromotive force (EMF) is formed in the turn . If such a turn is attached to an external circuit, then in it we obtain an alternating electric current.

When the plane of a rotating turn becomes perpendicular to the lines of force of a given magnetic field, the magnetic flux passing through it is the largest (Φ = Φmax), but its rate of change is zero (ΔΦ / Δt = 0), because, passing through this position, Conductors of the revolution slip through the field lines of force, without crossing them. So, the EMF of induction, formed in the turn, will become zero (E = 0).

When the plane of the turn is parallel to the force lines of the field, the stream that pierces it is equal to zero (Φ = 0), the rate of its change in this position is the greatest ((ΔΦ / Δt) max), since the conductors of the turn move perpendicularly relative to the lines of force.

The EMF that occurs in this case in the turn has the greatest value (E = Emax). With further rotation of the coil, the rate of change of the flux penetrating the coil will increase; Hence, the EMF will increase in absolute value from 0 to Emax. Thus, the level of EMF induction in a rotating coil changes from -Emax to + Emax in one turn.

Open the loop of the wire and attach it to the oscilloscope. When the turn rotates in a magnetic field, the oscilloscope will record all the current changes, along which it will be possible to judge the change in the electromotive force in the revolution during one turn.

The current that appears in the coil when it is uniformly reversed in a uniform magnetic field, as shown by the oscillogram, varies sinusoidally. This current is called alternating sinusoidal.

The time interval for which the electromotive force performs one oscillation is called the period of alternating current.

The letter designation of the oscillation period is T. The number of oscillations per second is the frequency of the current, which is denoted by the letter f. Its unit of measurement is hertz (Hz):

F = 1 / T, or T = 1 / f.

If the value of the EMF at some arbitrary time is denoted by e (its instantaneous value), and the largest value (amplitude) by Emax, the law expressing the dependence of e on time in the case of a sinusoidal current can be expressed as the following expression:

E = Emax˖sin (2 / T) t.

In most countries, an alternating current with a frequency of 50 Hz, a period duration of 0.02 seconds is used in industry and at home.

The production of an alternating electric current from mechanical energy is carried out by means of special machines, which are called generators. The principle of their work is based on the law of electromagnetic induction. The simplest circuit of the generator can be represented in the form of a frame rotating around an axis in the magnetic field of an electromagnet or a permanent magnet. When the frame rotates, it produces a variable electromotive force. By connecting the frame to the external circuit, we obtain an alternating electric current. An alternating current generator having a fixed magnetic system and rotating turns is constructed quite infrequently.

Almost in all such generators, the winding (armature) is fixed, and the magnetic system (inductor) rotates. The real part of the generator is called the stator, and the movable part is the rotor.