Talks on Electrical Engineering: Electricity - from simple to complex. Part 6.
The basis of modern electrical engineering is an alternating current, which, in turn, is caused by an alternating voltage. This is the voltage magnitude and polarity which have a periodic nature in function of time (Figure 16). The graph shows the most important characteristics of the AC voltage.
We note that the dependence of the amplitude of the AC voltage Ua from the time t is sinusoidal, as this process is one of vibrational wave. During each period, i.e. time interval, one accommodating a negative half-wave and a positive, or in other words, one positive and one negative half cycle, the amplitude of the AC voltage passes through zero at least (but not exceeding) three times.
The physical meaning of this is that at the point "0" (see. Figure 16), no voltage and hence the current in conductors no! Then, under the alternating voltage source appears a positive voltage, which reaches its maximum at the time period t1 (period "A").
During the time period t2 the voltage gradually decreases to zero, and then reappear, but now its polarity is negative. The maximum negative pressure is obtained in the "B" point. After that, during a time interval t4 the voltage again reaches zero. The whole cycle is completed within a time: T = t1 + t2 + t3 + t4. This is PERIOD. The reciprocal of the period, referred to as the oscillation frequency.
This parameter shows the number of times for one second AC voltage has changed its polarity, or how many periods (cycles) alternating voltage has undergone a second. Figure 17 shows some basic quantities characterizing the alternating voltage.
As you can see, the maximum and minimum voltage values (respectively Ua Ua + and -) are equal in magnitude (in absolute value) with each other. But in addition to the amplitude value of the AC voltage to a very important parameter is the effective stress. As seen in Figure 17, the shaded part of the positive half cycle is equal in area of the shaded part of the rectangle, in which "fit" the half-life. In this case the rectangle defined by the product of the Vrms and T / 2, the area is exactly equal to the positive half-cycle.
In other words, an AC voltage having an amplitude Ua, passing through the R resistor, allocates the same amount of heat as it releases during the same time the DC voltage, the magnitude of which is equal to Vrms! Therefore, when we say that the mains voltage is 220 V, then we are talking about the effective stress, which is associated with a maximal peak in the following equation: Vrms = 0.707 Ua.
Consequently, the peak voltage of the electrical grid is Ua = Vrms / 0,707 = 311 V. But the connection with the AC voltage AC is more complex and depends on the nature of the circuit, which is supplied with the voltage. If the load is purely active character (that can be approximated by a conventional resistor), the ratio between voltage and current shown in Fig.18.
In contrast to the previously discussed DC circuits, AC circuits contain, besides the active and reactive components further. This is primarily CONDENSER. This extremely popular and necessary component of the electrical circuit is generally two metal plates located at a certain distance from each other (Figure 19).
The capacitors are characterized by such important for the electrical parameter is electrical capacitance, which can be calculated from the formula C = 0,0885mS / r, where S - area of the plates; r - the distance between the plates, m - permittivity.
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