What voltage is, I think, not all electricians know a little bit more or less, figuratively speaking, it is the pressure of the electric charge. Conductors such as copper and aluminum contain a large number of electrons that can move freely (i.e., free electrons), and with voltage, these free electrons will move directionally under the same pressure, forming an electric current.
In other words, the pressure of the voltage to the charge has a direction, and this direction is expressed as positive and negative, that is, the positive direction of the voltage is from the positive to the negative. So, there is a positive and negative voltage, whether it is DC or AC. However, the positive and negative of the AC voltage is periodically changed, which makes it impossible to write its positive or negative in the circuit analysis.
But obviously, in the circuit analysis, the direction of the AC voltage and current is still marked, why is that? Quite simply, whether it is an AC circuit or a DC circuit, the positive direction of voltage and current is assumed, just as we assume that the unknown in the equation is x when we do a math problem.
Only by assuming the positive direction of voltage and current can we know the actual direction of voltage and current through the instantaneous value expression or change waveform of both voltages and currents. For example, the reference positive direction of an AC voltage u is shown in Figure 1-1 below, and its instantaneous value expression is u=um sin(2 ft), which is a sinusoidal function, so its waveform is the same as the waveform of sinx, as shown in the following figure.
Figure 1-1: The maximum value of the voltage instantaneous value is um, so the highest point of the waveform is for the um value. If the frequency f of this voltage is 50Hz, then the period t is 002 seconds, which corresponds to 2 of the sinx function. At this time, the horizontal axis of the coordinate axis is the time, from left to right indicates that time is advancing, and the origin corresponds to the starting point of time (0 time point or timing starting point).
Combined with the reference positive direction of the voltage, the instantaneous value expression, and the waveform, we can know the magnitude and actual direction of the AC voltage at any given moment. For example, at time 0, the value of the voltage is 0V, and it is from 0 seconds to 0Between 01 seconds, the voltage waveform is on the positive half axis of the vertical axis, indicating that the voltage is greater than zero, which means that the actual direction of the voltage is the same as the reference positive direction. That is, in 0 seconds to 0Between 01 seconds, the actual direction of the voltage is positive and negative.
Obviously, the positive direction of reference can be assumed arbitrarily, but the actual direction will not change with the assumption of the positive direction of reference, for example, if the AC voltage is the same as above, we assume that its positive direction is negative to the left and positive, as shown in Figure 1-2 below.
Figure 1-2 has the same starting point, i.e., the actual direction of the AC voltage is from 0 seconds to 0It is still positive and negative for 01 seconds, so the expression for the instantaneous value of the AC voltage is u=-um sin(2 ft). From the waveform, it is 0 seconds to 0Between 01 seconds, the voltage waveform is on the negative half axis of the vertical axis, indicating that the voltage is less than zero, which means that the actual direction of the voltage is opposite to the reference positive direction, and the reference positive direction is left-negative and right-positive, so the actual direction is left-positive and right-negative.
To sum up, AC voltage is also divided into positive and negative, although the actual direction of the voltage changes with time, but we can arbitrarily specify its reference positive direction, in our actual work, when writing the voltage is marked positive and negative, do you understand?
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