Why is voltage in direct current constant? And why does the voltage in A.C changing? Do we provide the voltage in that way?
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$\begingroup$ They are the definitions of DC and AC: DC is Direct (unchanging), AC is Alternating (varying). Are you asking why we do both? In what situations? $\endgroup$– Guy InchbaldFeb 16, 2021 at 13:47
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1$\begingroup$ As @GuyInchbald is explaining, you are asking the wrong question. Current is not constant because is is called direct. It is called direct because it is constant. We are able to apply a constant currect if we want. Or a varying current if we want. And we have simply invented names for them. $\endgroup$– SteevenFeb 16, 2021 at 14:03
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2 Answers
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AC does imply that the current is changing direction (usually in a periodic fashion). DC does not have to be constant. It can change with time (as in the slow charging of a capacitor).
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Why is voltage in direct current constant?
I'm guessing that when you say "direct current," you actually are asking about a D.C. power supply. Most D.C. power supplies provide constant voltage by design. That's because it's easier to design electronic circuits if you can depend on the supply voltage to always be the same.
why does the voltage in A.C changing?
Electrical engineers don't usually think of A.C. power supplies in that way. An EE would say that the power outlet in your house provides constant A.C. voltage. In fact, it's actually varying between some negative value and some positive value 60 or 50 times per second, but in most circuits (pretty much everything except A.C. motors and transformers) the power supply waveform is not at the forefront of their minds.
There's a long and famous story about why A.C. is used at all: https://en.wikipedia.org/wiki/War_of_the_currents
The TLDR version is, electric power is most efficiently transmitted over long distances at high voltage, but it is most efficiently used at lower voltages. When our power grids were constructed in the late 19th and early 20th centuries, the only practical means for changing the voltage was to use transformers, and transformers only work with A.C.
