Why is DC used to run equipment and not AC?
Answer:
You can use AC motors, but often you want to vary the motor speed and AC motors basically are governed by the frequency of the AC signal. While varying frequency is not impossible, it generally involves much more sophisticated and expensive circuitry than just varying the magnitude of the current as is done with DC motors.
Circuits is not my favourite subject, nor my best, by any stretch of the imagination, so forgive me for that. I'll do my best to answer without getting too sidetracked. For one, DC is generally used in very low voltage applications ...hence AA and AAA batteries are only 1.5V, and yet they are used for all kinds of things. DC is used, for the most part, in situations where all that is needed is the constant voltage source that the DC current provides. This, as I mentioned before, is most often in the form of batteries. ...Best way I can explain it, and probably the easiest to understand, is that batteries are primarily used as energy storage. It's like a water cooler - the amount available in the source goes down and either has to be replaced when empty, or refilled - recharged. You apparently know that AC is considered "better" - this is true in terms of general efficiency and power transmission. Unfortunately, AC power isn't as portable, by any means; rechargable batteries have made some great leaps recently.. the apparent switch from NiCd to NiMH has made rechargeable batteries a significantly more worthwhile investment. Additionally, you have to understand the basic premise of AC current - instead of a steady flow of electrons, the electrons are subject to repeated ..pushes. Periodic acceleration. There's a lot to get into about phase and power loss, but I think the issue that's more to answer your question is that use of AC current requires more hardware, in a sense. Because of the frequencies used in most applications, many electronics (consumer electronics) would be subject to the heat radiation and electromagnetic interference - both of which are difficult to design around, especially if trying to keep the devices small, as in cellphones.
So, ..in short, would use AC current if we could, but DC is sufficient for many situations, and DC is more convenient as we currently don't have much option in the way of portable AC current. Also, I think AC current tends to be 'reserved' more for those situations where it would be desired for the power to vary. Moreover, I think DC simplifies the same problems that such would bring up - you don't really get harmful power surges or fluctuations from a DC source, so you don't have to worry about safeguarding against that. Hope that clears things up a little.
As far as the radio is concerned, the circuitry is designed for DC. There are various circuits like diodes and transistors that can only accept one-way current, and if you push current the other way you can damage them. The primary reason for using AC is simply because electricity can travel much, much farther than with DC.
Audio and RF circuits require DC to operate without "buzz" that is caused by the AC 60 cycle frequency. I believe that if you could see the various waveforms in your equipment on an oscilloscope with the 60 Hz AC added to it, you might understand better the "why" of DC versus AC to power your equipment. The AC would modulate everything which would make communications, and the digital control of the devices extremely difficult. For digital control, the AC could cause false information pulses that would make the equipment unreliable, if it worked at all.
we can but we don't want, it may cause instability and unwanted performance. for example if a chip needs source voltage 5 volt and it is ac then in some voltages (if it is AC) it won't work cause it goes from -5 to 5 with a stanadar frequency but you need all the time 5V. Also the behaviour of the inductors and the capacitors will be uncertain. But the reason that the AC is used in the jack is that it's transported more easily in big distances in unlike the DC
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