標題:
Why a.c. and high voltage are used for long distance
發問:
Why a.c. and high voltage are used for long distance power transmittion? Please also come back because I might have more questions. 更新: But why a.c. is easily stepped up and down and d.c. is not? And please explain in terms of molecular/atomic motion why less current causes less energy loss in metal wires/cables. I really want to know why. 更新 2: Sorry for the delay. One more question. How does an a.c. change its current easily? Is it the magnitude of the current is fixed? As I know, a.c. is a sine wave pattern of magnitude changes, it is a fixed pattern. Is it not? Thanks.
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a.c. voltage is used in long distance power transmission because a.c. current is easily stepped up or stepped down by using a transformer. That is, the voltage supply can be altered due to different voltage demand. For a d.c. power supply, it is hard for the transformer to step up or down the voltage. High voltage is used because for the same power supply, a high voltage requires less current to transfer. (By P = IV) So, the power loss in the cable can be greatly reduced as power loss = I2R, where R is the resistance of the cable. Obviously, for a low voltage supply, then the current required to transmit the power is large, then the power loss is drastic. 2008-12-10 19:18:17 補充： Transformer makes use of electromagnetic induction to step up or down the voltage. This requires a changing magnetic field and hence changing current to do so. A.c. can achieve this easily while d.c. cannot. 2008-12-10 19:18:57 補充： For you 2nd question, this is nothing to deal with molecule. This is due to energy. When current is small, then the power loss is small. 2008-12-14 19:13:38 補充： a.c. is not necessarily a sine wave... As long as the current has a changing period, it can be defined as an a.c.
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