The transmission of electrical power is more efficient when high voltages are used. To take an example, 100 kW of power is carried by a 1000 A current at 100 V, and by a 100 A current at 1000 V (remembering that 1 W = 1 A × 1 V). Assuming that identical cables of resistance R are used for both, the power loss P will be 100 times greater in the first case than the second because the current I is ten times greater and P = I2R (Equation 25.12 on page 243). Since transformers provide a very efficient means of stepping the voltage up or down, it makes good sense to use alternating current to transmit electrical power over long distances. In practice, enormous voltages are used for this purpose, sometimes as high as 400 kV.
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