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The transmission of electric power over long distances can be done using either alternating current (AC) or direct current (DC). However, AC is generally preferred for long-distance transmission due to several key factors that reduce power loss:

Voltage Transformation: AC can be easily transformed to higher voltages and back to lower voltages using transformers. Higher voltage transmission reduces current for the same power, and lower current reduces resistive losses (power loss due to resistance in the wires). This makes AC transmission more efficient over long distances.
Skin Effect: AC experiences what is known as the skin effect, where alternating current tends to flow near the outer surface of the conductor. This effect increases the effective cross-sectional area of the conductor for AC, reducing resistance and hence power loss. However, this effect is more pronounced at higher frequencies and can increase losses if not properly managed.
Reactance and Phase Shift Management: AC systems can manage reactance (inductive and capacitive effects in the transmission line) and phase shifts between current and voltage. This management, through the use of capacitors and inductors, optimizes power flow and minimizes losses.

However, it's important to note that with advancements in technology, High Voltage Direct Current (HVDC) transmission is becoming more viable and is being used for specific applications where it can be more efficient than AC. HVDC is particularly advantageous for very long distances (typically hundreds of kilometers), underwater and underground cables, and for connecting asynchronous grids. HVDC minimizes resistive losses and eliminates the reactance and phase shift issues inherent in AC transmission.