Voltage Concept

A 1.5-volt AA battery and a 345,000-volt transmission line both push electrons through circuits. The difference is how hard they push. Voltage is the measure of that push, and it is the reason electricity can travel hundreds of miles from a power plant to your outlet.

Voltage (also called electric potential difference) measures the energy carried by each unit of charge, in joules per coulomb. One volt means each coulomb of charge carries one joule of energy. A 12-volt car battery gives each coulomb 12 joules. A 345 kV transmission line gives each coulomb 345,000 joules. The unit is the volt (V).

The water pressure analogy works well here. Voltage is like water pressure in a pipe. Higher pressure pushes water faster and harder. Higher voltage pushes electrons with more energy. But voltage alone does not determine how much total energy is delivered; you also need current (how many coulombs per second are flowing). Power, the rate of energy delivery, equals voltage times current: P = VI.

This relationship explains why transmission lines use high voltage. Delivering 1,000 MW at 345,000 volts requires only about 2,900 amps of current. Delivering the same power at 120 volts would require 8.3 million amps, which would melt any practical wire. High voltage, low current: less heat loss, thinner wires, cheaper transmission.

Worked Example

A power plant sends 500 MW across a 345 kV transmission line.

• Calculate current. I = P/V = 500,000,000 W / 345,000 V = 1,449 amps.

If the line voltage were reduced to 115 kV (one-third), what current would be needed for the same power?

I = 500,000,000 / 115,000 = 4,348 amps, three times higher. Since resistive heat losses scale with current squared, tripling the current increases heat losses by a factor of 9. This is why long-distance transmission uses the highest practical voltage.

Every step-up and step-down transformer between the plant and your outlet exists to keep that push high in the wires and low in your hand.

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