Which of the following modes is the most economical method of transmitting electrical power over a long distance?

Analyze the question: We want to send power over a long distance while wasting as little energy as possible, and at a practical cost.

Concept:

• Power transmitted: P = IV
• Power lost in the cables: P_loss = I²R

For a fixed power P, raising the voltage V lowers the current I (since I = P/V). Because the loss depends on I², even a small drop in current cuts the loss sharply.

Step by step:

1. Low current means a small I²R loss, so less energy is wasted as heat in the cables.
2. High voltage is therefore needed to deliver the same power with low current.
3. Alternating current or direct current? AC can be stepped up and stepped down easily and cheaply using transformers. This is how the National Grid in Nigeria works: power is stepped up to 330 kV or 132 kV for transmission, then stepped down for homes. Transformers do not work with DC, and converting DC voltages needs costly equipment, so DC is less economical for general use.

Eliminating the options:

• B: High current causes large I²R losses. ✗
• C: DC at high voltage works in special cases, but it cannot be easily transformed, so it is not the standard economical method. ✗
• D: DC at high current has large losses. ✗
• A: AC at high voltage and low current has low losses and easy transformation. ✓

Answer: A

Tip: Remember "HV–LC–AC": High Voltage, Low Current, Alternating Current is the combination for transmission. Whenever you see the words "economical" and "long distance," think of the I²R loss and transformers.