The main reason why a.c. is transmitted at very high voltage and low current is that

Analyze the question: It asks why electrical power is transmitted at very high voltage and low current over long distances (like the National Grid lines from power stations across Nigeria).

Concept: Power transmitted is P = IV. For a fixed power, raising the voltage V lets the current I be made small.

The power lost as heat in the transmission cables is:

P_loss = I²R

where R is the resistance of the cables. Heat loss depends on the square of the current, so lowering the current greatly cuts the loss.

Step-by-step solution:

1. Suppose we must transmit power P = 100 kW.
2. At 1,000 V, I = P/V = 100,000 / 1,000 = 100 A.
3. At 100,000 V, I = 100,000 / 100,000 = 1 A.
4. If the cable resistance is R = 10 Ω: • At 100 A: P_loss = 100² × 10 = 100,000 W (all the power is lost!) • At 1 A: P_loss = 1² × 10 = 10 W (almost nothing is lost)

So transmitting at high voltage and low current greatly reduces heat loss in the cables.

Checking the options:

• A. Current does not "flow faster" at high voltage, so this is wrong.
• B. Being able to be stepped down is a feature of a.c. (via transformers), but it is not the reason for using low current.
• C. Cost of generation has nothing to do with transmission voltage.
• D. Reduces heat loss. Correct, as shown by I²R.

Answer: D

Exam tip: Remember "High V, Low I, Low I²R." Whenever you see transmission lines in an exam, think of the I²R heat loss. Transformers step the voltage up at the power station and down near homes.