Explain why a transformer is used in electrical power lines.

Power stations use step-down transformers to decrease the voltage of the electrical output. The power lines have a constant resistance (say, R Ohms) which means there is a power loss across the lines given by P = IV. By using Ohm's law (V = IR), we can obtain the formula P = V^2 / R which shows that the power loss goes as the square of the voltage. By using a step down transformer, we can minimize the power loss that occurs during transmission.