What does a step-up transformer do?

A step-up transformer is designed to increase the voltage from the primary coil to the secondary coil. This is achieved through the principles of electromagnetic induction and the ratio of the number of turns of wire in the primary coil compared to the secondary coil. When the voltage is increased, the current is reduced in accordance with the law of conservation of energy, assuming that the transformer is ideal and there are no losses.

In a step-up transformer, the input voltage is less than the output voltage. If the voltage increases, the current must decrease proportionally to keep the power input and output equal, since power is the product of voltage and current (P = V × I). This relationship allows transformers to transmit electricity efficiently over long distances by increasing voltage to reduce current, which minimizes energy loss due to heating in the wires.

Thus, the correct description of a step-up transformer is that it increases the voltage while decreasing the current.