What distinguishes a step-up transformer?

A step-up transformer is specifically designed to increase voltage levels while decreasing the current in the circuit. This is achieved through a greater number of turns in the secondary coil compared to the primary coil. The principle behind this operation is derived from Faraday's law of electromagnetic induction, which states that the voltage induced in a coil is proportional to the rate of change of magnetic flux and the number of turns in the coil.

In the case of the step-up transformer, the secondary coil has more turns, enabling it to convert a lower primary voltage into a higher secondary voltage. Consequently, while the voltage increases, the current decreases according to the conservation of energy, maintaining the power balance in the transformer (assuming negligible losses).

The other options do not accurately describe the characteristics of a step-up transformer: fewer turns in the secondary coil would result in a step-down transformer; a description of decreasing voltage and increasing current indicates a different function entirely; and the operation of transformers is based on alternating current (AC), not limited to direct current (DC).