What happens to power in a circuit when a step-up transformer is used?

When a step-up transformer is used in a circuit, it increases the voltage while decreasing the current, which allows for the same amount of power to be transferred through the transformer under ideal conditions (ignoring losses). The principle behind transformers is that power input (in watts) equals power output (in watts). Since power is the product of voltage and current (P = V × I), a step-up transformer raises the voltage and proportionately lowers the current to maintain the power level.

Thus, even though the voltage is increased, the current is decreased to keep the overall power constant (assuming 100% efficiency). In practical applications, while there may be some loss due to resistance and other inefficiencies, the fundamental behavior of the transformer design ensures that the power remains approximately constant during the transformation process. This is a vital concept in electrical engineering, particularly in power distribution systems.