Power in a circuit is defined as the rate of change of energy, influenced by the power supply, which serves as the energy source. Changes in energy transfer can be achieved by manipulating current, voltage, or both while keeping resistance constant. For example, increasing the voltage to a bulb results in greater illumination, demonstrating the relationship between power and energy change. In DC circuits, while voltage and current may remain stable, component temperatures can fluctuate, affecting energy dynamics. Energy supplied can also manifest in various non-electrical forms, highlighting the versatility of energy transformation in circuits.