Electronics Engineering (ELEX) Board Practice Exam

What condition must be met for an instantaneous power to be calculated in a circuit?

1. Only alternating currents

2. Only direct currents

3. Both alternating and direct currents

4. N/A for power calculations

The correct answer is: Only alternating currents

The calculation of instantaneous power in a circuit is fundamental to understanding how power fluctuates over time, particularly in alternating current (AC) systems. Instantaneous power is defined as the product of the instantaneous voltage and instantaneous current at a particular moment in time. For an AC circuit, the voltage and current are usually sinusoidal and can change direction, leading to variations in power throughout each cycle. This allows for a dynamic assessment of power at every instant, hence the name "instantaneous power." The formulas used for instantaneous power in these systems take into account the time-varying nature of both voltage and current, thereby requiring measurements at specific points in time. In contrast, while instantaneous power can also be defined for direct current (DC) circuits, it is constant over time because both current and voltage do not change. The discussion around instantaneous power typically emphasizes AC circuits, making the emphasis on AC reasonable. Thus, from this context, the condition that relates particularly to standard definitions and discussions of instantaneous power is primarily focused on alternating currents.