Electronics Engineering (ELEX) Board Practice Exam

What characteristic of power varies in both magnitude and sign over a cycle?

1. Average power

2. Apparent power

3. Instantaneous power

4. Real power

The correct answer is: Instantaneous power

Instantaneous power is defined as the power at any given moment in time, and it varies continuously with time over a cycle. This power can change in both magnitude and sign because it is calculated as the product of instantaneous voltage and instantaneous current at that specific moment. In alternating current (AC) systems, both voltage and current alternate in direction, leading to oscillations in power. When the voltage and current are in phase, the instantaneous power is positive, indicating that energy is being delivered to the load. Conversely, when they are out of phase, the instantaneous power can become negative, which signifies that energy is being returned to the source. This characteristic distinguishes instantaneous power from average power, which is calculated over a full cycle and does not reflect these variations. Average power gives a single value representing the overall energy transfer but lacks the time-dependent detail of instantaneous power. Apparent power is also a measure that combines both real and reactive power components but does not vary in sign like instantaneous power does. Real power emphasizes the actual work done, remaining positive or zero, while instantaneous power fluctuates both in magnitude and sign.