why reactive power is present in AC not in DC

 DC systems also produce magnetic fields in components like inductors, motors, and electromagnets. However, reactive power is a concept specific to AC (alternating current) systems, and here's why DC systems don't have reactive power in the same sense:

1. DC vs. AC Behavior:

• In AC systems, the current and voltage alternate sinusoidally. As the current and voltage continuously change, inductors and capacitors store and release energy, creating a constant exchange of energy between the source and the reactive components. This exchange is what defines reactive power (Q) in an AC system.
• In DC systems, the current is constant (steady-state), meaning it doesn't alternate. Once a magnetic field is established in an inductor (like in a DC motor or electromagnet), the field remains constant, and there is no continuous exchange of energy between the source and the inductor, as there is in AC. Therefore, reactive power does not exist in DC.

2. Energy Storage in DC:

• When a DC current flows through an inductor, it still creates a magnetic field, but this field is created once and remains as long as the current is steady. There’s no need for continuous energy exchange as in AC.
• Similarly, if a capacitor is charged in a DC circuit, it stores energy in its electric field, but again, this happens once. There is no continuous charging and discharging, like in an AC system, where the voltage and current are constantly changing direction.

3. Reactive Power in AC:

• In AC systems, the constant changing of the current direction means that inductors and capacitors are continuously absorbing and returning energy to the circuit. This back-and-forth movement of energy is represented by reactive power (Q).
• Inductive loads in AC, like motors or transformers, require this reactive power to maintain their alternating magnetic fields because the magnetic field is being constantly established and collapsed with each AC cycle.
• In contrast, since DC current is unidirectional and constant, once the field is established, no further energy exchange is needed to maintain the field.

Example: DC Motor

• A DC motor also produces a magnetic field to create motion, but the field is constant after it is established. The inductor (or coil) creates a magnetic field, but because the current in DC doesn't fluctuate, there is no reactive power. The inductor just draws a constant amount of real power to maintain the field.

Why No Reactive Power in DC:

• Reactive power is tied to the alternating nature of AC systems, where energy is constantly stored and released due to the changing current and voltage.
• In DC systems, energy storage happens only once when an inductor or capacitor charges up, and after that, there is no continuous back-and-forth movement of energy, so reactive power isn't needed.

Summary:

In DC systems, magnetic fields are produced, but they are steady once created, so no reactive power is needed to maintain them. Reactive power is specific to AC systems, where the current and voltage are constantly alternating, causing inductors and capacitors to continuously store and release energy, leading to the concept of reactive power.