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# Ohm’s Laws for Direct and Alternating Current

In order to turn air into compressed air, you need power. This power comes in the form of electricity: Alternating Current or Direct Current. In this article we will give a short introduction to the laws of Ohm. These laws introduce resistance in the relationship between current and voltage.

## What is Ohm’s law for direct current?

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:
U = R x I
Where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

## What is Ohm’s law for alternating current (and what is self-induction)?

An alternating current that passes through a coil gives rise to a magnetic flow. This flow changes magnitude and direction in the same way that an electric current does. When the flow changes, an emf (electromotive force) is generated in the coil, according to the laws of induction.

This emf is counter-directed to the connected pole voltage. This phenomenon is called self-induction. Self-induction in an alternating current unit gives rise in part to phase displacement between the current and the voltage, and in part to an inductive voltage drop. The unit's resistance to the alternating current becomes apparently greater than that calculated or measured with direct current.

Phase displacement between the current and voltage is represented by the angle φ. Inductive resistance (called reactance) is represented by X. Resistance is represented by R. Apparent resistance in a unit or conductor is represented by Z.

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