What is the formula for calculating current if voltage and power are known, but resistance or impedance is unknown?

For DC Circuits (Using Power and Voltage)

In a direct - current (DC) circuit, the power P (in watts), voltage V (in volts), and current  (in amperes) are related by the formula P=VI

If we know the power P and voltage V, we can calculate the current using the formula I=P/V. For example, if a DC device has a power rating of 100 watts and is connected to a  20- volt source, then the current I=100/20=5 amperese.

In an alternating - current (AC) circuit, we deal with apparent power S (in volt - amperes), voltage V (in volts), and current I (in amperes). The relationship is given by S=VI.If we know the apparent power P and voltage V, we can calculate the current using I=S/V.

For example, if an AC circuit has an apparent power of 500 VA and is connected to a 100 - volt source, then the current I=500/100=5 amperes.

It should be noted that in AC circuits, if we are interested in the real power (in watts) P and want to calculate cosa , The relationship between real power P , apparent power S , and power factor is P=Scosa  . So, if we know P,V and  cosa, we first calculate  and then.S=P/cosa, and then I=S/V=P/Vcosa.