The fundamental Ohm's law formulas for
ac circuits are given by:



I = E / Z


Z = E / I



E = I*Z


P = E*I*cos Ø



Where:

I = current in amperes,


Z = impedance in Ohms,


E = volts across,


P = power in watts, 

Ø = phase angle in degrees.


Phase Angle

The phase angle is defined as the difference

in degrees by which current leads voltage in a

capacitive circuit, or lags voltage in an inductive

circuit, and in series circuits is equal to the

angle whose tangent is given by the ratio X/R and

is expressed by:

arc tan (X/R)

Where: 

X = the inductive or capacitive reactance in ohms,


R = the nonreactive resistance in ohms,

of the combined resistive and reactive components

of the circuit under consideration.

Therefore: 
in a purely resistive circuit, Ø = 0°

in a purely reactive circuit, Ø = 90°

and in a resonant. circuit,
Ø = 0°

also when: 

Ø = 0°, cos Ø = l and P = E*I,

Ø = 90°, cos Ø = 0 and P = 0.



Degrees x 0.0175 = radians. 1 radian = 57.3°


Power Factor

The powerfactor of any ac circuit is equal to
the true power in watts divided by the apparent
power in voltamperes which is equal to the
cosine of the phase angle, and is expressed by

E*I*cos Ø
p . f . =  = cos Ø
E*I

Where: 

p.f. = the circuit load power factor,


E*I*cos Ø = the true power in watts,


E*I* = the apparent power in voltamperes,


E = the applied potential in volts


I = load current in amperes.


Therefore: 
in a purely resistive circuit.

Ø = 0° and p.f. = 1

and in a reactive circuit,

Ø = 90° and p.f. = 0

and in a resonant circuit,

Ø = 0° and p.f. = 1


Ohm's Law for DC Circuits

The fundamental Ohm's law formulas for

dc circuits are given by,


E
I =  , R


E
R =  , I



E = I*R 

P = I*E 

where: 

I = current in Amperes,


R = resistance in ohms,


E = potential across R in volts,


P = power in watts.

