AC Theory Cheatsheet

1. Inductance is the relationship between the current and the magnetic field generated in a coil. Each is proportional to the delta of the other.

L = ( d2 n2) / l + 9.45d

d is diameter of coil in meters

n is number of coil turns

l is length of coil in meters

2. Resistance (R) is not affected by AC voltage or current frequency and remains the same for AC/DC.

R = V / I

3. Reactance (X) or Back EMF is the opposition to current flow in relation to the current frequency

        

a) Inductive reactance (XL) rises with an increase in frequency -

2πƒL

b) Capacitive reactance (XC) falls with a increase in frequency -  

1/2πƒC

4. Impedance describes the combined effect of resistance (R), inductive reactance (XL) and capacitive reactance (XC) in an AC circuit

For DC circuit =         Z = √ ( R² + (XC²) )

For AC circuit =         Z = √ ( R² + (XL - XC)²  )  

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