Definition – The impedance of a circuit is defined as the overall effect of opposition by the resistance & reactance in an ac circuit when ac voltage is applied across it. It is denoted by Z.

So, Impedance    Z = R + jX

Where       R = Resistance in circuit in Ω,

jX = Reactance in circuit in Ω which includes both inductive (XL) &    capacitive reactance (Xc)  .

So,    jX =  XL + (-XC)  (where Xc is negative)

Where   XL – Inductive Reactance = jwL  = 2πfL

Where Xc – Capacitive Reactance = 1 /wC, =  1/2πfC

Where f = frequency in Hz, L = Inductance in Henry, & C = Capacitance in Farad

Hence in other words we can say –

Ohms Law for DC Circuit – V = IR,       where R = Resistance,

Ohms Law for AC Circuit – V = I Z   where Z = Impedance which includes resistance R & Reactance X. So Z = R + jX

Series & parallel Connection of Impedance –

Series Connection – Z =    Г R²+ X²,

=    Г R²+ (XL – XC)²

Parallel Connection – Z = RX / Г R² + X² or 1/Z = 1/Z1 + 1/ Z2

Difference between Resistance, Reactance & Impedance

S. No.	Resistance (R)	Reactance (X)	Impedance (Z)
1	It opposes the flow of current in a circuit	It opposes the change of current & voltage in the circuit	It is the overall effect of oppose due to resistance & reactance both
2	It is denoted by R	It is denoted by X	It is denoted by Z
3	It is measured in Ohms (Ω)	It is measured in Ohms (Ω)	It is measured in Ohms (Ω)
4	Its value depends on types of material, conductive material offers less resistance while non conducting material offers more resistance. Ex Copper, aluminum etc are conducting material while rubber, plastic, wood etc are non-conducting material	Its value depends on two factors -1) Inductance & 2) capacitance. Capacitance factor reduces the effect of inductance in the circuit.	Its value depends on the resistance as well as reactance.