Impedance is the resistive parameter offered to the flow of current in a circuit. Whenever current flows or electrons move through a closed circuit, due to collisions of electrons, resistance is offered to the flow of current.
In a DC circuit, impedance is nothing but resistance. But in an AC circuit, along with resistance, inductance and capacitance are also present. Inductance occurs due to change in current which creates a magnetic field around the conductor. Due to this magnetic field, an opposition is offered to the change in current which is termed as inductance. Capacitance is due to the electric field of the current. Capacitance offers resistance to the change in voltage.
Impedance is the net resistance of the circuit when current flows.
We define impedance, Z, as
Z = R + j*(ωL + 1/ωC)
Where w is the angular frequency of supply, L is the self inductance and C is the capacitance.
Figure below shows the Impedance and Admittance Triangle using which one can get the value of Impedance and Admittance.
Admittance, Y, is the reciprocal of impedance.
We define Y as, Y = 1/Z
Y = G + jB
G is the conductance and B is the susceptance.
The synonymous unit of Admittance is mho, and the symbol ℧ (an upside-down uppercase omega Ω), are also in common use.
Normally we take impedance for series circuit to express voltage in terms of current. Admittance is taken in case of parallel circuit to express current in terms of voltage.