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*(wL + 1/wC)

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.

In transmission line, these
impedance and admittance are of very much importance. Based on these
parameters, various models are designed as discussed in post “Basis ofClassification of Transmission Line”.

Thank you!

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