What is impedance; is it the same as resistance?

[FAQ Team]

Resistance is a DC unit and Impedance is its AC counterpart which can vary with frequency.

(In the following formulae, we will use * to denote multiplication, / to denote division and + and - having their usual meanings.)

Resistance and impedance are measured in the same units (ohms) but they are not quite the same. Resistance is a measure of the ability of a material to oppose the flow of current. It is determined from Ohm's famous law which states that the resistance of a conductor is equal to the voltage across it divided by the current flowing through it. Resistance is usually applied to DC current circuits whereas Impedance, which is dependent on frequency, is used in AC circuits. Ohms Law is usually stated as....

V=I*R

So a device (resistor) with a resistance of 10 ohms and having a current of 1 amp flowing through it would have a voltage of 10 volts across its ends, thus

V=10 * 1

Resistance works identically for AC and DC.

There is another type of opposition to current flow which is dependent on whether the applied voltage is AC or DC and, indeed, on the frequency of the AC wave. This frequency-dependent opposition to current flow is called REACTANCE and it arises from physical mechanisms beyond the scope of this answer. Inductors and capacitors exhibit this reactance but differ in the way it is determined.

For an inductor

Xi = 2*pi*f*L

where Xi denotes inductive reactance, pi is 3.142, f is the frequency of the AC supply in Hertz and L is the inductance measured in Henries. So inductive reactance increases with frequency.

For a capacitor

Xc = 1/(2*pi*f*C)

where Xc denotes capacitive reactance, pi is 3.142, f is the frequency of the AC supply in Hertz and C is the inductance measured in Farads. So capacitive reactance decreases with frequency.

IMPEDANCE is simply the sum of the reactance of a component and any resistance it may have.

Example:

A 10 Henry inductor has a DC resistance of 200 ohms. What is its impedance at a supply frequency of 100 Hz?

Xi = 2*pi*f*L

Substituting

Xi = 2*3.142*100*10 = 6284 ohms

Impedance = 6284 + 200 = 6484 ohms