How do I know the difference between Impedance and Ohms?

Question

I get mixed up between the two of them. I know how to measure Ohms, but how do you measure impedance.

Answer 1

They are similar, and at DC they are identical. For AC circuits, it is the complex sum of the reactances of any inductance and capacitance.

With a series circuit of a resistor, a capacitor and an inductor,

Z = R + jXL - jXc

XL = 2πfL

Xc = 1/(2πfC)

j = √(-1)

wikipedia:

http://en.wikipedia.org/wiki/Electrical_...

Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. Impedance is a complex quantity  and the term complex impedance may be used interchangeably.

Dimensionally, impedance is the same as resistance; the SI unit is the ohm. The term impedance was coined by Oliver Heaviside in July 1886.

The reciprocal of impedance is admittance.

Answer 2

Impedance is resistance offered by an electric circuit or component, with both a real and imaginary part (thus, a complex entity).  However, not all components' impedance is represented by a complex number. For example, the resistance offered by a resistor consists of only the real part, and no imaginary part. In most texts, this is referred to as RESISTANCE, and not impedance and is denoted by Ohms.

Other components, like capacitors have resistance that consists of both a real and imaginary part. So, this is referred to as IMPEDANCE and not resistance.

The SI unit for both impedance and resistance is Ohms.

But if you're "measuring" Ohms of a resistor with a ohmmeter (or multimeter), you're measuring the resistance part (real part) of the resistor (remember the imaginary part of resistor is zero). But if you're measuring the Ohms of a capacitor using the multimeter, you'll get only the real part. The imaginary part cannot be measured using multimeter, but will have to be calculated by you in paper.

So, to answer the 2nd part of your question, in order to measure impedance, you can use multimeter, but you'll also have to calculate the imaginary part by hand.

Answer 3

There is a considerable amount of test equipment available just for this purpose. If you can measure the magnitude of the voltage and the current then measure the phase angle then you can calculate. Such an instrument is the Vector Impedance Meter.

Answer 4

Isn't it the same thing?  Another word for impedance is resistance, isn't it?  Resistance is measured in ohms...

Answer 5

Impedance and resistance are both measured in Ohms, however, Impedance can be represented as a complex number.  Resistance is real part of impedance.  Capcitance and inductance add to the imaginary portion of impedance.  There are impedance analyzers, that will tell you both the real and imaginary parts of impedance.  I don't think the average multimeter can do it.