written by: Miguel Leiva-Gomez
• edited by: KennethSleight
• updated: 8/29/2009
Many people have heard this term, but few know what it is, or how it affects them in their daily lives. Find out more about impedance and why it is important to learn about it.
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What is Electrical Impendance?
Electrical impedance (also called "impedance" in short) is an extension of the definition of resistance to alternating currents (AC). What is meant by this is that impedance includes both resistance (the opposition of the electric current that causes heat) and reactance (the measure of such an opposition as the current alternates) in detailing the opposition against electric currents. In direct currents (DC), electrical impedance is the same as resistance, but this does not hold true in AC circuits.
Impedance can also be different from resistance when a DC circuit changes flow in one manner or another, like the opening and closing of an electrical switch, as is observed in computers when they open and close switches to represent ones and zeros (binary language). The opposite of impedance is admittance, which is the measure of the allowance of current. The figure to the left is a complex impedance plane, in which impedance is represented by a Z, resistance is depicted as R, and reactance is depicted with X.
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Why is Impedance Important?
Impedance and resistance both have applications that, believe it or not, exist in your own home. Your home electricity is controlled by a panel which has fuses in it. When you go through an electrical surge, the fuses are there to interrupt the power so that the damage is minimized. Your fuses are like very high-capacity resistors that will be able to take the blow. Without them, your house's electrical system would fry and you would have to build it up from scratch.
This issue is solved thanks to impedance and resistance. Another situation in which impedance has importance is in capacitors. In capacitors, impedance is used to control the flow of electricity in a circuit board. Without the capacitors controlling and regulating electrical flow, your electronics that use alternating currents will either fry or go berserk. Since alternating current delivers electricity at a fluctuating pulse, there needs to be a gate that holds back all the electricity and lets it go smoothly so that the electrical circuit is not overloaded or underloaded.
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A Deeper Look at Reactance
You have seen earlier the word reactance mentioned and probably might not be comfortably familiar with the term. Reactance comes hand in hand when one defines impedance, and this is because it is exactly what sets impedance apart from resistance.
To make it easier to understand, when electricity is flowing in one direction (like in a standard direct current), there is some resistance experienced, but no reactance at all. Reactance will occur when the electricity changes direction, as is seen in an alternating current. Reactance has two different types: inductive and capacitive. When people speak of inductive reactance, an example would be an electromagnet running with an alternating current. Reactance in this case is observed in the changing of the magnetic field. The other type of reactance (capacitive reactance) is related to the changing electrical field in between two conductive surfaces. These two surfaces usually are separated by some sort of insulation. An example of where capacitive reactance is observed is in capacitors, which resist any changes in voltage and let extra electricity slowly leak out.
So basically it can be summarised that impedance is the sum total of the opposing force which consists of two components, namely resistance and reactance. The first is due to physical properties of the matter and other factors such as temperature, whilst the latter is an unwanted phenomenon which is inherent in certain electrical circuits having capacitive and inductive values.