Resistance, Impedance, and Reactance

The three terms, resistance, impedance, and reactance are often used interchangeably. These three words actually have different meanings in an electrical circuit. Resistance is the opposition to the flow of electric current through a material or device (like a resistor) that provides less conductivity to electrons than other materials. Impedance is defined as the combination of resistance and reactance in series with one another within an alternating current circuit, such as an AC power supply or transmission line. Reactance can be thought of as being similar to capacitance but for alternating currents instead of direct currents; it’s characterized by its ability to oppose changes in voltage at its terminals when those changes follow sinusoidal patterns.

Resistance, impedance, and reactance are all related, but they also have some differences that make them unique. It can be confusing to understand these three since they seem very similar in definition at times, but each serves its own purpose within circuits.

Examples of Resistance 

Resistance is a property of all materials, and it’s basically the degree to which they impede or resist current flow. It can be measured in ohms (Ω), with low resistance being less than one, and high resistance in the millions of ohms or more (often represented with the symbol for infinity). The higher the resistance an object offers to oppose current flow, the more it begins to become an insulator. As we know, even some of the best insulators can be overcome by enough current and voltage, so really insulators are just very highly resistive materials. Similarly, the lower a material’s resistance is, the more it begins to look like a regular conductor. 

Examples of Impedance 

Electrical impedance deals specifically with alternating currents, not direct currents like resistance does. One way to look at it is that impedance is the opposition to AC current flow, where resistance is the opposition of DC current flow. It’s basically just a measure of how well an object can handle alternating currents.

For example, a coil of wire with very low resistance will have an impedance of near zero. Conversely, materials like rubber or Styrofoam could be said to have high electrical impedance because it’s hard for alternating currents to flow through them well. The higher the frequency is in relation to the natural resonant frequency of an object, the more power there is going into that object as opposed to being transferred out from it. This means that the object uses less energy, since it is using some internally and not wasting much by transferring it out. 

Examples of Reactance 

Reactances are characterized by their ability to oppose changes in voltage at its terminals, when those changes follow sinusoidal patterns. There are two types of reactance that occur in AC circuits; these are called capacitive reactance and inductive reactance. Circuits with capacitors experience capacitive reactance, and circuits with inductors experience inductive reactance.

Capacitive Reactance is the opposition to changes in voltage caused by a reactive component (capacitor), or a circuit containing one. Capacitive reactance is proportional to the frequency of the applied alternating current and its capacitance.

Inductive Reactance is the opposition to changes in voltage caused by an inductive component (inductor), or a circuit containing one. Inductive reactance is also proportional to frequency, but only when that frequency exceeds some minimum value called the ‘inductive threshold’. Below this value, inductive reactances are negligible; above it, they increase rapidly with increasing frequencies. This behavior gives rise to an important principle: the lower the supply’s available power at high frequencies, the higher will be its impedance as seen from low-frequency circuits connected across it. For DC circuits, there may not even exist any notion of impedance – not because these machines have no resistance (it is a law of nature), but rather because their resistances would cause so little current flow in them that their effect on circuit operation tends to become irrelevant.

Resistances add together in a series circuit, but not in a parallel circuit. In a parallel circuit, the total resistance is always lower than any individual one. A good way to understand the difference between these two circuits is that in series, there’s only one path for current and it must pass through each resistor so they all share its burden equally. In a parallel circuit each component has its own voltage drop, but they are not shared.

The reactance of an inductor increases with frequency, whereas capacitors have their reactances decrease with frequency. Resistance opposes changes in current flow when viewed from ‘a particular direction,’ or more accurately, ‘opposes the change of electric flux’. The higher the opposition (measured by Ohm’s Law), the greater impedance you will find at high frequencies.

The impedance of an electrical device is defined as the total opposition to current flow within a circuit or network, and it combines both resistance and reactance (resistors, inductors, and capacitors) but with opposite signs so that they cancel each other out when added together. Likewise, if you have a resistor in parallel with an inductor then their respective impedances are subtracted from one another, yielding zero net impedance at any given frequency since they cancel one another out.

The canceling out of inductive reactance by using a capacitor (and vice versa) is called Power Factor Correction. We won’t dive into Power Factor in this article, but just note for the time being that inductors and capacitors are very similar things —  they just have opposite effects on a circuit.

Summary

Resistance, Impedance, and Reactance all affect how much energy flows through a circuit over time. They can be thought of as little hurdles, ponds, or patches of sand that a sprinter may come across while running a race. There’s something happening in the circuit that is opposing current flow, making it more difficult for the current to flow quickly. Since electrical current (amperes) is a rate, this means it is a quantity over time. So resistance, impedance, and reactance all slow current flow down to a lesser quantity over time. Just like rush hour traffic does to cars! It’s important to note that when this happens, there is typically a power loss that occurs — this can be useful, depending on whether it’s intended or not. We’ll get into that in a later article on power in electrical circuits.

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