If you’ve ever worked with copper or aluminum wiring, you’ve probably heard about “resistivity.”
Resistivity relates to how difficult it is for electrical currents to flow through a material. Although nearly everything on Earth has the potential to resist current, except for superconductors, the amount depends on several factors, including temperature, purity, and the current’s frequency.
Even within a particular material like copper, there are different resistivities based on wire type and composition. Having this information can make it easier to efficiently control the flow of electrical energy through an installed system.
Let’s get this out of the way – resistivity and resistance are NOT THE SAME.
When we mention resistivity, it’s a standard constant measurement, known as an intrinsic property, given to a material at a particular temperature, typically 20° C. Resistance is slightly different because it factors in resistivity, along with conductor size and wire length, to measure a wire or cable’s ability to allow electricity to flow through it.
|Metal||Resistivity (Ω⋅m) @ 20° C|
|Silver||1.59 × 10⁻⁸|
|Copper||1.68 × 10⁻⁸|
|Gold||2.44 × 10⁻⁸|
|Aluminum||2.82 × 10⁻⁸|
|Iron||9.71 × 10⁻⁸|
As you can see, the resistivity of copper is less than other metals like iron and aluminum, making it better for transporting electricity. This doesn’t mean aluminum is a poor transporter of electricity – it simply requires a larger gauge wire to move the same amount of current.
Although resistivity is a consistent measurement, it can change based on the metals’ purity and temperature. For example, copper might be measured at 1.68 × 10⁻⁸ at 20° C, but metal impurities or a higher temperature will make it more difficult for electrons to move through.
Like peanut butter needs jelly, resistivity needs an opposite measurement. This is where conductivity comes in.
Conductivity is another intrinsic property measuring how well a material conducts an electrical current. It’s inversely proportional to resistivity, meaning that the higher the resistance is, the less conductive the material is.
Some great conductors include gold, silver, copper, aluminum, and steel. Wire and cable are typically made using copper, aluminum, and sometimes steel, copper-clad steel tracer wire for example, because they are inexpensive and durable.
If a material can carry a current, you can measure the resistivity of that material – it all comes down to the type of atoms there are.
Materials like copper and aluminum are good conductors of electricity because they have low resistivity and allow electrical currents to flow easily. On the other side of the coin, materials like rubber, glass, and plastic are much more difficult for electricity to flow through. These materials would be called insulators.
So, how does one figure out a wire’s resistivity? The easiest way to find the answer is to do some quick math using a simple formula.
Calculate the resistance of the wire (measured in ohms) and find the cross-sectional area of the wire (measured in square meters). If you know how long the wire is, you can create an equation that looks something like this:
Resistivity (Ωm) = Resistance (Ω) * Area (m²) / Length of wire (m)
The answer can change based on several factors, including how pure the metal is, the temperature, and how the wire was formed.
It’s important to remember that resistivity is a constant measurement of a material in a controlled environment. Not every sample will be the same, opening the door for deviations.
Depending on several factors, materials that were good insulators or conductors can become even better under the right conditions.
Material quality has a massive impact on resistivity. Impurities make it harder for electrons to easily flow through the material.
An easy way to think of this is a freshly paved road versus one full of potholes. Cars will not move nearly as fast if the road is an utter mess and difficult to navigate.
Electrons need a fast path from point A to point B, but it’s harder to do as temperatures rise. The reason is that as heat is applied to the material, its atoms begin to vibrate faster, making it more difficult for the current to move smoothly.
Think of it as trying to move through a crowd at a jam band concert (low temperatures) versus pushing through a mosh pit at a metal show (higher temperatures).
Everything slows down with age, and the same can be said for materials that have to move electricity. As the material ages, defects can begin to form. Whether it’s rust, dirt, chips, damage, etc., resistivity increases over time.
Properties of a material matter.
Pure materials have naturally lower resistivities than alloys, and that’s because alloys combine two or more materials together into one new one. When the crystal structure is all one material, it’s easier for electrons to flow through. The process is slower when they must navigate between two or more materials to accomplish the same goal.
Stress can pose real trouble for an electron moving through a material. When mechanical stress occurs, the material’s crystal structure can change, slowing electrons down.
To combat stress, copper and aluminum wires are subjected to an annealing process to strengthen them. Annealing requires the metal to reach and maintain a particular temperature before slowly cooling down.
It might not seem like a big deal but knowing the resistivity of the materials you’re working with, including aluminum or copper wire, can go a long way.
For example, it’s entirely possible to use aluminum as a transmission cable. We don’t because the gauge size needed to move the same amount of electricity through aluminum cable from the generation plant to homes and businesses would be humongous. Material choices also prevent unnecessary overheating damage caused by high resistivity.
In other cases, knowing how electricity flows through a material can help when trying to develop new resistors, measure temperatures with a thermocouple, or choose fuses for electrical products.
Whether you’re trying to get as much electricity flowing as possible or stop it, it’s vital to know how resistivity impacts an electrical system.
Depending on the material, conditions, and application, knowing how it affects the overall flow of electricity will ensure every installation and action is done as efficiently and safely as possible.
And, as with anything, a little preparation can reduce mistakes and improve performance for years.
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