Last Updated on April 20, 2022 by Kevin Chen
Are you aspiring to be an electrician or do you just want to enrich yourself with knowledge on matters of electricity? Even if you are an ordinary buyer of electrical components and devices, there are certain terms that you should know.
Among them includes resistance and resistivity.
In this guide, we are going to break down the difference between resistance and resistivity. At the end of the guide, you will be able to explain what they mean and when to use them.
What is resistance?
Resistance can be defined as the opposition to the flow of electric current in a conductor. It is denoted by the symbol ‘R’. The unit used for measuring resistance is called Ohm and the symbol used for it is ‘Ω’.
Resistance can be seen as a force that pushes back when you are trying to move something.
When you try to push a heavy object, there will be friction between your hand and the object. Similarly, when an electric current tries to flow through a conductor, there will be some resistance between them.
Resistance arises due to collisions of electrons with ions and atoms in their path. The collisions cause electrons to lose energy which results in heat generation. This is why resistance leads to heat generation and most resistors have high heatsink ratings because of this property.
What is resistivity?
Resistivity can be defined as the ability of a conductor to resist the flow of electric current. It is denoted by the symbol ‘ρ’. The unit used for measuring resistivity is called Ohm-meter and the symbol used for it is ‘Ω-m’.
Resistivity can be seen as a measure of how much a material opposes the flow of electric current.
The higher the resistivity, the more will be the opposition to electric current and vice versa.
Resistivity can be thought of as a measure of how easily electrons can flow through a material. The lower the resistivity, the more electrons will flow (more current) and vice versa.
So, we see that resistivity is directly proportional to resistance. This explains why we can also define resistivity as the measure of the resistance of an object. When you hear somebody inquiring about the resistivity of an object, they simply want to know the specific electric resistance that the object has.
Is resistivity the same as specific resistance?
The resistivity of a material is a measure of how much it resists the flow of electricity. In other words, resistivity is the reciprocal of conductivity.
Resistivity is sometimes called specific resistance, but resistivity and specific resistance are not the same thing. Resistivity is measured in ohm-meters per square meter (ohm·m) or ohm·cm. Specific resistance is measured in ohm·cm or ohm·cm/W.
Resistance vs Resistivity: What’s The Difference?
Now let’s dissect the key differences between resistance and resistivity.
Definition
Resistance is the measure of the opposition to the flow of an electric current.
Resistivity is the measure of how strongly a material opposes the flow of an electric current.
Resistance can be defined as:
R=R(I, V) where R(I, V) is called the resistance function.
Resistivity can be defined as:
ρ=ρ(I, V) where ρ(I, V) is called the resistivity function.
Symbol
Resistance is denoted by the Greek letter ‘Ω’ while resistivity is denoted by the Latin letter ‘ρ’.
Unit
Resistance is measured in Ohm (Ω) while resistivity is measured in Ohm-meter (Ω-m).
Proportionality
Resistance is directly proportional to resistivity.
This means that if you double the resistance, you will also double the resistivity.
On the other hand, we can say that resistivity is inversely proportional to resistance.
This means that if you double the resistivity, you will also halve the resistance.
Effect of temperature on resistance
Resistance has a positive temperature coefficient, which means the resistance increases with an increase in temperature.
On the other hand, resistivity has a negative temperature coefficient, which means that resistivity decreases with an increase in temperature.
This can be explained by the fact that at higher temperatures electrons move faster and hence are less likely to collide with the nucleus of an atom.
Hence, it will be easier for electrons to move through a material due to its lower resistivity at higher temperatures.
Effect of length on resistance and resistivity
The resistance of a wire is directly proportional to its length.
This can be explained by the fact that the length of a wire decreases the number of collisions with the nucleus of an atom.
Hence, it will be easier for electrons to move through a material due to its lower resistivity at higher temperatures.
On the other hand, length has no impact on the resistivity of an object. So, if the length of the cable that you are buying is a key issue, the next thing that you should look at is its resistance.
Cross section area of the material
The cross-section area of a material is directly proportional to its resistance.
This can be explained by the fact that the cross-section area of a wire decreases the number of collisions with the nucleus of an atom.
Hence, it will be easier for electrons to move through a material due to its lower resistivity at higher temperatures.
On the other hand, the cross-section area has no impact on resistivity.
If you are considering buying a wire based on its cross-section area, then you should also look at its resistance before making your purchase decision.
Potential difference (Voltage drop)
The potential difference is the voltage that is lost in a wire. The voltage drop occurs due to resistance and heat. If the potential difference is low, the power will be high. If the potential difference is high, then the power will be low.
For example, if a wire has lower resistance, then it will have a lower potential difference.
But, what is the relationship between voltage drop and resistivity?
Voltage drop is directly proportional to the material’s resistivity. So, if the voltage drop is low, then the wire is highly conductive and highly resistive.
On the other hand, if the voltage drop is high then it will be a good conductor with a low resistivity.
Applications
In which applications should I care about resistance and in which one should I care about resistivity?
You will worry about resistance when dealing with electrical devices such as heaters, fuses, or transformers. You will worry about resistivity when you are dealing with heat transfer.
For example, heat transfer from a heater to a house is an application where resistance should be considered. On the other hand, you will think of resistivity in applications that involve measurement and quality control.
How do I measure resistance?
You can measure the resistance of a material by:
- Measuring the voltage drop across the material and relating this value to the material’s electrical conductivity. The most common method of measuring voltage drop is using an ammeter or voltmeter, but there are other ways as well. For example, one way to measure voltage drop is using a galvanometer (a type of voltmeter). A galvanometer has two electrodes connected in series that can be used to measure potential difference and thus voltage drop across a resistor. The galvanometer measures the potential difference in millivolts (mV). The most commonly used value for measuring resistance is the ohmic value.
- Measuring the current across the material and relating this value to the material’s electrical conductivity. The most common method of measuring current is using a bridge circuit, but there are other ways as well. For example, one way to measure current is using an ammeter or voltmeter, but there are other ways as well. For example, one way to measure current is using an ammeter or voltmeter, but there are other ways as well.
- Measuring temperature and applying Ohm’s Law to calculate the resistance of a material by either measuring the resistance of a piece of wire that has been heated or applying the Law of Conservation of Energy to calculate resistance based on the temperature difference between two points.
How to measure resistivity?
The easiest way to find out the resistivity of an object is by using a collinear probe. A collinear probe is a high-precision probe that uses two small probes to measure the resistance of a material. The two probes are held at right angles to each other. One is called the “reference” and the other is called the “probe”. The reference and probe are connected by an insulated wire that carries a current. By measuring the voltage drop across this wire, it’s possible to calculate the resistivity of a material.
Another way to find resistivity is to use a metal probe and electrical conductivity meter. A metal probe is a very small metal rod that you can insert into your material sample. The probe is connected by an insulated wire that carries a current. By measuring the voltage drop across this wire, it’s possible to calculate the resistivity of a material.
Conclusion
I hope you now have in-depth knowledge on the difference between resistance and resistivity. So, next time you are undertaking an electrical engineering project, you should be able to differentiate the two.
And when buying electrical components in China, ask the sourcing agent ICRFQ to ascertain the resistance and resistivity of the object.
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