Specific resistance is the measure of the conductivity of a particular element and it is also known as electrical resistivity. Based on the value, the conductivity, and hence the electrical characteristics of the element are evaluated. This parameter is often used to analyze the electrical conductivity of a particular element, and hence have prime importance while selecting the elements. Some applications need elements that have superior electrical characteristics. The selection of those materials is done based on resistance. Thermal stress, strain, etc, are other aspects of the selection of materials.
What is Specific Resistance?
Definition: It is defined as resistance offered per unit length and unit cross-sectional area when the applied potential difference is known. The mathematical expression for specific resistance is given as ρ= RA/L
Where ‘ρ’ is the specific resistance, ‘R’ is the resistance, ‘A’ is the area of the conductor, and ‘L’ is the length of the conductor. As can be seen from the expression, ‘ρ’ is directly proportional to the area of the conductor and inversely proportional to the length of the conductor.
It can be also said that when resistance increases with an increase in the cross-sectional area of the conductor and a decrease in the length of the conductor. It also depends on one important parameter i.e. temperature. With the increase in temperature, the resistance may increase or decrease depending on the element. The inverse or reciprocal of this resistance is defined as specific conductance. The unit of specific resistance is ohm per meter.
Specific Resistance Formula
The formula for this is given by
Where ‘ρ’ is is also called electrical resistivity. R is the resistance of the material, A is the cross-sectional area and L is the length of the conductor.
As mentioned before, this resistance is directly proportional to the resistance of the elements, it’s the cross-sectional area, and inversely proportional to the length of the conductor. It also depends on the temperature of the element.
Specific Resistance for Different Materials
The specific resistance for materials at 20 degrees centigrade are listed below.
- Silver 9.8
- Copper 10.37
- Gold 14.7
- Tungsten 33.2
- Steel 95.8
It can be seen that silver has the least resistance at a particular temperature. For this reason, silver is considered one of the best conductors. The examples of other good conductors are Gold, Aluminum, etc.
The factors affecting the specific resistance are explained in brief
- Area – The resistance is directly proportional to the cross-sectional area of the conductor. As the area of the conductor increases, the resistance also increases. Based on this factor, the current carrying capacity of the element is evaluated. If the cross-sectional area of the conductor is more, then it can carry more current. In other words, the thickness of 5 Amp wire would be more than 1 Amp.
- Length – The resistance is inversely proportional to the length of the conductor. When the length of the conductor increases, then the resistance decreases. For a particular application, the length of the conductor is constant.
- Temperature – The resistance also depends on temperature. For some materials, for an increase in temperature, their resistance increases. Such elements are called to have a positive coefficient of temperature for resistance. For such elements with an increase in temperature, it will increases. Similarly, for some elements for an increase in temperature, their resistance decreases. Such elements are said to have a negative coefficient. For such elements, this resistance decreases with an increase in temperature
It may be noted that Ohm’s Law is valid only at a constant temperature. For a change in temperature, the ohm’s law will not hold true. That means that for a change in temperature the applied voltage is not directly proportional to current. For constant temperature only, voltage and current are directly proportional to each other, and resistance acts as the proportionality constant.
What is Airway Resistance?
Airway resistance deals with the science of respiratory physiology. It can be defined as the resistance of the respiratory tract with respect to airflow during inhaling and exhaling of air through our respiratory system. Airway resistance is the opposition to the flow caused by the forces of friction. It can be defined as the ratio of driving pressure to the rate of airflow. Similar to ohm’s law, the expression for airway resistance can be given as R= ∆P/V ̇
Where ‘R’ is the airway resistance, ∆P is the pressure difference which causes the airflow. And V represents the volumetric airflow. The pressure difference ∆P is the difference between atmospheric pressure and alveolar pressure.
Specific Resistance of Copper
It depends on the cross-sectional area, length, and temperature. For a specific temperature, the resistance of copper is 10.37-ohm meter. This has been calculated at a temperature of 200 C. From this, it can be evaluated that, copper is one of the good conductors after silver.
For this reason, copper is used in most of the electrical wiring due to its low specific resistance and abundance at a low rate. On the other hand, silver which is a better conductor than copper, cannot be used because of high cost and less abundance.
Copper is also used for winding purposes in almost all electrical machines. For example the transformer winding, induction motor stator winding, etc. The use of copper in electrical machines reduces the eddy current losses. Further, these windings are laminated to reduce losses.
1). What is the unit of Resistance?
The unit of resistance is ohms. Given by the symbolic representation Ω
2). What is Ohm’s Law formula?
Ohm’s Law states that the flow of charges or current flowing through an element is directly proportional to the potential difference applied across it.
The resistance becomes the proportionality constant between voltage and current. More is the resistance, more would be the voltage drop and less would be current. However, Ohm’s law is valid only at a constant temperature. For varying temperatures, ohm’s law is not valid.
3). How is voltage defined?
Voltage is the potential difference across two points. It is equivalent to pressure applied. This causes the charges to flow from one point to another, or other words, the charges flow from high pressure to low pressure. Hence the voltage causes the current to flow from one end to another. Resistance is the parameter that defines the proportionality constant between voltage and current.
4). Does Specific Resistance depend on temperature?
Yes, it depends on the temperature.
5). Does specific resistance depend on length?
Yes, it is inversely proportional to the length of the conductor. With an increase in length, it decreases.
Hence we have seen the basics of specific resistance, and how it depends on length, cross-sectional area, and temperature of the conductor. Here is a question for you, what could be the specific resistance of superconductors?