Unit of Specific Resistance - Definition, Resistance Vs Electrical Resistance, FAQs
Specific resistance, also known as resistivity, is an important concept in current electricity that describes how strongly a material opposes the flow of electric current. It is defined as the resistance offered by a conductor of unit length and unit cross-sectional area. Represented by the symbol ρ (rho), specific resistance depends only on the nature of the material and temperature, not on its shape or size. The SI unit of specific resistance is ohm metre (Ω·m). Materials with low resistivity, such as copper and silver, are good conductors, while materials like glass and rubber have very high resistivity and act as insulators. Understanding specific resistance helps students analyze electrical circuits and material properties effectively.
This Story also Contains
- What is Specific Resistance?
- SI Unit of Specific Resistance
- Other Popular Units of Specific Resistance (Resistivity)
- Specific Resistance (Resistivity) of Some Materials at 20°C
What is Specific Resistance?
Specific Resistance:- When a voltage is applied to a substance, its specific resistance to a current is defined as the resistance provided by a unit length and unit cross-section of the substance. The unit of specific resistance is Ω m. Specific resistance, often known as electrical resistivity, is a measurement of a particular element's conductivity.
The Specific resistance of a conducting substance can be stated mathematically as,
$\rho=\frac{R A}{L}$
Where,
R denotes the conductor's resistance.
L is the conductor's length.
A = conductor's cross-sectional area.
ρ = the material's proportionality constant, often known as its specific resistance or resistivity.
The Ohm-meters (Ω m) is the unit of specific resistance.
SI Unit of Specific Resistance
The Ohm-meters (Ω m) is the unit of specific resistance.
Ohm-meter symbol: Ω m
Other Popular Units of Specific Resistance (Resistivity)
The SI unit of specific resistance (resistivity) is ohm metre ( $\mathbf{\Omega} \cdot \mathbf{m}$ ). However, in practical and laboratory measurements, some other units are also used depending on the system of units and the scale of measurement.
1. Ohm-centimetre ( $\Omega \cdot \mathrm{cm}$ )
This is a commonly used unit in laboratories and semiconductor physics.
$
1 \Omega \cdot \mathrm{~m}=100 \Omega \cdot \mathrm{~cm}
$
It is widely used for measuring resistivity of semiconductors and insulating materials.
2. Micro-ohm metre ( $\boldsymbol{\mu} \boldsymbol{\Omega} \cdot \mathbf{m}$ )
Used for materials with very low resistivity such as metals.
$
1 \mu \Omega \cdot \mathrm{~m}=10^{-6} \Omega \cdot \mathrm{~m}
$
3. Ohm-millimetre ( $\Omega \cdot \mathrm{mm}$ )
Sometimes used in engineering applications for wires and cables.
4. CGS Unit (abohm-cm)
In the CGS system, resistivity may be expressed in abohm-centimetre.
Factors affecting the specific resistance
Specific resistance is dependent upon the temperature, composition, and pressure of the material. Temperature affects specific resistance. The specific resistance of various materials increases as the temperature rises. The resistance of such elements is considered as having a positive coefficient of temperature. It will increase for such elements as the temperature rises. Similarly, as the temperature rises, the resistance of particular elements decreases. The coefficient of such elements is considered to be negative. The resistance of such elements decreases as the temperature rises.
Specific Resistance (Resistivity) of Some Materials at 20°C
The following table shows the approximate values of specific resistance (resistivity) of various materials at 20°C.
The SI unit of specific resistance is ohm metre (Ω·m).
Conductors
| Material | Resistivity (Ω·m) | Type |
| Silver | 1.59 × 10⁻⁸ | Conductor |
| Copper | 1.68 × 10⁻⁸ | Conductor |
| Gold | 2.44 × 10⁻⁸ | Conductor |
| Tin | 1.09 × 10⁻⁷ | Conductor |
| Platinum | 1.06 × 10⁻⁷ | Conductor |
| Iron | 9.70 × 10⁻⁸ | Conductor |
Insulators
| Material | Resistivity (Ω·m) | Type |
| Glass | 10¹¹ – 10¹⁵ | Insulator |
| Rubber | 10¹³ | Insulator |
| Diamond | 10¹² | Insulator |
| Air | 10⁹ – 10¹⁵ | Insulator |
Also check-
- NCERT Exemplar Class 11th Physics Solutions
- NCERT Exemplar Class 12th Physics Solutions
- NCERT Exemplar Solutions for All Subjects
NCERT Physics Notes:
Frequently Asked Questions (FAQs)
Conductance is the reciprocal of resistance.
A material's specific resistance is an inherent property that is determined by its composition. However, resistance is an intrinsic property that is determined by the conductor's length and cross-section, as well as the applied potential difference. Ohm (. m) is the SI unit for specific resistance is ohm-meters (Ω m), whereas Ohm is the SI unit for resistance.
Copper has a very low specific resistance, with a value of 1.68* 10-8 Ω m at 20°C. Copper is a good conductor of electricity due to its low resistivity. As a result of its low specific resistance, copper is preferred in most electrical wiring.
The resistivity of glass is approximately 1014. Glass is usually formed when molten material cools too quickly to form a regular crystal lattice. Most of the common glasses are made of the same chemical composition as quartz.
