You must have had electric shocks! Haven’t you? Did you get the shock on a plastic wire? It is not possible. You can’t get shocks from plastic wires. But, why is it so? It is because of a phenomenon that we will read about in this chapter. We will study about resistivity of various materials.
What is Resistance?
We know that electric current that flows in a circuit is as similar to the water flowing through a river. In a river rock, branches and other particles resist the flow of water. in a very similar fashion, a circuit has elements to resist the flow of electrons.
Browse more Topics under Current Electricity
- Electric Current
- Ohm’s Law
- Electrical Energy and Power
- Temperature Dependence of Resistivity
- Drift of Electrons and the Origin of Resistivity
- Combination of Resistors – Series and Parallel
- Atmospheric Electricity and Kirchhoff’s Law
- Wheatstone Bridge, Meter Bridge and Potentiometer
- Cells, EMF, Internal Resistance
- Cells in Series and Parallel
Resistance is nothing but this property of resisting the flow of electrons or the current. The unit of resistance is ohm. One ohm is equal to volt per ampere. From Ohm’s law, we have seen that R = V / I, Where V is the voltage and I is the current.
Resistors are used to resist or control the flow of electrons by the conductive material. They do not provide any power to the circuit. They may reduce the voltage and current passing through the circuit. Hence, resistors are passive devices. Most of the resistors are made up of carbon, metal or metal oxide film.
Resistivity is the resistance per unit length and cross-sectional area. It is the property of the material that opposes the flow of charge or the flow of electric current. The unit of resistivity is ohm meter.
We know that R = ρ L / A. Thus we can derive the expression for resistivity from this formula. ρ = R A / L, where R is the resistance in ohms, A is the area of cross-section in square meters and L is the length in meters. When the values of L, the length, and A, the area is equal to one, we can say that the resistivity is equal to the resistance.
So, resistivity is the specific resistance of a material. When we have a thick wire, the resistance decreases. The resistance increases when the wire is thin as the area of cross-section is less. When the length of the wire increases, the resistance also increases. When the length of the wire decreases, the resistance decreases as the length is less.
The Resistivity of Various Materials
A material with high resistivity means it has got high resistance and will resist the flow of electrons. A material with low resistivity means it has low resistance and thus the electrons flow smoothly through the material.
For example, Copper and Aluminium have low resistivity. Good conductors have less resistivity. Insulators have a high resistivity. The resistivity of semiconductors lies between conductors and insulators. Gold is a good conductor of electricity and so it has low resistivity.
The glass is a good insulator which does not allow the flow of electrons. Hence, it has a high resistivity. Silicon is a semiconductor and so it allows partial movements of electrons. The Resistivity of Silicon comes between glass and gold. The resistivity for perfect conductors is zero and the resistivity for perfect insulators is infinite.
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Solved Examples for You
Question 1: The resistivity of alloys is ______ than its constituent elements.
Answer: Option A – Higher. Metal alloy has a greater resistivity than the corresponding metals because of lattice distortion from the alloying elements. A metal with no alloying elements would transport electron by drift oscillation over the lattice.
The difference in atomic radii of alloying elements and in electronegativity from base metal, the presence of alloying element changes the local electronic structure of the base metal. Such change modulates the typical drift oscillation mechanism in electron conduction by scattering and leads to higher resistance.
Question 2: Name three materials or substances that have good resistance.
Answer: Insulators have good resistance. Examples include glass, ceramics, wood etc.