The resistance of a conductor can be defined as the property of the conductor that opposes the flow of electric current through it. The SI unit of resistance is ohm. It is denoted by the upper case letter 'R'. When an electric current of 1 Ampere passes through a conductor, then the potential difference becomes 1 volt, this resistance of a conductor is known as 1Ω (ohm).
Metals like silver, copper, gold etc. have low resistance. Copper is used mostly in the wire as it offers low resistance. Some substances offer very high resistance. Some mostly used substances are given below with their composition, uses and resistance,
Names of Substances | Composition (Alloy) | Uses | Resistance at 20 degree celsius ohm metres |
Constantan | 55% copper and 45% nickel | Used in construction of pyrometer and thermocouple | 5 \(\times\) 10^{-7} |
Manganin | 83% copper, 13% manganese and 4% nickel | Used in the construction of rheostats and resistors | 4.2 \(\times\) 10^{-7} |
Nichrome | 60% nickel and 40% chromium | Used in construction of heating element of heaters | 1.1 \(\times\) 10^{-6} |
As we know that, the resistance of a conductor is directly proportional to its length and inversely proportional to its cross- sectional area. If 'l' is the length of a wire, 'A' is it's the cross- sectional area and 'R' is its resistance, then
R∝ l ........... (i)
R∝ \(\frac{1}{A}\) ................(ii)
From eq (i) and (ii) we have
R∝ \(\frac{l}{A}\)
∴ R =ρ \(\frac{l}{A}\)
Where,
ρ (Rho) = resistivity or specific resistance of the material
The SI unit of resistivity (ρ) is ohm- metre (Ωm).
If l = 1m, A = 1m^{2}, then R =ρ
ρ = R \(\frac{A}{l}\).
The resistivity of a material is numerically equal to the resistance of a conductor of unit area per unit length of a material.
The reciprocal of the resistivity (ρ) of a material is called its conductivity (σ) sigma.
Conductivity (σ) =\(\frac {1}{ resistivity (ρ)}\).
Any two differences between conductor and insulator are as follows,
Conductor | Insulator |
Those substances that allow heat and electricity to flow through them are called conductors. | Those substances that do not allow heat and electricity to flow through them are called insulators. |
There are free electrons. | There are not free electrons. |
Eg: brass, copper, silver etc | Eg: silk, rubber etc. |
The resistance of a conductor depends on the following factors:
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Sanju
Give the relation of resistance and wire with it's length and thickness.
Feb 05, 2017
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Anjali
What do you mean by resitivity and conductivity defination.
Jan 02, 2017
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