Electrical Engineering - What is Resistance and Resistivity?

Electrical Engineering ⇒ Topic : Resistance and Resistivity

What is Resistance and Resistivity?

Gopal said on : 2018-03-16 08:59:38

Resistance is the property of a substance due to which it opposes the flow of current through it. Its unit is ohm (e). Let us consider a conductor of length / m and area of cross-section a m² (Figure). If R he its resistance, then

Figure : Conductor having length / m and area a m².

R ∝ 1 (when a is constant)

and R ∝	1	(when / is constant)
	a

Hence, R ∝	1
	a

(when both / and a are not constant)

R = ρ	l
	a

ρ is the constant of proportionality and is known as resistivity of the material. Substituting 1 = 1 m and a = 1 m² in above, we get
R = ρ

we get ρ =	Ra
	l

Its Unit is	Ωm²	i.e.,Ωm
	m

The unit of resistivity p is ohm metre (Ωm).

Ayaan said on : 2018-03-21 23:47:08

Units of resistance : The unit of reistance is ohm and is denoted by the symbol Ω.
A wire is said to have a resistance of one ohm if it develops 0.24 calories of heat when 1 ampere of current flows through it for one second.
For example, when we say that a wire has a resistance of R ohms, it means that if one ampere current flows through this wire for one second, then R X 0.24 calories of heat will be produced.

Amir said on : 2018-03-27 04:11:59

Resistance is the property of a material by virtue of which it opposes the flow of electric current.

Since current is the flow of electrons, hence 'resistance is an opposition to the flow of electrons. This opposition occurs because of the presence of a large number of atoms and molecules in the substance through which current flows.
The electrons while moving through a substance collide with these atoms and molecules, these collisions offer opposition to the flow of electric current. Moreover, the atomic structure of the substance also decides the extent of opposition. If there are a large number of free electrons as in a copper wire, then by a small potential difference, these free electrons can be moved and therefore, opposition and hence resistance is less. On the other lland, there are other substances which have fewer free electrons, for example, carbon. When the same potential difference is applied (as was applied to copper wire), far fewer electrons will flow because there are fewer free electrons. Hence, carbon has high resistance.

Leo said on : 2018-07-16 23:18:02

RESISTANCE:- The unit of electrical resistance is ohm. Its symbols is only. Ohm is defined as the resistance between two points of a conductor when a potential difference of 1 volt, applied between these points, produces in this conductor a current of 1 ampere, the conductor not being a source of any electromotive force.

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