what is resistance? State and explain the laws of resistance.
Answers
☆Resistance-Resistance is the measure of opposition to current flow in an electric circuit.
The following are the main laws of resistance:
(i) Resistance of a conductor is directly proportional to its length, provided temperature and other physical conditions remain unchanged.
It means that R ∝ l i.e., if the length increases, the resistance also increases and if length decreases, its resistance also decreases.
(ii) Resistance of a conductor is inversely proportional to its area of cross section, other conditions remaining the same.
If A is the area of cross section, then :
R α 1/A
Or R α 1/πr2 where r is radius of the wire
Keeping the length same, if the radius of the wire is doubled then :
R α 1(2r)2 α 1/4r2
Or R α ¼.1/πr2, or R becomes one fourth.
Similarly if r is made half, then :
R α 1/(r/2)2
α 4/r2 α 4(1/r2), or R becomes 4 times
This shows that R is α 1/A
(iii) R depends on the nature of the material of the conductor. It means, if we take equal lengths of wires of copper, aluminium and iron and all of the same cross-sectional area, their resistance are different from each other since they are of different materials.
What is Resistance?
Definition: It is the opposition force experienced by the flowing electrons in some substances. this opposes the flow of electricity in a material. When a current of one ampere flows through a material that has a potential difference of one volt across it, then the resistance of that material is said to be one Ohm. The SI unit of resistance is represented by a greek symbol Ω.
First Law
The First Law states that ” conductive material is directly proportional to the length of the material”. According to this law, the resistance of the material increases with the increase in the length of the material and decreases with the decrease in the length of the material. .i.e.
R ∝ L—–(1)
Second Law
The Second Law states that ” the conducting material is inversely proportional to the cross-sectional area of the material”. According to this law, its material increases with the decrease in the cross-sectional area of the conductor and decreases with an increase in the cross-sectional area.
Third Law
The Third Law states that ” the conducting material depends on the nature of the material”. According to this law, the resistance value of the material varies depending upon the type of material. Two wires made up of different materials and having the same length and cross-sectional area will have different values.
Fourth Law
The Fourth Law states that “the conducting material depends on its temperature”. According to this law when the temperature of a metallic conductor is increased, it’s value also increases.
From the first, second and third law, the resistance of a material can be given as R ∝ L/A
i.e R = ρL/A