Resistance

Question 1 A piece of wire is redrawn by pulling it until its length is doubles. Compare the new resistance with the original value?

Question 2 How is the resistivity of a material related to the use of that material?

Question 3 On what factors does the resistance of a conductor depends?

Question 4 Why connecting wires in an electric circuit are made of Cu and Al?

Question 5 Why Filament of an electric bulb is made up of tungsten metal?

Cause of Resistance

A conductor has large number of free electrons. When a potential difference is maintained across the ends of conductor, the free electrons move from one end to other end. During their movement, they collide with each other and with atoms of conductor. These collision opposes the movement of electrons  from one end to other end. This opposition to the flow of electrons due to collision is the resistance. More the collision suffered by electron in a conductor, more is the resistance.

Factors on which resistance of a conductor depends

1) Resistance of a conductor is directly proportional to length

Copper wire(A)= length(l), area of cross section X

copper wire (B)=length (l),area of cross section X

Connect wire A between the terminal, note ammeter reading.

Connect wire B between the terminal, note ammeter reading.

Reading of ammeter in second case is half that of reading in 1st case.

Resistance of second wire is double than 1st wire.

 

2) Resistance of a conductor is inversely proportional to area of cross section

Copper wire(A)= length(l) , area of cross section X

copper wire (B)=length (l),area of cross section is more than X

Connect wire A between the terminal, note ammeter reading.

Connect wire B between the terminal, note ammeter reading.

Reading of ammeter in second case is more that of reading in 1st case.

Resistance of a conductor is inversely proportional to area of cross section 

Resistance of a conductor also depends upon nature of material of conductor, temperature of conductor.

Resistivity of a conductor is defined as resistance of the conductor of unit length and unit area of cross-section.

S.I. unit of resistivity is ‎Ω m

Order of resistivity

Metals < Alloys <Insulators

A material whose resistivity is low is a good conductor i.e. low resistivity means less resistance to flow of current.

For Example : Silver has lowest resistivity and is the best conductor.

Resistivity does not depend on l and A but on nature of material of which substance is made of.

Uses of Different Material

1) Connecting wires in electric circuit are made up of Cu and Al.

a) Resistivity of Cu and Al is low(act as a good conductor)

b) Cheaper than silver

2) Filament of an electric bulb is made up of tungsten metal.

Tungsten has low resistivity, does not burn even at higher temperature(melting point is very high)

3) Heating elements of electrical appliances like electric iron, heater, toaster, immersion rod are made of nichrome (alloy of Ni, Cr, Fe, Mn)

Melting point is high or does not burn at higher temperature.

4) Insulators are used to protect ourselves from severe shock of electric current.