Q24 of 35 Page 93

What is electrical resistivity? In a series electrical circuit comprising a resistor made up of a metallic wire, the ammeter reads 5 A. The reading of the ammeter decreases to half when the

length of the wire is doubled. Why?

Resistance of a uniform metallic conductor is directly proportional to the length (l) and inversely proportional to the area of cross-section.


i.e. R


R


Or R =


ρ is constant of proportionality and is known as electrical resistivity of the material


When length of wire is doubled, the current decreases because Resistance of a uniform metallic conductor is directly proportional to the length (l) and according to Ohm’s law, Resistance is inversely proportional to the current and therefore when the length of wire is increased, there is decrease in the value of current.


More from this chapter

All 35 →
22

Draw a circuit diagram of an electric circuit containing a cell, a key, an ammeter, a resistor of 2 Ω in series with a combination of two resistors (4 Ω each) in parallel and a voltmeter across the parallel combination. Will the potential difference across the 2 Ω resistor be the same as that across the parallel combination of 4Ω resistors? Give reason.

 23

How does use of a fuse wire protect electrical appliances?

 25

What is the commercial unit of electrical energy? Represent it in terms of joules.

 26

A current of 1 ampere flows in a series circuit containing an electric lamp and a conductor of 5 Ω when connected to a 10 V battery. Calculate the resistance of the electric lamp. Now if a resistance of 10 Ω is connected in parallel with this series combination, what change (if any) in current flowing through 5 Ω conductor and potential difference across the lamp will take place? Give reason.