Resistance And Resistivity - Practice Questions & MCQ

NEET Seats in India 2024 - Government and Private Colleges For MBBS, BDS

Quick Facts

Resistance and Resistivity is considered one the most difficult concept.
52 Questions around this concept.

Solve by difficulty

An electric current is passed through a circuit containing two wires of the same material, connected in parallel. If the lengths and radii of the wires are in the ratio of 4/3 and 2/3, then the ratio of the currents passing through the wire will be

$3$

$1/3$

$8/9$

$2$

View Solution

The thermistors are usually made of

metals with low temperature coefficient of resistivity

metals with high temperature coefficient of resistivity

metal oxides with high temperature coefficient of resistivity

semiconducting materials having low temperature coefficient of resistivity

View Solution

Concepts Covered - 1

Resistance and Resistivity

Resistance

The resistance is known as the property of substance by virtue of which it opposes the flow of current through it.
Formula-

For a conductor of resistivity $\rho$ having a length of a conductor= l

and Area of a crosssection of conductor= A

Then the resistance of a conductor is given as

$R=\rho \frac{l}{A}$

Where $\rho\rightarrow$ Resistivity

Its S.I unit is $Volt/Amp$ or ohm $(\Omega)$
Its Dimensions is $ML^{2}T^{-3}A^{-2}$
Reciprocal of resistance is known as conductance.
Resistance of a conductor depends on the following factors

Length -

As $R=\rho \frac{l}{A}$

So Resistance of a conductor is directly proportional to its length

i.e. $R \ \alpha \ l$

Area of cross-section-

As $R=\rho \frac{l}{A}$

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

i.e. $R \ \alpha \ \frac{1}{A}$

Material of the conductor-

As $R=\rho \frac{l}{A}$

And For a conductor, if n = No. of free electrons per unit volume in the conductor, $\tau$ = relaxation time then the resistance of conductor

Then $\rho=\frac{m}{ne^2\tau}$

for different conductors n is different

And $\rho$ depends on n

So R is also different.

Temperature-

As $R=\rho \frac{l}{A}$

And $\rho=\frac{m}{ne^2\tau}$

So $R \ \alpha \ \frac{1}{\tau}$

And as temperature increases $\tau$ decrease

So as the temperature increases resistance increases

Temperature-dependent resistance is given by

$R_{T}-$ Resistance at temperature $T$

$R_{0}-$ Resistance at temperature $T_{o}$

$\alpha-$ temperature coefficient of resistance

Where the value of $\alpha$ is different at different temperatures

From Ohm's law

$V=IR$

Where $R-$ Electric Resistance

Ohmic Substance: The substance which obeys Ohm's law are known as Ohmic substance. I-V graph is linear and the slope gives conductance which is reciprocal of resistance

2. Non-ohmic substances

Those substances which don't obey Ohm's law are known as Non-ohmic or non-linear conductors.

For example gases, crystal rectifiers, etc.

3. Superconductor: For certain materials resistivity suddenly becomes zero below a certain temperature (critical temperature). The material in this state is called a superconductor.

In Superconductor, resistivity is zero