What is the difference between resistance and resistivity?

Resistance ($R$) is a property of a specific object or component, depending on its material, length, and cross-sectional area. It quantifies the overall opposition to current flow for that particular piece of material. Resistivity ($\rho$), on the other hand, is an intrinsic property of the material itself, independent of its shape or size. It describes how strongly a material resists electric current per unit length and unit cross-sectional area. Think of it this way: resistivity is like the 'inherent difficulty' of the material, while resistance is the 'total difficulty' of a specific path made from that material.

Why does resistance increase with temperature for metals?

In metals, free electrons carry the current, and they collide with the vibrating atoms of the crystal lattice. As temperature increases, the atoms in the lattice vibrate with greater amplitude and frequency. This increased thermal vibration makes it more likely for electrons to collide with these atoms, and the collisions become more frequent and effective in impeding the electron's directed motion. Consequently, the average drift velocity of electrons decreases for a given electric field, leading to a reduction in current and thus an increase in resistance.

Are all materials Ohmic? What are non-Ohmic materials?

No, not all materials are Ohmic. Ohmic materials are those that strictly obey Ohm's Law, meaning their resistance remains constant over a wide range of applied voltages and currents, provided physical conditions like temperature are constant. Their voltage-current (V-I) graph is a straight line passing through the origin. Non-Ohmic materials, however, do not follow Ohm's Law. Their resistance changes with the applied voltage or current, resulting in a non-linear V-I graph. Examples include semiconductor diodes, transistors, and thermistors, where resistance can vary significantly with operating conditions.

How does stretching a wire affect its resistance?

When a wire is stretched, its length increases, but its volume remains constant. If the length ($L$) increases, its cross-sectional area ($A$) must decrease proportionally to maintain constant volume ($V = A \cdot L$). Since resistance $R = \rho L/A$, if $L$ increases to $nL$, then $A$ must decrease to $A/n$. Substituting these into the formula, the new resistance $R' = \rho (nL)/(A/n) = n^2 (\rho L/A) = n^2 R$. So, stretching a wire by a factor of 'n' times its original length increases its resistance by a factor of $n^2$. This is a common NEET question type.

What is the role of resistance in a circuit?

Resistance plays several crucial roles in electrical circuits. Primarily, it controls the amount of current flowing through a circuit or a specific part of it, according to Ohm's Law ($I = V/R$). Resistors are used to limit current to safe levels for sensitive components, to divide voltage in a circuit, and to create specific time delays when combined with capacitors or inductors. Additionally, resistance is responsible for converting electrical energy into heat, which is utilized in heating elements, light bulbs, and fuses for protection.

Electrical Resistance

Physics

NEET UG

Version 1Updated 22 Mar 2026

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Electrical resistance is a fundamental property of a material that quantifies its opposition to the flow of electric current. Defined by Ohm's Law, it is the ratio of the voltage across a conductor to the current flowing through it, provided the physical conditions (like temperature) remain constant. The SI unit for electrical resistance is the Ohm ( $Omega$ ), named after Georg Simon Ohm. This intr…

Quick Summary

Electrical resistance is a fundamental property of materials that quantifies their opposition to the flow of electric current. It arises from collisions between moving electrons and the atoms within the material's structure, converting electrical energy into heat.

Defined by Ohm's Law as the ratio of voltage ( $V$ ) to current ( $I$ ), $R = V/I$ , its SI unit is the Ohm ( $\Omega$ ). Resistance depends on four key factors: directly proportional to the conductor's length ( $L$ ) and its material's resistivity ( $\rho$ ), and inversely proportional to its cross-sectional area ( $A$ ).

This relationship is expressed as $R = \rho L/A$ . Resistivity is an intrinsic material property, while resistance is specific to a given object. For most metals, resistance increases with temperature due to increased atomic vibrations.

Materials obeying Ohm's Law are called Ohmic, while those that don't are non-Ohmic. Resistance is vital for controlling current, dividing voltage, and generating heat in various electrical applications.

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Key Concepts

Ohm's Law and Resistance Calculation

Ohm's Law, $V = IR$ , is the most fundamental relationship involving resistance. It allows us to calculate any…

Factors Affecting Resistance:

R = \rho L/A

The resistance of a conductor is not just a fixed value; it depends on its physical dimensions and the…

Temperature Dependence of Resistance

For most metallic conductors, resistance increases with temperature. This is because increased thermal energy…

Definition: — Opposition to current flow.

Ohm's Law: —

V = IR \implies R = V/I

SI Unit: — Ohm (

\Omega

Factors: —

R \propto L

R \propto 1/A

R \propto \rho

R \propto T

(for metals).

Formula: —

R = \rho \frac{L}{A}

Resistivity ($\rho$): — Intrinsic material property. Unit:

\Omega \cdot \text{m}

Conductivity ($\sigma$): —

\sigma = 1/\rho

. Unit: S/m.

Temperature Dependence: —

R_T = R_0 [1 + \alpha(T - T_0)]

Stretching Wire: — If length increases by factor

n

, resistance increases by

n^2

(volume constant).

To remember factors affecting resistance: 'Really Long And Thin Materials Resist Too'

Resistance is proportional to Length.

Area (cross-sectional) is inversely proportional.

Temperature affects it.

Material (resistivity) is key.

Resistivity is an Intrinsic property (I for Intrinsic).