Name: Ohm’s Law & Introduction to Electric Resistance
Uploaded: 2019-07-19
Duration: 8 min 43 s
Description: In our attempt to establish a relationship between current and voltage, we will discover a new physical quantity in this video. Let's watch and learn.

Question

What is electrical resistivity of a material? What is its unit? Describe an experiment to study the factors on which the resistance of conducting wire depends.

Accepted Answer

Resistivity of a material is the resistance offered by a cube of unit length made up of that material when current flow perpendicular to the opposite faces.

The equation for resistivity is,

$ρ = R . \frac{A}{L}$

Unit of $R =$ ohm, Area $= m^{2}$ and $L = m$ .

$ρ = \frac{o h m \times m^{2}}{m} = o h m \times m .$

Thus, the SI unit of $ρ$ is $Ω m$

Object: To demonstrate the dependence of resistance on length, area of cross-section and material of the wire.

Requirements: Ammeter, wires of nichrome having

(i) length $L$ and area of cross-section $A$ ,

(ii) length $2 L$ and area of cross-section $A$ ,

(iii) length $L$ and area of cross section $2 A$ ,

(iv) one wire of copper having length $L$ and area of cross-section $A$ .

Method: Complete the electric circuit as shown in figure. Connect at $A B$ the above-mentioned wires one by one observe the current passing through the circuit.

Wire at AB Observation

(i) Wire of nichrome of length $L$ and area of cross-section $A$ . $x$ ampere current flows

(ii) Wire of nichrome of length $2 L$ and area of cross-section $A$ . $\frac{x}{2}$ ampere current flows.

(iii) Wire of nichrome of length $L$ and area of cross-section $2 A$ . $2 x$ amp of current flows.

(iv) Wire of copper of length $L$ and area of cross-section $A$ . Current $> > x$

Result: The above experiment shows that

(i) The resistance of a wire is directly proportional to its length.

(ii) The resistance of a wire is inversely proportional to its area of cross-section.

(iii) Resistance depends on the material of the wire.

Or

$R \propto L R \propto \frac{1}{A} R \propto \frac{L}{A} R = ρ \frac{L}{A}$

$ρ$ (rho) is called the resistivity of the material.

(iv) Further, the resistivity of a material depends on temperature also. In the case of metals and alloys, the resistivity increases (and therefore resistance) with temperature. Thus, resistance depends on the temperature also.

What is electrical resistivity of a material? What is its unit? Describe an experiment to study the factors on which the resistance of conducting wire depends.

Important Questions on Electricity

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