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Class 10, Science, Chapter-11, Lecture-2, Ohm's Law (Notes)

OHM’S LAW

At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

${\rm{V}} \propto {\rm{I}}$
${\rm{V}} = {\rm{I}} \cdot {\rm{R}}$

where

${\rm{V}} = $ Potential difference
${\rm{I}} = $ Current
${\rm{R}} = $ Constant of proportionality (Resistance)

EXPERIMENTAL VERIFICATION OF OHM’S LAW:

Procedure:

  1. A circuit consisting of wire AB, an ammeter, a voltmeter, a switch, and a number of cells of equal potential difference is set up.
  2. The current $\left( {\rm{I}} \right)$ in the circuit and the potential difference $\left( {\rm{V}} \right)$ across the wire are noted for one cell, two cells, and three cells, respectively.
  3. The graph is plotted between current $\left( {\rm{I}} \right)$ and p.d. $\left( {\rm{V}} \right)$.

Observations:

  1. The graph is a straight line.
  2. The ratio of V and I is constant.

Conclusion:

$${{\rm{V}} \over {\rm{I}}} = {\rm{R, ~a ~constant}}$$

This verifies Ohm’s Law.

RESISTANCE:

The property of a conductor due to which it opposes the flow of current through it is called resistance.

S.I unit – ohm $\left( {\Omega} \right)$

ONE OHM:

The resistance of a conductor through which a current of 1 ampere flows when a potential difference of 1 volt is applied to its ends is one ohm $\left( {1 \Omega} \right)$.

FACTORS ON WHICH RESISTANCE OF A WIRE DEPENDS:

  1. Material of wire
  2. Length $\left( \rm{L} \right)$
  3. Cross-sectional area $\left( \rm{A} \right)$

RESISTIVITY:

The resistance offered by a cube of material of side 1 m when current flows perpendicular to the opposite faces is called resistivity.

${\rm{R}} = \rho \left( {{{\rm{L}} \over {\rm{A}}}} \right)$ where $\rho $ is the constant of proportionality, called resistivity.

Mathematically,  $\rho  = {{{\rm{R}} \cdot {\rm{A}}} \over {\rm{L}}}$

SI unit of resistivity is ohm metre $\left( {\Omega m} \right)$

Alloys are preferred over metals for making the conductors of electric heating devices.

Reasons:

  1. Alloys produce more heat as they have higher resistivity than metals.
  2. Alloys do not oxidize (burn) readily at high temperatures.