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:
- A circuit consisting of wire AB, an ammeter, a voltmeter, a switch, and a number of cells of equal potential difference is set up.
- 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.
- The graph is plotted between current $\left( {\rm{I}} \right)$ and p.d. $\left( {\rm{V}} \right)$.
Observations:
- The graph is a straight line.
- 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:
- Material of wire
- Length $\left( \rm{L} \right)$
- 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:
- Alloys produce more heat as they have higher resistivity than metals.
- Alloys do not oxidize (burn) readily at high temperatures.