RESISTANCE

It may be defined as the property of a substance. due to which it opposes the flow of electricity through it.

Metals acids and salt solutions are good conductors of electricity. Among st pure metals, silver, copper and aluminum are very good conductors in the given order. This, as discussed earlier, is due to the presence of a large number of free or loosely attached electrons on their atoms. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide with other atoms and electrons, thereby producing heat.

Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like Bakelite, mica, glass, rubber, p.v.c

It is helpful to remember that electric friction is similar to friction in mechanics.


                                            THE UNIT OF RESISTANCE

The practical unit of resistance is ohm. A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals.

For insulators whose resistances are very high, a much bigger unit is used megohm=10 to the power 6 ohm or kilohm= 10 to the power 3 ohm. In the case of very small resistances, smaller units like milliohm= 10 to the power -3 ohm or microhm= 10 to the power -6 ohm are used.