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Of course, all along the path of conductor electrons have random 'collisions' with the ions; as explained in detail later. Under such conditions, net flow of electrons across a cross-section does not remain zero, so one has an electric current flowing across the cross-section.
Amount of charge passing through a cross-section of a conductor per unit time is measure of electric current and is called the 'ampere'.
Q (coulomb) / t (second)
Ampere: If the net of charge flowing across a cross-section of a conductor is one coulomb per second, then the electric current passing is called one ampere. If the charge passing in time t second is Q coulomb, then the electric current is, I (ampere) Resistance: We referred to term "random collisions" between the drifting electrons and the ions located at the lattice points. Actually, the ions are not stationary, but execute an oscillatory motion about their mean position, energy of which increases with the increase in the temperature of the conductor. Electrons drifting between the ions have to move through the randomly varying electric fields due to these oscillations and hence are frequently deflected from their paths. These deflections can be effectively considered as results of the "collisions" with ions. Such random deflections suffered by the drifting electrons act so as to offer a resistance to the drift motion. This factor is a major contributor to the property called electrical resistance of a conductor. Other factors that contribute to the resistance are the impurities in the conductor and its constitutional and mechanical defects.
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2. Ohm's law and the Resistivity
A German scientist named Ohm deduced in 1828 a law that "under a given (fixed) physical conditions, the ratio of the electrical potential difference to the current is constant." This constant is known as the resistance of the conductor. Mathematically expressed, the law is,
R = VЛI
where V is the potential difference between the ends of a conductor and I is the current flowing through it. With V measured in volts and I in amperes, we have R in units called ohm, Thus,
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