Electric Current and Ohm's LawBASIC DATA ABOUT ATOM :Mass of electron = 9.11 x 10-31 kgMass of proton = 1.67 x 10-27 kgMass of neutron = mass of protonMass of electron = 1/1840 mass of protonDiameter of nucleus is of the order of 10-14 mDiameter of orbits = 104 times dia of moleculeDiameter of electron = 10-15 mCharge on electron = - 1.602 x 10-19 coulombCharge on proton = + 1.602 x 10-19 coulomb. UNIT OF CURRENT :The charge on an electron is measured in terms of coulomb. The unit of current is coulomb per second and is called ampere.Thus I (Ampere) = coulomb/second = ∆ q / ∆ tOne coulomb is equivalent to the charge of 6.28 x 1018 electrons.1 emu of current = 3 x 1010 esu of current. ELECTROMOTIVE FORCE :Electromotive force or potential of a body is the work done in joules to bring a unit electric charge from infinity to the body. It is expressed in terms of volts (V).The potential difference is defined as that which causes current to flow in the closed circuit. RESISTANCE :Resistance is the property of a substance due to which it opposes the flow of electrons (i.e., electric current) through it. The unit of resistance is ohm (Ω).Metals, acids and salt solutions are good conductors of electricity. Silver, copper and aluminium offer least resistance to flow of current and are called very good conductor of electricity. The electrons while flowing through the molecules or the atoms of the conductor, collide with other atoms and electrons, thereby producing heat.Some substances offer relatively greater difficulty or hindrance to the passage of these electrons. Such substances are called poor conductors or insulators of electricity. Some of the insulators are glass, bakelite, mica, rubber, polyvinyl chloride (P.V.C.), dry wood, etc. The resistance of a conductor depends on: 1. Length of conductor- it varies directly with the length2. Cross-sectional area of the conductor - it varies inversely with the cross-sectional area3. Its resistivity i.e. the nature of composition, etc., of the material of which the conductor is made up4. Temperature of the conductor - it almost varies directly with the temperature. Thus R, the resistance of a conductor is given by R = ρ l / A whereρ = specific resistance or resistivity of the material,l = length of the conductors,A = cross-sectional area of conductor. Ohm's Law :If the temperature and other conditions remain constant, the current through a conductor is proportional to the applied potential difference and it remains constant. Thus Current = Applied Voltage / Resistance of the circuit Resistance = Applied voltage / Current in the circuit Potential across resistance = Current x Resistance. Conditions for Ohm's Law :1. Ohm's law can be applied either to the entire circuit or a part of a circuit.

2. When ohm's law is applied to a part circuit, part resistance and the potential across the part resistance should be used.3. The Ohm's law can be applied to DC as well as AC circuits. However, in case of AC circuits impedance Z, is used in place of resistance. Thus I = E / Z = Applied voltage / Impedance in the circuit Conductance (G ) :Conductance is the reciprocal of ( R ) and is measure of the ease with which the current will flow through a substance. Thus G= 1 / R The unit of conductance is mho.