03(T)%20-%20Current%20Electricity - 3 CURRENT ELECTRICITY...

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3 - CURRENT ELECTRICITY Page 1 3.1 Electric Current 1 ampere of electric current is produced when 1 coulomb of electric charge passes through the cross-section of the conductor in 1 second. In a steady circuit, electrical potential at all points of a conductor remain constant with respect to time. In such a steady circuit, the amount of electric charge entering any cross- sectional area of a conductor in a given time interval is equal to the amount of electric charge leaving that cross-sectional area in the same interval of time. In other words, charge never accumulates at any point in the conductor. Also the electric charge is neither created nor destroyed at any point in the conductor. This means that electric charge is conserved. Let Q amount of electric charge flow through any cross - sectional area of the conductor in time interval t. The average electric current flowing in time interval t is given by t Q = > < ΙΙ The instantaneous electric current at time t is given by dt dQ = t Q 0 t lim = Ι Ι Electric current is a fundamental quantity in S.I. unit and its symbol is A meaning ampere. Smaller units of the current are milliampere ( mA = 10 - 3 A 29 29 29 and microampere A = 10 - 6 A ). Current density The electric current density near any point is defined as the amount of electric current flowing perpendicularly through the unit cross - sectional area near that point. Let J be the electric current density at every point on the cross - sectional area, P be some point of the curved cross - sectional area of the conductor as shown in the figure, a be the area vector of the surface near point P, component of which in the direction of the current is a cos θ, where θ is the angle between the area vector and the direction of the current. Therefore, average current density near point P, θ cos a = > J < Ι where ∆Ι ΙΙ is electric current flowing through the small area element near point P. Therefore, electric current density, θ cos da d = θ cos a 0 a lim = J Ι Ι da . J = θ cos da J = d Ι
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3 - CURRENT ELECTRICITY Page 2 Integrating, JA = da J = a da . J = d = Ι Ι area sectional - cross entire the = A where A = J Ι 3.2 Electromotive Force and Terminal Voltage The potential difference between the two poles of an electric cell is defined as the work done by the non-electrical force ( due to chemical process taking place inside the cell ) in moving a unit positive electric charge from a negative pole towards the positive pole. The energy gained by the unit positive charge due to the non-electrical force, in moving it from the negative pole towards the positive pole, is called the emf ( electromotive force ) of the electric cell. Its unit is joule / coulomb ( = volt ). When
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03(T)%20-%20Current%20Electricity - 3 CURRENT ELECTRICITY...

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