Current Electricity

Objectives

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• Flow of charges in a metallic conductor • Ohm’s law, electrical resistance, V-I characteristics • Carbon resistors and combination of resistors • Kirchhoff ’s laws - Wheatstone’s bridge and its applications • Electric power and Electric energy • Heating effect - Joule’s law – applications • Thermoelectric effects – Seebeck effect – Peltier effect – Thomson effect

Summary

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„ The current, I flowing in a conductor I dQ dt = ,where dQ is the charge that flows through a cross-section in a time interval dt. SI unit of current is ampere (A). 1A = 1 C s–1. „ The current density J in a conductor is the current flowing per unit area. J I A =           „ Current is a scalar but current density is a vector. „ The general form of Ohm’s law   J E = σ „ Practical form of Ohm’s law states that V ∝ I, or V = IR where I is the current and R is the resistance, V potential difference between ends of the conductor. „ The resistance R of a conductor is R V I = . SI unit of resistance is ohm (Ω) and 1 Ω =1 1 V A „ The resistance of a conductor R l A = ρ where l is length of the conductor and A is its area of cross section. „ The resistivity of a material determines how much resistance it offers to the flow of current. „ The equivalent resistance (RS ) of several resistances (R1 , R2 , R3 ……..) connected in series is RS = (R1 +R2 +R3 ……..) „ The equivalent resistance (RP ) of several resistances (R1 , R2 , R3 ……..) connected in parallel is 1 1 1 1 R R P 1 2 R R3 =+++...... „ Kirchhoff ’s first rule (Current rule or junction rule): The algebraic sum of the currents at any junction is zero. „ Kirchhoff ’s second rule (Voltage rule or loop rule): In a closed circuit the algebraic sum of the products of the current and resistance of each part of the circuit is equal to the total emf included in the circuit. „ Electric power is the rate at which electric energy is delivered. „ If a current I flows across a potential difference V, the power delivered to the circuit is P = IV. „ In a resistor R, the electrical power converted to heat is P = I2 R =V R 2 „ The energy equivalent of one kilowatt-hour (kWh) is 1kWh = 3.6 X 106 J. „ Metre bridge is one form of Wheatstone’s network. „ Potentiometer is used to compare emf of cells. „ Joule’s law of heating is H = VIt (or) H = I2 Rt.