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PHYS-102Ch-25and26-NOTES

PHYS-102Ch-25and26-NOTES - CHAPTERS-25 and 26 NOTES CURRENT...

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1 CHAPTERS-25 and 26 NOTES CURRENT AND DIRECT-CURRENT CIRCUITS _______________________________________________________________________ 1. Current and current density: Current I = Q/ t, is the amount of charge flowing past a certain cross sectional area per unit time. The unit for current is the Ampere (A). (1 A = 1C/s) The current can also be expressed as I = nqv d A, where n is the number of charge carriers per unit volume, q is the carrier charge, v d is the drift velocity, and A is the cross sectional area. The current density J = nq v d . The magnitude J = I/A. 2. Ohm’s Law: V = IR. 3. Resitivity: ρ = RA/l , where R is the resistance of a conductor of length l and of uniform cross-sectional area A. The resistivity, ρ = RA/l = VA/Il = E/J. The resistance R is a property of a given resistor whereas ρ is characteristic of the material from which the resistor is fabricated. 4. Emf, ε and terminal voltage V ab of a battery with internal resistance r. ε = V ab + ir, where i is the current flowing through the circuit containing the battery. 5. Power: P = V ab I ( for a general circuit element) = I 2 R ( power into a resistor) 6. Kirchhoff’s junction and loop rules (Also see Concept question 7) [a] Junction rule : The algebraic sum of the currents into any junction is zero 1 2 3 ..... 0 i i I I I I = + + + = a I 1 I 3 I 2 At junction ‘a’ I 1 +I 2 – I 3 = 0

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