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Phys chapter 21 notes

# Phys chapter 21 notes - CHAPTER-21 NOTES CURRENT AND...

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CHAPTER-21 NOTES CURRENT AND DIRECT-CURRENT CIRCUITS _______________________________________________________________________ 1. Current and current density: Current I = Δ Q/ Δ 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 [a] Junction rule : The algebraic sum of the currents into any junction is zero Water pipe analogy for the junction rule: the flow rate of water entering a pipe equals the rate of water leaving.

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