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 = nqvdA, where n is the number of charge carriers per unit volume, q is the carrier charge, vdis the drift velocity, and A is the cross sectional area. The current density J = nqvd. The magnitude J = I/A.2. Ohm’s Law: V = IR. 3. Resitivity:ρ= RA/l, whereR 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 ρis characteristic of the material from which the resistor is fabricated. 4. Emf, εand terminal voltage Vabof a battery with internal resistance r.ε= Vab+ ir, where i is the current flowing through the circuit containing the battery. 5. Power: P = VabI ( for a general circuit element) = I2R ( 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 zero123.....0iiIIII=+++=∑aI1I3I2At junction ‘a’ I1+I2– I3= 0
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