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Unformatted text preview: 1 Physics 1200 Lecture 2  F2011 Capacitance Calculating capacitance Energy storage Current and Current density Resistivity and Resistance Power dissipation 2 Current and Current Density Resistivity and Resistance Ohms Law Power dissipation 3 Electric Current Charge flow is produced when an electric field is applied to charges. Positive charges flow in the direction of the electric field, negative charges move opposite to the field. Current is defined as the rate of flow of charge: E v d dq i dt Current flows in the direction that positive charge moves (and opposite to the flow of negative charge.) Current always flows from high potential to low, in the direction of the field. 4 Current Density: J Current density: The rate of current flow per unit area through a cross section to the flow. area drift drift i J i J dA A J nqv v n q or drift velocity carrier density charge per particle i + q conductor v A J (Current is the flux of current density) 5 P2F09 5 Kirchhoffs Junction Rule Because electric charge is conserved, the current through any closed surface must always add up to the change of charge enclosed. At any point in an electrical circuit where charge density is not changing in time, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point. I 1 I 2 I 3 I 1 =I 2 +I 3 wires in steady state. encl dQ J dA dt J dA I 1 I 2 I 3 I 1 =I 2 +I 3 wires 6 iClicker Question 3.1 Across which line above is the current the greatest? A=aa B=bb C=cc D=They are the same. E. None of the above. I am a maverick. 7 iClicker Question 3.2 Through which line above is the current density the greatest? A=aa B=bb D=They are the same. E. N. o. t. A. (maverick) 8 Resistivity The resistivity is the scaling factor between applied field and current density....
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This note was uploaded on 03/21/2011 for the course PHYS 1200 taught by Professor Stoler during the Spring '06 term at Rensselaer Polytechnic Institute.
 Spring '06
 Stoler
 Physics, Capacitance, Current, Resistance, Energy, Power

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