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**Unformatted text preview: **Class 8 Resistivity and Reistance Circuits Physics 106 Fall 2011 Press CTRL-L to view as a slide show. Learning Outcomes Last time we learned about I Capacitance Problems I Parallel-Plate Capacitors I Capacitors in Circuits I Current I Resistance I Ohm’s Law Learning Outcomes Today we will learn about I Resitivity I How resistivity changes with temperature I Series and parallel circuits with resistors I Kirchoff’s Laws Resistivity Resistance and Length I Resistance is proportional to the energy lost in a conductor. I The longer a wire, the greater the resistance of the wire. Resistance and Drift Speed I Earlier we found: I = nev d A I Hence: v d = I / ( neA ) I If the cross sectional area is big, the electrons go more slowly. I If the electrons go more slowly, they lose less energy in collisions. I If the electrons lose less energy, the resistance is smaller. Resistivity I In summary: R = ρ L A I ρ is the resistivity of the material Resistance and Temperature Temperature Variation of Resistance I For most metals, resistance increases with increasing temperature I With a higher temperature, the metal’s atoms vibrate with increasing amplitude I The electrons lose more energy in collisions to the faster-moving atoms Temperature Variation of Resistance I Approximately linear: R = A + BT where A and B are constants. Temperature Variation of Resistance I We usually rewrite the constants in a more convenient form: R = R [ 1 + α ( T- T )] R is the resistance at temperature T R is the resistance at the reference temperature T (often T = 20 ◦ C) α is the temperature coefficient of resistivity Temperature Variation of Resistance I Since R = ρ L / A : ρ = ρ [ 1 + α ( T- T )] Energy and Power Electrical Energy and Power I A battery converts chemical potential energy to electrical energy I Pushing charges on the wire increases potential energy, much like compressing a spring. I As charge moves through a resistor, it loses this potential energy during collisions with atoms in the resistor A Simple Circuit I A battery is connected to a resistor in series. A Simple Circuit I The ground sets the voltage on the lower wire to zero....

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