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Unformatted text preview: 1 Lecture 17 Chapter 20. Capacitors, Resistors and Batteries Many elements in a circuit act as resistors: prevent current from rising above a certain value. Goal: find a simple parameter which can predict V and I in such elements. Need to combine the properties of material and geometry. Resistance Conventional current: Group the material properties together: Current density: Conductivity ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⋅ = m V A nu q σ Conductivity Combining the properties of a material In copper at room temperature, the mobility of electrons is about 4.5 . 10-3 (m/s)/(V/m) and the density of electrons is n= 8 . 10 28 m-3 . What is ? What is the strength of E required to drive a current of 0.3 A through a copper wire which has a cross-section of 1 mm 2 ? Exercise 2 The conductivity of tungsten at RT is =1.8 . 10 7 (A/m 2 )/(V/m) and it decreases 18 times at a temperature of a glowing filament (3000 K). The tungsten filament has a radius of 0.015 mm. What is E required to dive 0.3A through it?...
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