The Line of Resistance. APS Teachers Day Workshop Los Angeles, CA March 22, 2005 Dr. Larry Woolf General Atomics Larry.Woolf@gat.com www.sci-ed-ga.org (click on Presentations to see all these slides). Multimeter Operation. Work with your group With leads together, R = 0
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APS Teachers Day Workshop
Los Angeles, CA
March 22, 2005
Dr. Larry Woolf
www.sci-ed-ga.org (click on Presentations to see all these slides)
Discuss possible ways to do this with your group
Generally: 2 resistors in parallel
R = L/(Wt) = L/A (A=Wt) is called the electrical resistivity(t is thickness)
Resistivity and resistors-in-series relationship 2 resistors in parallelR = L/AIf L = L1 + L2R = (L1 + L2)/A = L1/A + L2/A = R1 + R2
Resistivity and resistors-in-parallel relationship 2 resistors in parallelR = L/AIf A = A1 + A2R = L/ (A1 + A2) 1/R = (A1 + A2)/ L1/R = A1/ L + A2/ L 1/R = 1/R1 + 1/R2
Resistor with resistance R
Measurements confirm constant I in the resistor.
Therefore charges in wire move with constant velocity.
But charges are subject to F=ma=qE=qV/L, so they should accelerate, not move with constant velocity!
Charges do not move freely from one end of the resistor to the other – they have lots of collisions, on average every time .
Vfinal ~ a
Therefore, charges move along the resistor with constant average “drift velocity - vD” that is proportional to the acceleration. (vD = a , not ½ a ; see references for details)
Electrical/Mechanical Analogy electrons in a resistor
Pegboard with Pegs electrons in a resistor