Question 209297
The resistance of a wire varies directly with its length and inversely with the square of its diameter. If 100 feet of wire with diameter 0.01 inch has a resistance of 80 ohms, what is the resistance of 90 feet of the same type of wire if its diameter is 0.04 inch? 
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r = k*L/D^2
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Find "k":
80 = k*100/0.01^2
80 = k*100/0.0001
80 = k/1,000,000
k = 80,000,000
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Now you know the equation is 
r = (80,000,000)L/D^2
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what is the resistance of 90 feet of the same type of wire if its diameter is 0.04 inch? 
r = (80,000,000)90/(0.04)^2
r = 4.5x10^12 ohms
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Cheers,
Stan H.