The answer is 9 : 4. But why, how to calculate it ?? thx for answering.
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Electricity about resistance
2007-02-25 3:40 am
Two copper wires, A and B, of the same length have the ratio of mass of 4 : 9. Assuming the cross-sectional are uniform, what is the ratio of the resistance of A to that of B ?
The answer is 9 : 4. But why, how to calculate it ?? thx for answering.
The answer is 9 : 4. But why, how to calculate it ?? thx for answering.
回答 (1)
2007-02-25 9:05 am
✔ 最佳答案
Let Ma, Mb be the masses of wire A and B respectivelyAa and Ab be their respective cross-sectional areas
and L be their lengths
Thus resistance of wire A, Ra = pL/Aa
where p is the resistivity of copper
resistance of wire B, Rb = pL/Ab
Ra/Rb = [pL/Aa]/[pL/Ab] = Ab/Aa --------------------- (1)
But Ma = density x volume = d.(Aa.L) (where d is the density of copper)
and Mb = d.Ab.L
thus Ma/Mb = Aa/Ab = 4/9
substitute into (1),
Ra/Rb = 9/4
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