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where r came from?

  • #1
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Thread moved from the technical forums, so no Homework Template is shown.
Summary:: I have a question that I don't understand but you may know where r was come from.

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I mentioned about my question on the addition.
 

Answers and Replies

  • #2
kuruman
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Lower case ##r## is radial distance from the axis. Upper case ##R## is resistance. Two different quantities. Besides, you can't have ##dR## on both sides of the equation you are questioning.
 
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  • #3
BvU
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  • #4
BvU
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##R = {\rho\;l\over A} ## is for a flat sheet with thickness ##l##.
Here ##dR = {\rho \, dr \over A}## is for a tube with radius ##r## and thickness ##dr##. Its area ##A## is ##2\pi Lr##
 
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  • #5
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Okay I get it now. Thanks both of you for attention :)
 

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