1.

Two wires having same length and material are stretched by same force. Their diameters are in the ratio 1:3. The ratio of strain energy per unit volume for these two wires (smaller to larger diameter) when stretched isA. `3 : 1`B. `9 : 1`C. `27 : 1`D. `81 : 1`

Answer» Correct Answer - b
According to question, energy in string per unit volume is given by `(1)/(2) xx` stress `xx` strain
i.e., `(U)/(V) = (1)/(2) xx (F)/(A) xx (Delta l)/(l)`
[Symbols have their usual meanings]
`= (1)/(2) xx V xx ("strain")^(2)`
`:. (U_(s))/(S_(L)) = (Y_(s))/(Y_(L)) xx ("Stress"(S))/("Strain"(L))^(2)`
`= (A_(L))/(A_(S)) = ((l)/(l))^(2)`
`implies (r_(L)^(2))/(r_(s)^(2)) = (3^(2))/(1^(2)) = 9 : 1`


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