1.

The packing efficiency of the two dimensional square unit cell show in the adjoining fig ,. Is (##PR_CHE_V01_XII_C01_E20_009_Q01.png" width="80%">

Answer»

0.3927
0.6802
0.7405
0.7854

Solution :In the two - dimensional packing , packing FRACTION
Area occupied by circles
` (" within the square")/("Area of the square") `
` ( 2 xx pir^(2))/a^(2) `
AC = `SQRT(AB^(2) + BC^(2)) = sqrt(a^(2) + a^(2)) = sqr2 a `
but AC = 4 r( r= radius of each sphere)
` sqrt2 a= 4R or a = 4/sqrt2 r = 2 sqrt2r`
packing fraction
` ( 2XX pi r^(2))/((2 sqrt2 r)^(2))= pi/4 = ( 3.143)/4 = 0.785i.e, 78.5%`


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