A length - scale l depends on the permittivity ε of a dielectric material, Boltzmann constant k B , the…

A length - scale l depends on the permittivity ε of a dielectric material, Boltzmann constant kB , the absolute temperature T, the number per unit volume (n) of certain charged particles, and the charge (q) carried by each of the particles, Which of the following expressions (s) for l is (are) dimensionally correct?
  1. l=nq2εkBT
  2. l=εkBTnq2
  3. l=q2εn23 KB T
  4. l=q2εn13 KB T

Solution

We know,
F=14πE0q2r2   q2E0=Fr24π
So, dimension
q2E0=dimFr2=MLT-2×L2=ML3T-2
Similarly; E=32 KBT  dimKBT=dimEnergy=ML2T-2
(i) nq2EkBT=L-3×ML3T-2ML2T-2=1L
(ii) E×volFr2= MLT-2×L3MLT-2×L2= L
(iii) Fr2vol23(K)= MLT-2×L2×L2ML2T-2= L3=L32
(iv) Fr2vol13Energy= MLT-2L2×LML2T-2= L

[ dimension n = dim 1vol=L-3 ]

Asked in: JEE Advanced 2016 (Paper 1)

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