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Units and Measurements question

2016 · 9 Apr · Shift 1 · Q66
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Units and Measurements question

2016 · 9 Apr · Shift 1 · Q66

JEE MainPhysicsUnits and MeasurementsMCQ+4 / −1
In the following ‘I’ refers to current and other symbols have their usual meaning. Choose the option that corresponds to the dimensions of electrical conductivity :
  1. A
    ML −-− 3 T −-− 3 I2
  2. B
    M −-− 1 L3 T3 I
  3. C
    M −-− 1 L −-− 3 T3 I2
  4. D
    M −-− 1 L −-− 3 T3 I
View written solutionFree

Correct answer: C

  1. Use the relation between conductivity and resistivity

Electrical conductivity σ\sigmaσ is the reciprocal of resistivity ρ\rhoρ:

σ=1ρ\sigma = \frac{1}{\rho}σ=ρ1​

So we first find the dimensions of resistivity.

  1. Dimensions of resistance

From Ohm’s law,

V=IR⇒R=VIV = IR \Rightarrow R = \frac{V}{I}V=IR⇒R=IV​

Now,

V=workchargeV = \frac{\text{work}}{\text{charge}}V=chargework​

Dimensions of work:

[W]=ML2T−2[W] = ML^2T^{-2}[W]=ML2T−2

Dimensions of charge:

[Q]=IT[Q] = IT[Q]=IT

Hence dimensions of potential difference:

[V]=ML2T−2IT=ML2T−3I−1[V] = \frac{ML^2T^{-2}}{IT} = ML^2T^{-3}I^{-1}[V]=ITML2T−2​=ML2T−3I−1

Therefore,

[R]=[V][I]=ML2T−3I−2[R] = \frac{[V]}{[I]} = ML^2T^{-3}I^{-2}[R]=[I][V]​=ML2T−3I−2

  1. Dimensions of resistivity

Using

R=ρlA⇒ρ=RAlR = \rho \frac{l}{A} \Rightarrow \rho = R\frac{A}{l}R=ρAl​⇒ρ=RlA​

where area AAA has dimensions L2L^2L2 and length lll has dimensions LLL.

So,

[ρ]=[R]⋅L2L=[R]⋅L[\rho] = [R]\cdot \frac{L^2}{L} = [R]\cdot L[ρ]=[R]⋅LL2​=[R]⋅L

[ρ]=ML2T−3I−2⋅L=ML3T−3I−2[\rho] = ML^2T^{-3}I^{-2} \cdot L = ML^3T^{-3}I^{-2}[ρ]=ML2T−3I−2⋅L=ML3T−3I−2

  1. Dimensions of conductivity

Since

σ=1ρ\sigma = \frac{1}{\rho}σ=ρ1​

we get

[σ]=M−1L−3T3I2[\sigma] = M^{-1}L^{-3}T^3I^2[σ]=M−1L−3T3I2

  1. Match with the options

The required dimensions are:

M−1L−3T3I2M^{-1}L^{-3}T^3I^2M−1L−3T3I2

This matches Option C.

  1. Verification with stored answer

Stored correct answer: C

Derived answer: C

So the derived answer agrees with the stored answer.

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