arthham

JEE Advanced 1998 · Units and Measurements

Let [ε0][\varepsilon_0] denote the dimensional formula of the permittivity of the vacuum, and [μ0][\mu_0] that of the permeability of the vacuum. If M=massM = \text{mass}, L=lengthL = \text{length}, T=timeT = \text{time} and I=electric currentI = \text{electric current},

Answers: B. [ε0]=M−1L−3T4I2[\varepsilon_0] = M^{-1} L^{-3} T^{4} I^{2} · C. [μ0]=MLT−2I−2[\mu_0] = M L T^{-2} I^{-2}

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Written solution

Step 1: Identify the given quantities and their dimensions.

The fundamental dimensions are mass MM, length LL, time TT, and electric current II.

Step 2: Determine the dimensional formula of [ε0][\varepsilon_0].

Use Coulomb's law for the electrostatic force between two charges.

F=14πε0q1q2r2F = \frac{1}{4\pi\varepsilon_0}\frac{q_1 q_2}{r^2}

Rearrange the equation to express ε0\varepsilon_0 in terms of the other variables.

ε0=q1q24πFr2\varepsilon_0 = \frac{q_1 q_2}{4\pi F r^2}

Express the dimensions of charge qq, force FF, and distance rr.

[q]=IT[q] = I T [F]=MLT−2[F] = M L T^{-2} [r]=L[r] = L

Substitute these dimensions into the expression for [ε0][\varepsilon_0].

[ε0]=(IT)2(MLT−2)(L)2[\varepsilon_0] = \frac{(I T)^2}{(M L T^{-2})(L)^2} [ε0]=M−1L−3T4I2[\varepsilon_0] = M^{-1}L^{-3}T^{4}I^{2}

Step 3: Determine the dimensional formula of [μ0][\mu_0].

Use the formula for the magnetic force per unit length between two parallel current-carrying conductors.

FL=μ0I1I22πd\frac{F}{L} = \frac{\mu_0 I_1 I_2}{2\pi d}

Rearrange the equation to express μ0\mu_0.

μ0=2πdFLI1I2\mu_0 = \frac{2\pi d F}{L I_1 I_2}

Since dd and LL both represent length, their dimensions cancel out.

[μ0]=[F][I]2[\mu_0] = \frac{[F]}{[I]^2}

Substitute the dimensions of force and current.

[μ0]=MLT−2I2[\mu_0] = \frac{M L T^{-2}}{I^2} [μ0]=MLT−2I−2[\mu_0] = M L T^{-2}I^{-2}

Step 4: Verify the relationship using the speed of light.

The speed of light in vacuum cc is related to ε0\varepsilon_0 and μ0\mu_0 by c=1μ0ε0c = \frac{1}{\sqrt{\mu_0 \varepsilon_0}}.

[ε0][μ0]=(M−1L−3T4I2)(MLT−2I−2)[\varepsilon_0][\mu_0] = \left(M^{-1}L^{-3}T^{4}I^{2}\right)\left(M L T^{-2}I^{-2}\right) [ε0][μ0]=L−2T2[\varepsilon_0][\mu_0] = L^{-2}T^{2}

This matches the dimension of [c−2][c^{-2}].

Answer: [ε0]=M−1L−3T4I2[\varepsilon_0] = M^{-1}L^{-3}T^{4}I^{2} and [μ0]=MLT−2I−2[\mu_0] = M L T^{-2}I^{-2}

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