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JEE Advanced 2024 · Units and Measurements

A dimensionless quantity is constructed in terms of electronic charge ee, permittivity of free space ε0\varepsilon_0, Planck's constant hh, and speed of light cc. If the dimensionless quantity is written as eαε0βhγcδe^{\alpha}\varepsilon_0^{\beta}h^{\gamma}c^{\delta} and nn is a non-zero integer, then (α,β,γ,δ)(\alpha, \beta, \gamma, \delta) is given by

Answer: A. (2n,−n,−n,−n)(2n, -n, -n, -n)

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Written solution

Step 1: Write down the dimensions of each physical quantity.

The dimensions of electronic charge ee, speed of light cc, Planck's constant hh, and permittivity of free space ε0\varepsilon_0 are given by:

[e]=A T[e] = \text{A}\,\text{T} [c]=L T−1[c] = \text{L}\,\text{T}^{-1} [h]=M L2 T−1[h] = \text{M}\,\text{L}^{2}\,\text{T}^{-1} [ε0]=M−1 L−3 T4 A2[\varepsilon_0] = \text{M}^{-1}\,\text{L}^{-3}\,\text{T}^{4}\,\text{A}^{2}

Step 2: Set up the dimensionless condition.

The quantity eαε0βhγcδe^{\alpha}\varepsilon_0^{\beta}h^{\gamma}c^{\delta} must be dimensionless.

[eαε0βhγcδ]=M0 L0 T0 A0[e^{\alpha}\varepsilon_0^{\beta}h^{\gamma}c^{\delta}] = \text{M}^{0}\,\text{L}^{0}\,\text{T}^{0}\,\text{A}^{0}

Substitute the dimensions of each quantity into the expression.

(A T)α(M−1 L−3 T4 A2)β(M L2 T−1)γ(L T−1)δ=M0 L0 T0 A0(\text{A}\,\text{T})^{\alpha} (\text{M}^{-1}\,\text{L}^{-3}\,\text{T}^{4}\,\text{A}^{2})^{\beta} (\text{M}\,\text{L}^{2}\,\text{T}^{-1})^{\gamma} (\text{L}\,\text{T}^{-1})^{\delta} = \text{M}^{0}\,\text{L}^{0}\,\text{T}^{0}\,\text{A}^{0}

Step 3: Equate the exponents of each base dimension.

Equating the powers of M\text{M}, L\text{L}, T\text{T}, and A\text{A} on both sides yields a system of linear equations.

For A\text{A}:

α+2β=0\alpha + 2\beta = 0

For M\text{M}:

−β+γ=0-\beta + \gamma = 0

For L\text{L}:

−3β+2γ+δ=0-3\beta + 2\gamma + \delta = 0

For T\text{T}:

α+4β−γ−δ=0\alpha + 4\beta - \gamma - \delta = 0

Step 4: Solve the system of equations.

Express α\alpha, γ\gamma, and δ\delta in terms of β\beta.

From the equation for A\text{A}:

α=−2β\alpha = -2\beta

From the equation for M\text{M}:

γ=β\gamma = \beta

Substitute γ=β\gamma = \beta into the equation for L\text{L}:

−3β+2β+δ=0-3\beta + 2\beta + \delta = 0 δ=β\delta = \beta

Verify these relations in the equation for T\text{T}.

−2β+4β−β−β=0-2\beta + 4\beta - \beta - \beta = 0 0=00 = 0

Step 5: Express the solution in terms of the non-zero integer nn.

Let β=−n\beta = -n, where nn is a non-zero integer.

α=2n\alpha = 2n β=−n\beta = -n γ=−n\gamma = -n δ=−n\delta = -n

This gives the ordered quadruple.

(α,β,γ,δ)=(2n,−n,−n,−n)(\alpha, \beta, \gamma, \delta) = (2n, -n, -n, -n)

Answer: (α,β,γ,δ)=(2n,−n,−n,−n)(\alpha, \beta, \gamma, \delta) = (2n, -n, -n, -n)

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