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JEE Advanced 2004 · Units and Measurements

Pressure depends on distance as, P=(α/β)exp⁡(−αz/(kθ))P = (\alpha/\beta)\exp(-\alpha z/(k\theta)), where α\alpha, β\beta are constants, zz is distance, kk is Boltzman's constant and θ\theta is temperature. The dimension of β\beta are

Answer: C. M0L2T0M^0 L^2 T^0

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Written solution

Step 1: Analyze the exponential term.

The argument of an exponential function must be dimensionless.

[αzkθ]=M0L0T0\left[\frac{\alpha z}{k\theta}\right] = M^0 L^0 T^0

Step 2: Find the dimensions of the known quantities in the exponent.

The dimension of distance zz is length, and the product of Boltzmann's constant kk and temperature θ\theta has the dimensions of energy.

[z]=L[z] = L [kθ]=ML2T−2[k\theta] = M L^2 T^{-2}

Step 3: Determine the dimensions of α\alpha.

Solve for the dimensions of α\alpha using the dimensionless condition of the exponent.

[α]=[kθ][z][\alpha] = \frac{[k\theta]}{[z]} [α]=ML2T−2L[\alpha] = \frac{M L^2 T^{-2}}{L} [α]=MLT−2[\alpha] = M L T^{-2}

Step 4: Determine the dimensions of pressure PP.

Pressure is defined as force per unit area.

[P]=ML−1T−2[P] = M L^{-1} T^{-2}

Step 5: Determine the dimensions of β\beta.

Since the exponential term is dimensionless, the pre-factor αβ\frac{\alpha}{\beta} must have the same dimensions as pressure PP.

[αβ]=[P]\left[\frac{\alpha}{\beta}\right] = [P] [β]=[α][P][\beta] = \frac{[\alpha]}{[P]} [β]=MLT−2ML−1T−2[\beta] = \frac{M L T^{-2}}{M L^{-1} T^{-2}} [β]=L2[\beta] = L^2 [β]=M0L2T0[\beta] = M^0 L^2 T^0

Answer: The dimensions of β\beta are M0L2T0M^0 L^2 T^0.

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