UPSC Physics Optional PYQs (CSE & IFoS)PYQs

Write down the Maxwell-Boltzmann law for the distribution of speeds c of molecules in a gas. Show the distribution graphically for the temperatures T and 2T; also write down the expression for average value of c^3.

Calculate the mean free path of molecules of \text{H}_2 gas at 20^\circ\text{ C} at atmospheric pressure. Assume the molecular diameter to be 2.00 \times 10^{-10}\text{ m}.

Explain, using the kinetic modal, why peaks in the curves showing Maxwell distribution of molecular speeds move towards higher speed at higher temperature.

How much faster does a cup of coffee cool when it is at 100^\circ\text{ C} than when it is at 30^\circ\text{ C}, if room temperature is 20^\circ\text{ C}?

Obtain the Clausius-Clapeyron equation. \frac{dp}{dt} = \frac{L}{T(v_2 - v_1)} where the symbols have their usual meanings

Use the Maxwell relation \left(\frac{\partial s}{\partial v}\right)_T = \left(\frac{\partial p}{\partial T}\right)_v to show that the internal energy, U, of a thermodynamic system depends on the volume according to the expression \left(\frac{\partial U}{\partial v}\right)_T = T\left(\frac{\partial p}{\partial T}\right)_v - p where the symbols have their usual meanings. Use this result to derive Stefan's law for a black-body radiator.

1 kg of ice at 0^\circ\text{ C} is melted and converted to water at 0^\circ\text{ C}. Compute the change in entropy.

Write a short note on Thermodynamic potentials.

A volume of one gm-mole of an ideal gas expands isothermally to four times its initial volume. Calculate the change in its entropy in terms of gas constant.

Write a short note on Production of low temperature using adiabatic demagnetisation.

Find the temperature at which root mean square velocity of nitrogen molecules in earth’s atmosphere equals the velocity of escape from the earth’s gravitational field. Mass of N_2 atom = 23.24 \times 10^{24}\text{ g}. Mean radius of earth = 6370\text{ km}.

Calculate the mean free path of helium atoms at NTP, the coefficient of viscosity being 190 \times 10^{-7}\text{ kg m}^{-1}\text{ s}^{-1}. 1\text{ atmospheric pressure} = 0.076 \times 136 \times 10^3 \times 9.81\text{ Nm}^{-2}.

Write a short note on Negative temperature.

A Carnot’s engine is made to work between 0^\circ\text{C} and -200^\circ\text{C}. Calculate us efficiency. Derive the expression you use of calculation.

A gas possesses a Maxwellian velocity distribution function. Show that the fraction of molecules in a given volume that possess a velocity (+v_x) in one direction only and whose magnitude is greater than some selected value v_o is \int_{v_o}^{\infty} f(V_x) dv_x = \frac{1}{2} \left[ 1 - \frac{1}{\sqrt{\pi}} \text{erf} \left( \frac{1/4 m v_o^2}{KT} \right)^{1/2} \right] Symbols have their usual meanings.

Mention some experiment which establishes that electrons in metals obey Fermi-Dirac statistics and justify your answer in some detail.

Starting from van der Waals equation for a gas, deduce expressions for its critical temperature, critical pressure and critical volume.

Write a note on Production of temperature below 1 K.

Find the density of hydrogen gas at temperature of 25^\circ\text{C} and pressure of 750\text{ mm Hg}.

Derive the expression for the energy distribution of particles according to Bose-Einstein statistics.