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BITSAT 2015
Exam
BITSAT
Year
2015
Subject
Physics
Chapter
Thermal Properties of Matter

BITSAT 2015 · Physics · Thermal Properties of Matter

0.5 mole of an ideal gas at constant temperature 27^ ^ C kept...

0.5 mole of an ideal gas at constant temperature 27∘C27^{{}^{\circ}} \mathrm{C} kept inside a cylinder of length LL and crosssection area A closed by a massless piston. 0.5 mole of an ideal gas at constant temperature 27^ ^ C kept inside a cylinder of length L and crosssection area A
The cylinder is attached with a conducting rod of length L,L, cross-section area (1/9)\mathrmm2(1 / 9) \mathrmm^{2} and thermal conductivity k\mathrm{k}, whose other end is maintained at 0∘C0^{{}^{\circ}} \mathrm{C}. If piston is moved such that rate of heat flow through the conducing rod is constant then velocity of piston when it is at height L/2L / 2 from the bottom of cylinder is : [Neglect any kind of heat loss from system

Options

  1. A(kR)m/sec\left(\frac{\mathrm{k}}{R}\right) \mathrm{m} / \mathrm{sec}
  2. B(k10R)m/sec\left(\frac{\mathrm{k}}{10 R}\right) \mathrm{m} / \mathrm{sec}
  3. C(k100R)m/sec\left(\frac{\mathrm{k}}{100 R}\right) \mathrm{m} / \mathrm{sec}
  4. D(k1000R)m/sec\left(\frac{\mathrm{k}}{1000 R}\right) \mathrm{m} / \mathrm{sec}

Correct answer

C. (k100R)m/sec\left(\frac{\mathrm{k}}{100 R}\right) \mathrm{m} / \mathrm{sec}

Step-by-step solution

ΔQΔt=ΔWΔt=\frac{\Delta Q}{\Delta t}=\frac{\Delta W}{\Delta t}= work done per unit time = kaθLdWdt=\frac{\mathrm{k} a \theta}{L}\frac{d W}{d t}= P dvdt=kaθL,\frac{d v}{d t}=\mathrm{k} \frac{a \theta}{L}, P=nRTV⇒0.5R(300)VA⋅dℓdt=kaθL⇒0.5R(300)A⋅L2\frac{n R T}{V}\Rightarrow \quad \frac{0.5 R(300)}{V} A \cdot \frac{d \ell}{d t}=\frac{\mathrm{k} a \theta}{L} ⇒ \frac{0.5 R(300)}{A \cdot \frac{L}{2}} A ⋅\cdot v=kaθL⇒v=kaR(27300)=k100R\frac{\mathrm{k} a \theta}{L}\Rightarrow \quad v=\frac{\mathrm{k} a}{R}\left(\frac{27}{300}\right)=\frac{\mathrm{k}}{100 R}
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