Why does thermodynamics feel like two different subjects in Physics and Chemistry?

5 answers & replies

• [+][–]CPCommons Physics DeskTeam· 1mo ago

  Think of Physics thermodynamics as a machine story: a gas in a cylinder does work on a piston, takes in heat, and you keep a ledger of the energy. Chemistry thermodynamics is a reaction story: what does a reaction do to energy and disorder? Same law, different cast.

  Kinetic theory is the bridge, because it tells you what internal energy is for an ideal gas: the motion energy of the molecules.

  Idea	Result
  Average KE per molecule	$\frac{3}{2}kT$
  Internal energy, $n$ moles, $f$ degrees of freedom	$U = \frac{f}{2}nRT$
  Molar heat capacities	$C_V = \frac{f}{2}R$, $\;C_P = C_V + R$
  Ratio	$\gamma = 1 + \frac{2}{f}$

  Monatomic gas: $f = 3$. Diatomic gas at ordinary temperatures (vibration not active): $f = 5$. For an ideal gas, $U$ depends only on temperature, so in an isothermal process $\Delta U = 0$ and $Q = W$.

  Learn them in this order: kinetic theory, then the first law, then the four processes (isothermal, isobaric, isochoric, adiabatic). In the adiabatic case $Q = 0$ and $PV^{\gamma} = \text{constant}$, so the work done by the gas comes entirely out of its internal energy, which is why an expanding gas cools.

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• [+][–]CCCommons Chemistry DeskTeam· 1mo ago

  The trap is the sign of work, and it is a trap because both conventions are legitimate.

  • Physics: $W$ is work done by the gas. Expansion gives positive $W$. First law: $Q = \Delta U + W$.
  • Chemistry (current NCERT/IUPAC style): $w$ is work done on the system. Expansion gives negative $w$. First law: $\Delta U = q + w$.

  Take the same ideal gas expanding isothermally and reversibly. $\Delta U = 0$. Physics: $Q = W$, both positive. Chemistry: $q = -w$, with $q$ positive and $w$ negative. One physical event, two sign sets.

  A habit that saves marks: before touching a number, write the convention in the margin ("work BY gas" or "work ON system"), based on the subject the question sits in and the wording it uses.

  Then the Chemistry-only layer. Enthalpy is $H = U + pV$, so at constant pressure $\Delta H = q_p$, and for reactions involving gases $\Delta H = \Delta U + \Delta n_g RT$. On top of that comes entropy and

  $\Delta G = \Delta H - T\Delta S$

  where the sign of $\Delta G$ decides spontaneity at a given temperature. Physics seldom asks for $\Delta G$; Chemistry chapters seldom build engine cycles. That is the real split.

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  • [+][–]CPCommons Physics DeskTeam· 1mo ago

    One link worth making: at constant pressure, the Physics heat $Q = nC_P\Delta T$ is the very quantity Chemistry calls $\Delta H$ (when only pressure-volume work is involved). So the heat-capacity table above and the enthalpy chapter describe the same energy with different names.

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• [+][–]CMCommons Mock & PYQ DeskTeam· 1mo ago

  Two question styles keep appearing in mocks, and they punish different habits.

  Quick conceptual or ratio questions. "Ratio of rms speeds of two gases at the same temperature" is a classic. From $v_{rms} = \sqrt{3RT/M}$, the lighter gas is faster and $v_1/v_2 = \sqrt{M_2/M_1}$. "$\gamma$ for a diatomic gas" is the same type: if the table above is in your head, these take seconds.

  P-V graph questions. Area under the curve is work done by the gas. A closed loop's area is the net work. Clockwise loops on a P-V diagram mean net work is done by the gas (engine-like); anticlockwise means work is done on it. A common slip is to read the graph before identifying which process each segment is.

  For mock analysis, give each thermodynamics mistake one of three labels: sign, formula, or graph reading. After ten or so mistakes one label usually dominates, and then you know exactly what to revise instead of re-reading the whole chapter.

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• [+][–]CSCommons Strategy DeskTeam· 1mo ago

  Do not study the two versions as separate islands. An order many students settle on:

  1. Kinetic theory of gases first, since it explains what internal energy is.
  2. Physics thermodynamics: first law, the four processes, and the second law with reversible and irreversible processes. (The Carnot engine has been dropped from the rationalised NCERT text, so check the syllabus notice before spending time on it.)
  3. Some weeks later, Chemistry thermodynamics, with a one-page translation sheet next to your notes: sign of work, $U$ versus $H$, and what $\Delta G$ means.

  The translation sheet is two columns, Physics and Chemistry, describing the same events with matching signs. Rewrite it from memory once a week for a month. It takes about five minutes and it is the cheapest insurance against the sign flip.

  One caveat: topic lists get revised. Check the current NTA syllabus notice to see exactly what is in scope for each subject before you spend a week on a sub-topic.

  If you have your own trick for keeping the two sign conventions apart, post it below. Someone reading this thread tonight is hunting for exactly that.

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