Ch 4Thermodynamics — Class 12 Physics, concept-first.

Thermodynamics is the branch of physics that studies heat, temperature, and the ways in which heat can be converted into other forms of energy — and other forms of energy converted into heat.

Thermal equilibrium is the condition two objects reach when heat stops flowing between them. Picture a piece of ice dropped into a glass of water at room temperature: heat flows from the warmer water…

The Zeroth Law of Thermodynamics states: if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.

So far we have only discussed thermal equilibrium — objects settling to a common temperature. But this alone does not explain everyday observations like why rubbing your palms together makes them warm…

Every physical system — however large or small — is made up of a huge number of molecules, and those molecules are always in some form of random, disordered motion (and, in liquids and solids, are als…

Before internal energy changes can be discussed quantitatively, two pieces of vocabulary need to be pinned down precisely: what exactly counts as a 'system', and what counts as a 'process'.

With the sign convention for energy transfer now fixed, we can define heat precisely. Consider a glass of water on a table (the system), at temperature , with the table and the rest of the room as its…

Now consider a concrete thermodynamic system: gas confined inside a cylinder fitted with a movable, massless, frictionless piston.

During the mid-nineteenth century, James Joule demonstrated experimentally that mechanical work and the heat produced by that work are equivalent quantities — related by , where is called the mechanic…

Beyond simple temperature change, thermodynamics is really about tracking how a whole set of a system's measurable properties change together as it gains or loses energy.

A property, or equivalently a state variable, of a thermodynamic system is any measurable or observable characteristic of the system while it remains in equilibrium — pressure, volume, temperature, de…

Every equilibrium state of a thermodynamic system is completely and uniquely described by specific values of its macroscopic state variables.

Recall the work-done integral from Section 4.5, (Eq. 4.2). This integral can only be evaluated once the specific relationship between and — that is, the path — connecting the initial and final volumes…

A thermodynamic process is the procedure by which a system's initial state is changed into a different, final state. During the process, heat may be transferred into the system (positive heat, e.g.

Consider a system that starts at state , with coordinates on a p-V diagram, and ends at state , with coordinates , where and .

Just as the work done depends on the path taken, so does the heat transferred to a system — even between the same two endpoint states.

With the general groundwork of Sections 4.7.1 and 4.7.2 in place, thermodynamic processes are now classified into named categories, based on which state variable is held fixed.

Consider two objects at different temperatures brought into thermal contact: heat flows from the hotter object to the colder one until they reach a common temperature (Section 4.2).

An isothermal process is one in which a system's pressure and volume change while its temperature is held exactly constant, so .

An isobaric process is one that occurs at constant pressure, so . Water boiling at a fixed (typically atmospheric) pressure is the standard everyday example.

An isochoric process (also called an isovolumic process) occurs at constant volume, so . Two familiar examples are heating a gas that is sealed inside a rigid, fixed-volume container, and the diffusio…

An adiabatic process is one in which there is absolutely no transfer of heat between the system and its environment: throughout.

A cyclic process is one that eventually returns a system to its original starting state — the initial state and the final state of the overall process are identical.

Free expansion was already introduced concretely in Section 4.7.2 (Fig. 4.13(b)) as one of two routes to the same final volume; here it is named formally as its own category within the classification…

A heat engine is a practical machine, built around a repeating cyclic process, that converts part of an absorbed quantity of heat into useful mechanical work.

Every heat engine, regardless of its specific design, shares three essential elements:

A heat engine's repeating cycle is a well-defined sequence of operations and, being a cyclic process, can be represented on a p-V diagram exactly as described generally in Section 4.7.3.6.

So far, every heat engine discussed has taken heat from a hot source and rejected some of it to a cold sink, converting the rest into useful mechanical work — heat flowing 'downhill' (from hot to cold…

As will be stated formally in Section 4.10 (the Clausius statement of the Second Law), heat can never flow spontaneously, on its own, from a region of lower temperature to a region of higher temperatu…

Refrigeration is the process of cooling a space, or a substance, and/or maintaining its temperature below the temperature of its surroundings — in short, refrigeration is simply artificial cooling.

Applying the First Law to a refrigerator's repeating cycle is exactly the same as applying it to a heat engine's cycle, since a refrigerator IS just a heat engine run backward: over one full cycle, ,…

An air conditioner works on exactly the same principle as a refrigerator (Section 4.9.2) — the only real difference is the scale of the space being cooled: a refrigerator cools a small, enclosed chamb…

A heat pump is a device that works like a refrigerator, but 'operating inside out' — instead of being used to cool a small chamber (like a refrigerator) or a room (like an air conditioner), it is used…

The First Law of Thermodynamics is an extremely powerful bookkeeping tool — it insists that energy is always conserved in every process — but, as the next subsection shows, it leaves two important, ve…

The First Law of Thermodynamics tells us that heat can be converted into work, and work can be converted into heat — it is, at its core, simply a quantitative statement of the equivalence between heat…

The Second Law of Thermodynamics is a general principle that puts firm constraints both on the direction in which heat can spontaneously flow, and on the maximum possible efficiency any heat engine ca…

Sections 4.7.3.1 and 4.10 have already established that reversibility is a special, idealised property that real processes only approximate, and that the Second Law caps how efficient any real heat en…

Reversibility (Section 4.7.3.1) is central to the Carnot cycle's theoretical importance, so it is worth restating precisely why.

Converting mechanical work into heat (as happens, for example, in a refrigerator, Section 4.9.2) is an inherently irreversible process.

Because every individual step of the Carnot cycle is, by construction, reversible (Section 4.11.2), the ENTIRE Carnot cycle — run as a whole — is also reversible.

Two central theorems connect the Carnot cycle directly back to the Second Law of Thermodynamics, and both can be stated together in a single, classic sentence: 'The Carnot engine is the most efficient…

The Sterling cycle (more commonly spelled Stirling cycle) is a further example of a closed thermodynamic cycle, distinct from both the ordinary heat-engine cycle of Section 4.8.2 and the Carnot cycle…

The chapter-end exercises of the Balbharati Physics Std-XII textbook for Thermodynamics, in the book's own printed order and grouping: five multiple-choice questions, five short 'answer in brief' item…

Sample papers and previous-year board questions for this subject.