11.4 Heat, internal energy and which in turn rotate the wheels of the train. work In physics, we need to define the notions of heat, 11.5 First law of temperature, work, etc. more carefully. Historically, it took a thermodynamics long time to arrive at the proper concept of ‘heat’. Before the 11.6 Specific heat capacity modern picture, heat was regarded as a fine invisible fluid 11.7 Thermodynamic state filling in the pores of a substance. On contact between a hot variables and equation of body and a cold body, the fluid (called caloric) flowed from state the colder to the hotter body ! This is similar to what happens 11.8 Thermodynamic processes when a horizontal pipe connects two tanks containing water 11.9 Second law of up to different heights. The flow continues until the levels of thermodynamics water in the two tanks are the same. Likewise, in the ‘caloric’ 11.10 Reversible and irreversible picture of heat, heat flows until the ‘caloric levels’ (i.e., the processes temperatures) equalise. 11.11 Carnot engine In time, the picture of heat as a fluid was discarded in favour of the modern concept of heat as a form of energy. An Summary important experiment in this connection was due to Benjamin Points to ponder Thomson (also known as Count Rumford) in 1798. He Exercises observed that boring of a brass cannon generated a lot of heat, indeed enough to boil water. More significantly, the amount of heat produced depended on the work done (by the horses employed for turning the drill) but not on the sharpness of the drill. In the caloric picture, a sharper drill would scoop out more heat fluid from the pores; but this was not observed. A most natural explanation of the observations was that heat was a form of energy and the experiment demonstrated conversion of energy from one form to another–from work to heat. Reprint 2025-26 THERMODYNAMICS 227 Thermodynamics is the branch of physics that in a different context : we say the state of a system deals with the concepts of heat and temperature is an equilibrium state if the macroscopic and the inter-conversion of heat and other forms variables that characterise the system do not of energy. Thermodynamics is a macroscopic change in time. For example, a gas inside a closed science. It deals with bulk systems and does not rigid container, completely insulated from its go into the molecular constitution of matter. In surroundings, with fixed values of pressure, fact, its concepts and laws were formulated in the volume, temperature, mass and composition that nineteenth century before the molecular picture do not change with time, is in a state of of matter was firmly established. Thermodynamic thermodynamic equilibrium. description involves relatively few macroscopic variables of the system, which are suggested by common sense and can be usually measured directly. A microscopic description of a gas, for example, would involve specifying the co-ordinates and velocities of the huge number of molecules constituting the gas. The description in kinetic theory of gases is not so detailed but it does involve molecular distribution of velocities. Thermodynamic description of a gas, on the other (a) hand, avoids the molecular description altogether. Instead, the state of a gas in thermodynamics is specified by macroscopic variables such as pressure, volume, temperature, mass and composition that are felt by our sense perceptions and are measurable*. The distinction between mechanics and thermodynamics is worth bearing in mind. In mechanics, our interest is in the motion of particles (b) or bodies under the action of forces and torques. Fig. 11.1 (a) Systems A and B (two gases) separated Thermodynamics is not concerned with the by an adiabatic wall – an insulating wall motion of the system as a whole. It is concerned that does not allow flow of heat. (b) The with the internal macroscopic state of the body. same systems A and B separated by a When a bullet is fired from a gun, what changes diathermic wall – a conducting wall that is the mechanical state of the bullet (its kinetic allows heat to flow from one to another. In this case, thermal equilibrium is attainedenergy, in particular), not its temperature. When in due course. the bullet pierces a wood and stops, the kinetic energy of the bullet gets converted into heat, In general, whether or not a system is in a state changing the temperature of the bullet and the of equilibrium depends on the surroundings and surrounding layers of wood. Temperature is the nature of the wall that separates the system related to the energy of the internal (disordered) from the surroundings. Consider two gases A and motion of the bullet, not to the motion of the bullet B occupying two different containers. We know as a whole. experimentally that pressure and volume of a given mass of gas can be chosen to be its two

3.1 The storage battery of a car has an emf of 12 V. If the internal resistance of the battery is 0.4 Ω, what is the maximum current that can be drawn from the battery?

11.3 ZEROTH LAW OF THERMODYNAMICS (a) Imagine two systems A and B, separated by an adiabatic wall, while each is in contact with a third system C, via a conducting wall [Fig. 11.2(a)]. The states of the systems (i.e., their macroscopic variables) will change until both A and B come to thermal equilibrium with C. After this is achieved, suppose that the adiabatic wall between A and B is replaced by a conducting wall and C is insulated from A and B by an adiabatic wall [Fig.11.2(b)]. It is found that the states of A and B change no (b) further i.e. they are found to be in thermal Fig. 11.2 (a) Systems A and B are separated by an equilibrium with each other. This observation adiabatic wall, while each is in contact forms the basis of the Zeroth Law of with a third system C via a conducting Thermodynamics, which states that ‘two wall. (b) The adiabatic wall between A systems in thermal equilibrium with a third and B is replaced by a conducting wall, system separately are in thermal equilibrium while C is insulated from A and B by an adiabatic wall.with each other’. R.H. Fowler formulated this * Both the variables need not change. It depends on the constraints. For instance, if the gases are in containers of fixed volume, only the pressures of the gases would change to achieve thermal equilibrium. Reprint 2025-26 THERMODYNAMICS 229