The nature of heat

Theories of heat

(a)    The caloric theory of heat: This theory asserts that heat is a kind of matter – a hypothetical fluid called the caloric in order to explain the various thermal phenomena some unusual properties are assigned to this fluid. It is considered to be all pervading, indestructible and uncreatable, since it raises the temperature of a body to which it is supplied and lower that of a body from which it is removed. It is imagined to be very light, since a body does not gain in weight when heated. The fluid is again assumed to be highly elastic and is composed of particles which strongly repel each other. This assumption can satisfactorily explain the expansion of bodies when heated as well as the emission of heat and light by bodies in combustion.

Just as water at unequal heights in two vessels connected by a tube will flow from one to the other until the levels in the two vessels are equal, so the caloric may be regarded as following from the hot body to the cold one in contact till equilibrium is reached. Thus in this theory temperature may be regarded as the ‘level’ of the caloric. The generation of heat due to friction is compared to the oozing out of water from a sponge when squeezed; the caloric fluid when thus squeezed out manifests itself as heat. When a piece of metal is hammered it grows hot; this is because caloric is being hammered out of its pores. These facts show that caloric is a material fluid though weightless.

Dr. Black

An important part of the caloric theory was the doctrine of latent heat propounded by Dr. Black in 1760. When ice is contained in a vessel ad heat is communicated to it, there is no change in temperature till the whole of the ice is melted. This experimental observation presents a difficulty which was got round by supposing that caloric is able to enter into combination with matter and remain latent as it could not be detected by a thermometer. Thus water must be regarded as a compound of ice and caloric. Similarly, steam must be a compound of boiling water and caloric.

Rumford

(b)    The dynamical theory of heat: Serious doubts on the caloric theory of heat began to be cast towards the end of the 18^th century. Rumford in 1798 observed that a large amount of heat could be produced as a result of friction while boring the barrel of gun. The amount of heat, thus produced, did not depend on the quantity of metal scraped, but was proportional to the amount of motion lost by the boring machine. He also found that heat obtainable in the process appeared to be inexhaustible in quantity. This indicated an unlimited caloric content of the material which is highly improbable. Again, the granules of material, scooped out through the boring were found to have the same specific heat as that of the parent material. This fact was quite in opposition to the caloric theory which asserted that the generation of heat through friction should be due to the formation of a material having a lower heat capacity. Hence, Rumford rightly rejected the caloric theory and stated that heat is a kind of motion. Whenever motion disappears it reappears as heat and there is an exact proportionality between the two. He even made an estimate of the ratio between the work done by the friction and amount of heat thereby produced, which we now call the mechanical equivalent of heat. His value is close to the one now accepted as standard.

Humphry Davy

In 1799, Humphry Davy noted that water could be produced by rubbing two blocks of ice together. It is an admitted fact that water has greater capacity for heat than ice. But in accordance with the caloric theory, water produced by friction of ice should have a lower heat capacity than ice, which is, therefore, not true to facts. Hence the caloric theory became untenable. The correct notion is that heat is produced by the motion of material particles caused by friction.

Dr. Joule

But the works of Rumford and Davy and their valuable conclusions were soon forgotten, until, in 1840, Dr. Joule performed a series of elaborate experiment s following the suggestion of Rumford that there exists a relation of proportionality between the mechanical work done and the amount of heat thereby produced. Thus Joule established that heat and work are mutually convertible which is now known as the first law of thermodynamics.

Mechanical equivalent of heat

The exact relationship between mechanical work and heat was established by Dr. Joule. Whenever mechanical work is converted into heat or heat into mechanical work, one is equivalent to the other. This principle of equivalence is otherwise known as the first law of thermodynamics. If W is the quantity of work done, H the amount of heat produced, then, W=JH, where J is a content, known as the mechanical equivalent of heat or Joule’s equivalent in honor of joule who first determined its value.

Since W=J, when H=I, mechanical equivalent of heat is the work done to produce unit quantity of heat.