The Nature of Heat

CSEC Physics Syllabus - Effective for examinations from May - June 2015
Section B - Thermal Physics and Kinetic Theory

Nature of Heat

Specific Objective 1.1

differentiate between the caloric and kinetic theories of heat as they existed in the eighteenth century;

Specific Objective 1.2

discuss the role of Joule's experiments in establishing the principle of conservation of energy;

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Eighteenth Century Caloric Theory of Heat

In the eighteenth century, heat was considered an invisible substance called caloric. When an object was heated, it was considered to have gained caloric. Although, there was no experimental evidence to show that a heated object increased its mass and a colder object decreased its mass. This should have been the case if caloric was a real substance with weight. 

The Caloric Theory 

• Heat is a fluid called caloric.
• Caloric flows from a hot object to a cold object when they are in contact. This could be proved because the hot object gets cooler and the cold object gets hotter. 
• Friction creates small particles caused by the rubbing process and these particles produce more caloric. 

Eighteenth Century Kinetic Theory of Heat

Count Rumford

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Benjamin Thompson was born in North America and fled to Europe where, in recognition of his scientific work, he was made Count Rumford by the Bavarians. In 1798, he made an observation that could not be explained by the Caloric Theory of Heat. While supervising the boring of a gun barrel at a factory in Bavaria when he noticed that a lot of heat was generated while only a small quantity of brass chips were removed from the barrel. He inferred that it was unlikely that all the heat (or caloric) could have been stored in such a small mass of brass chips. 

He demonstrated and further inferred:

1. The longer the boring took place, the greater was the amount of heat produced
2. When the barrel was bored in a tank of water, the water came to a boil even though no flame was used
3. Heat was created when mechanical work was done against friction and that heat is related to mechanical energy
4. Heat could be created and therefore it could not be a material substance.

The Kinetic Theory

• All atoms have some form of motion.
• Heat is the kinetic energy of moving atoms.
• The heat produced by friction is explained as the energy supplied when mechanical work is done.

Sir Humphry Davy

In 1799, Sir Humphry Davy showed that when two pieces of ice below freezing point were rubbed together, they melted and produced water at +2 °C. This could only be explained by saying that the work of rubbing had produced the heat. This evidence strongly supported the kinetic theory of heat. 

James Prescott Joule

In 1842, James Joule of Manchester, England began a series of important experiments, which proved that heat was not a material substance. He converted different forms of energy, such as mechanical and electrical energy, into heat. He measured the amount of energy expended or converted and the quantity of heat produced and found that they were always in a constant ratio. Joule realised that friction converted the energy required for work into energy in the form of heat and the whole process neither gained nor loss energy, in fact the total energy was always conserved. Joule concluded that heat is a form of energy.