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→First Law of Thermodynamics: more to come real soon.
Scientists had mostly figured out a linear relationship between temperature rise and change in heat energy. This is called the ''specific heat'', in joules per degree of temperature rise per gram of substance. For example, the specific heat of water is 4.2 joules per gram per degree of temperature rise.
Scientists had mostly figured out a linear relationship between temperature rise and change in heat energy. This is called the ''specific heat'', in joules per degree of temperature rise per gram of substance. For example, the specific heat of water is 4.2 joules per gram per degree of temperature rise.
Heat engines (that is, steam engines) were being used on an industrial scale, but scientists still didn't know what temperature really meant. And they didn't know why it only flows "downhill". Also, while other forms of energy (as in running electricity through a resistor) could be converted to heat with essentially perfect efficiency, converting the other way (as in a steam engine) was very inefficient.
Heat engines (that is, steam engines) were being used on an industrial scale by then, but scientists still didn't know what temperature really meant. And they didn't know why it only flows "downhill". Also, while other forms of energy (as in running electricity through a resistor) could be converted to heat with essentially perfect efficiency, converting the other way (as in a steam engine) was very inefficient. No one knew why.
The clues that unraveled this mystery came from the study of gases, which had been going on for some time before. [[Boyle's Law]], formulated in the 1660's, stated that, for a given sample of gas at a fixed temperature, the pressure was inversely proportional to the volume. That is,
:<math>PV = constant</math>
[[Charles' Law]], formulated in the 1780's, stated that, for a given sample of gas at a fixed pressure, the volume was directly proportional the the "absolute" temperature. That is,
:<math>V/T = constant</math>
This required that the temperature scale be modified. The necessary scale was known as absolute temperature, now known as [[International System of Units#Kelvin|Kelvin scale]]. All thermodynamic measurements are in Kelvins.
Putting these together, we get, for a given sample of gas
:<math>PV = ZT</math>
Where Z is some constant that is characteristic of the gas sample. It's easy to see that Z is proportional to the amount of gas (2 grams of gas will have twice the volume of 1 gram). So Z is actually the amount of gas, measured in some convenient units (grams, moles, molecules), time some number that is characteristic of the gas.
In the 1810's, Gay-Lussac and Avogadro made an amazing discovery: The mysterious constant is just the molecular weight of the gas.
==[[Second Law of Thermodynamics]]==
==[[Second Law of Thermodynamics]]==