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→First Law of Thermodynamics
Of course, in all experiments that attempt to tally energy (or other properties) accurately, one must be careful to avoid outside interference. Newton's formula <math>F = ma\,</math> requires that no unaccounted-for forces are acting on the object. Similarly, in thermodynamics, one must take into account any possible sources of heat into or out of the entity under test. This is often described by saying that the law of thermodynamics apply only to "isolated", or "closed", systems. If a system can interact with some external entity, that entity's properties must be taken into account.
Of course, in all experiments that attempt to tally energy (or other properties) accurately, one must be careful to avoid outside interference. Newton's formula <math>F = ma\,</math> requires that no unaccounted-for forces are acting on the object. Similarly, in thermodynamics, one must take into account any possible sources of heat into or out of the entity under test. This is often described by saying that the law of thermodynamics apply only to "isolated", or "closed", systems. If a system can interact with some external entity, that entity's properties must be taken into account.
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.
==[[Second Law of Thermodynamics]]==
==[[Second Law of Thermodynamics]]==