 Welcome to the first video in this series on thermodynamics. Here we consider the phenomenon of heat and the concept of temperature. Heat is essential to our existence, so it's not surprising that throughout human history, people have pondered its nature. For Earth's ecosystem, the sun is the most important source of heat and light. So much so that in many cultures the sun was considered a god. We don't control the sun, obviously, but when our ancestors learned how to start and maintain fires, they obtained a source of heat and light that they could control for their own purposes, from heating their dwellings and cooking to, eventually, smelting and smithing metals and building steam engines. It was especially this last innovation that motivated the development of a rigorous theory of thermodynamics. Various explanations of the nature of heat, fire and light have been proposed through the centuries. Fire was one of the four classical elements, earth, water, air and fire, which were thought to be the fundamental components of all matter. In 1667, Johann Besscher proposed that fire consists of the release of a substance he called phlegiston. At this time, no clear distinction between fire and heat had been established. In 1783, Antoine Lavossier proposed that heat, as distinct from fire, was due to the presence of a substance called caloric. The more caloric an object contains, the hotter it is. Caloric must be massless, since the mass of a body does not change with temperature, that is, as it gains or loses caloric. Caloric must also be self-repellent. This explains why hot and cold bodies in contact equalize their temperature, as caloric flows from the hot to the cold body. A powerful argument against the idea of heat as a substance was presented in 1798 by Benjamin Thompson in his an experimental inquiry concerning the source of the heat, which is excited by friction. Thompson observed that during the boring of a cannon, a tremendous amount of heat is generated by friction, and there is no limit to this process. If heat was a substance, presumably, it would eventually all be driven off, and friction would then no longer produce heat. In fact, many scientists had already expressed the opinion that heat is not a substance, but a process, the motion of the particles making up a body. Bacon's opinion was, heat itself is motion and nothing else. Descartes wrote, It is this motion that is now called heat and now light according to the different effects it produces. Hook saw heat as being nothing else but a very brisk and vehement agitation of the parts of a body, while Boyle had written a treatise of the mechanical origin of heat and cold, and Newton had proposed that heat consists in a minute vibratory motion. Regardless of its nature, we are led to the concept of temperature, when we consider how to quantify the intuitive concepts of hot and cold. A standard approach is to find various physical phenomena that can serve as temperature references. In the Celsius temperature scale, zero degrees is essentially the freezing point of water. Here we have an ice-water mixture in equilibrium. My thermometer registers zero degrees Celsius. If it didn't, I could use this mixture to calibrate its zero degree reading. 100 degrees Celsius is the boiling point of water. Here the thermometer inserted into boiling water registers 98.8 degrees Celsius. This might indicate it needs to be recalibrated, but in fact, this illustrates something that early researchers soon discovered. The boiling point of water varies with atmospheric pressure. At lower or higher pressure, water boils at a lower or higher temperature. The 100 degree reference is specific to so-called standard atmospheric pressure. The calculated boiling point for the barometric pressure during the measurements shown here corresponded to the measured water temperature. Intermediate temperatures can be produced in so-called mixing experiments. Carlo Renaldini was a pioneer of this technique. Later, Daniel Fahrenheit extended the process. In a simple mixing experiment, we take a volume V1 of water at a temperature T1. Mix it with a volume V2 of water at a temperature T2 to produce a volume V of water at a temperature T. The final volume is found to be the sum of V1 and V2. Actually, this is only an approximation, although a very good one. Water changes volume slightly with temperature. The final temperature is found to be a weighted average of the original temperatures. The weighting factors are the volumes. Again, this is only a very good approximation. Starting with freezing water and boiling water, that is, T1 equals 0 and T2 equals 100, Renaldini used this idea to produce water of any desired intermediate temperature by varying the volumes accordingly. In effect, this equation served to define temperatures between 0 and 100 degrees Celsius. Regardless of the ultimate nature of heat, it seemed clear that in this process the hot water cooled by giving up some heat, and the cold water warmed by absorbing that heat. The amount of heat transferred was precisely that needed to bring the two volumes to the same temperature. This leads us to define the calorie as that amount of heat which raises the temperature of 1 gram of water by 1 degree Celsius. Considerable effort was spent attempting to construct a device that could provide a continuous reading of temperature. This is what we now call a thermometer. The gas thermoscope, thought to have been invented by Galileo, was an important step toward this goal. A glass bulb containing a gas had a long thin neck which is inserted into an open volume of water. As the temperature changes, the enclosed gas expands or contracts. This causes the water level in the neck to rise or fall. A high level indicates a low temperature and a low level a high temperature. Here's a museum replica of this type of device. The gas thermoscope is, in fact, a type of gas thermometer. All we need to do is add volume markings and calibrate these to known temperatures. Let's do some kitchen science and try to make a gas thermometer using a 100 milliliter syringe. Sealed airtight at both ends. For calibration, we'll cheat and use a modern electronic thermometer. At room temperature, about 24 degrees, the syringe volume was 60 milliliters. In a freezer at about minus 12 degrees, the volume was 52 milliliters. And in hot tap water at about 57 degrees, the volume was 66 milliliters. The milliliter markings are not very precise, but we do get three measurements that fairly closely lie on the line. This line can be used to convert arbitrary syringe volume measurements to temperature. The gas thermoscope and thermometer have a common drawback. The volume of the enclosed gas depends not only on the temperature, but also on atmospheric pressure. These devices are actually a type of barometer-thermometer combination. Nevertheless, over short periods of time, they can provide a useful measure of temperature changes. But they will not give repeatedly accurate results in different locations and at very different times. Our fit line shows gas volume decreasing with decreasing temperature. It's interesting to extrapolate the line all the way to zero volume. This happens at around minus 270 degrees. Since volume cannot decrease beyond this limit, this suggests that temperature cannot either. This is presumably the coldest possible temperature. If we use this quote absolute zero as our zero temperature and keep the Celsius degree increment, then we have the so-called absolute or Kelvin temperature scale. Scientifically useful thermometers were first produced by Daniel Fahrenheit with his development of the mercury and glass thermometer in 1714. This had a volume of mercury enclosed in an evacuated sealed glass tube. The sealed design makes it insensitive to barometric pressure. The result was the first practical thermometer that was repeatedly accurate. It was now possible for anyone, anywhere, under any conditions, to make reliable temperature measurements. These precision instruments enabled the development of quantitative thermal science. After being state of the art for centuries, only recently they've been increasingly replaced by electronic thermometers.