Q: How does Joule’s experiments show the fallacy in caloric nature of heat?

A: Concept of heat as caloric satisfactorily explains the flow of heat from hotter to colder objects. Caloric theory however fails to describe how work creates heat as in the Joule’s experiments. Production of heat required the flow of caloric, which neither could be created nor destroyed. The only conclusion can be made is that no such fluids could exist and heat was not a conserved quantity like mass.

Q: Caloric theory considered heat as a substance that flows from one body to another. Kinetic theory proclaims heat as motion. Which theory explains the heating of two blocks rubbing against each other better?

A: Two blocks are being heated up by the same mount when they are rubbed against each other. If the cause of the heating were a fluid, then it would have flowed from a body at higher energy (hotter) to one at a lower energy (colder). Blocks are heated up because the kinetic energy of motion (rubbing) has been converted to heat in a process called "friction".

Q: How would caloric theory justify the evaporation of water at constant temperature?

A: If caloric is the fluid flowing from a hotter to a colder object, then heating the water must accompany a rise in temperature. This is the case as water is heated from a low temperature to its boiling point. When water reaches it’s boiling point (100 ^oC at one atmosphere), it boils while the temperature remains unchanged. This condition continues until all water is turned to steam. Caloric theory explains this behavior by arguing that heat is going into a latent or hidden and invisible form. The existence of caloric is however evident by noting that steam occupies a larger volume. The question is how could caloric theory explain the freezing of water!!

Q: A volume of gas is expanded to a vacuum chamber. How does the temperature of the gas change, does it increase, decrease or remain the same?

A: Experiments performed by French physicist Gay Lussac in 1807 showed that the temperature remained constant. If caloric theory were valid, and they thought it to be so at the time, then one would expect that since concentration of the caloric fluid has dropped, temperature, which is a measure of it should also drop. Contradiction between caloric theory and these results were considered to cast one of the most serious blows to the concept of heat as a caloric. Lussac’s experiments can be easily interpreted by kinetic theory, however. When gas expands, it’s volume increases while the total energy Q and total mass remains unchanged. Although the number of collisions (pressure) is reduced, the speed (kinetic energy) of particles has not changed. Temperature as a measure of the average kinetic energy of the particles is also the same.

Q: Why thermometers are constructed with a narrow tube running up from the center of the bulb?

A: When the temperature increases, the fluid expands and pushes up the stem. Narrow tubes are advantageous, because small changes in volumes are easily noticeable.

Q: What was the main problem with Galileo’s thermoscope?

A: The level of the liquid in capillary tube changed with barometric pressure. Furthermore, the tube had no scale and therefore no absolute measurement of temperature could be made. To be useful, thermometers have to be calibrated. Earlier thermometers used a single reference temperature for calibration. The problem with a single reference point is that there is no way of comparing the readings between two thermometers employing different fluids. Using two reference points made it possible to divide the distance between the reference marks into a number of equal intervals. First thermometers were divided into 360 divisions, like that of a circle, thus the term degree. Gabriel Fahrenheit (1708) used spirits as the fluid, and a mixture of ice, water, and salt, the lowest attainable in laboratory setting at the time, as zero mark. He arbitrarily used 60 to the boiling point of water. A few years later he introduced mercury, enabling him to achieve temperatures much higher than that of boiling water. Furthermore, he changed the second reference point to the temperature of the body who assigned the value of 96. The choice of 96 is believed to be that it is divisible by 2, 3, 4, 8, 12, 16, and 32. The choice of body temperature for the upper reference point was because of its intended use in meteorology. On this scale water boiled at a temperature around (but not exactly) 212 degrees. Today, the Fahrenheit scale uses ice and steam points of water fixed at 32^o F and 212^o F. On this scale, body temperature is 98.6^o F.

Q: What are the advantages of alcohol and mercury thermometers over air thermometers?

A: Air shrinks or expands by changing the pressure. Higher pressures push air molecules together and reduces the volume it occupies. Mercury and alcohol cannot be compressed as easily, so they are more accurate.

Q: Heat energy is measured with a thermometer, which is a device for measuring the average random motion of the molecules of matters such as liquids or gases. How can Brownian motions of these molecules result in observed rise of a column of mercury or other fluids commonly indicated by a thermometer?

A: When a thermometer is placed in a hot liquid, for example, the molecules of the liquid shake the molecules in the thermometer. The reverse will be true if the liquid is at a lower temperature than the thermometer. As the molecules of the thermometer get agitated, they dance faster and travel farther apart from each other, resulting in apparent expansion of the thermometer. The glass in thermometer stem does not expand as fast as the liquid it contains. The net effect will be the rise (or fall) of the level of the fluid in thermometer.

Heat Capacity

Q: The calorie is defined as the amount of heat required to change the temperature of 1 g of water by 1° C from 14.5° C to 15.5° C. What is so special about these temperatures?

A: Heat capacity depends somewhat on the temperature. The differences are small at low temperatures but rapidly increases at higher temperatures. Kinetic theory is particularly useful for explaining these differences. At low temperatures, any energy added to the system would mainly goes into increase in the kinetic energies of the molecules. As temperature is raised, molecules are agitated and would start rotating about their center of mass. The motion resembles that of dumbbells. At even higher temperatures, the bonds connecting the atoms become less rigid and atoms start vibrating. At each step of the way, a larger fraction of the energy will be required to support such motions. In another way, resistance to flow of heat increases at higher temperatures.

Heat Expansion

Q: How does kinetic theory accounts for different rates of expansion in solids, liquids, and gases?

A: Kinetic energy describes the state of matter in term of motion of its constituent particles. At sufficiently small temperatures, molecular motion is much localized and molecules are "frozen" in their space. As temperature rises, the quivering of molecules becomes more and more intense until the crystalline lattice structure of the solid is broken and solid turns into liquid. Liquid molecules, although enjoy a certain degree of freedom, move sluggishly for the most part in a group. The higher the temperature is this dependency is smaller and particles can move more independently. At sufficiently elevated temperatures, called the boiling temperature, molecules become increasingly restless until they are separated from each other. Under these conditions, motion is completely independent and the matter is in a gaseous state.

Q: If you wish to loosen the metal lid on glass jar, should you run hot or cold water on it?

A: Hot water. Metal lid expands more than the glass jar, so it should loosen somewhat.