Perhaps the greatest difficulty for students in their first efforts to study temperature and heat comes in making the distinction between the two quantities. Heat is a form of energy that is transferred between two objects because of a temperature difference between the two objects. The most common perception of temperature is in terms of the hotness and coldness of an object. This identification of temperature is unsatisfactory for physics, because it involves a subjective sensing of the temperature and, frankly, our ability to distinguish whether something is hotter or colder than something else is not that reliable. In addition, when we define a quantity in physics we prefer to express it in terms of physical parameters that we can measure. We can accomplish this in the case of temperature by using such physical properties as the height of a liquid such as mercury in a glass tube. The higher the temperature, the higher the mercury level in the glass tube. If we put marks on the tube for certain reference temperatures that we can reliably reproduce, we have a temperature scale. We will encounter three such reference scales. The one with which you may be most familiar is the Fahrenheit scale, the temperature scale used in the United States (and almost nowhere else in the world). On this scale the freezing point of water is 32^o, and the boiling point of water is 212^o. The Celsius scale (sometimes referred to as the Centigrade scale because of its use of 100 divisions) uses 0^o for the freezing point of water and 100^o for the boiling point of water. With 100 divisions between these two fixed reference points for the Celsius scale and 180 divisions for the Fahrenheit scale a Celsius degree is 180 /100 = 9 / 5 times as big as a Fahrenheit degree. We can use this fact and the fact that 32^o F and 0^oC both represent the freezing point of water to obtain the conversion formulas between these two systems. Thus the temperature in Celsius is given by T_c = (5 / 9) (T_f - 32) and the temperature in Fahrenheit is given by the expression T_F = (9 / 5) T_c + 32.

The Kelvin scale is designed to have its zero point represent the temperature at which an ideal gas would have zero pressure. There is an easy conversion between the Kelvin scale and the Celsius scale. The Kelvin temperature is obtained by adding 273.2 to the Celsius temperature. The Kelvin scale is defined in terms of the properties of an ideal gas, so it has physical significance. It is the temperature scale that must be used in several equations in physics. One of those equations is the equation of state for an ideal gas.

Temperature is the measure of the internal energy of molecules. This is most easily visualized for an ideal gas. The higher the temperature the more energy each molecule has and the more collisions that occur. We express this in terms of the ideal gas equation of state, P V = N k T, where P is the pressure (defined in Chapter 9 as the force per unit area), V is the volume, N is the number of molecules, k is the Boltzman constant, and T is the absolute temperature measured on the Kelvin scale. When we talk about an ideal gas we make the assumption that the only forces between the molecules of an idea gas are those that occur during collisions. As a result of this behavior, the Kelvin temperature is a direct measure of the internal energy of an ideal gas as illustrated in Figure 10.16 on page 198 in the text.

Heat is a form of energy that is transferred between two objects solely because of a temperature difference between the objects. Experimentally we find that this energy always is transferred from the higher temperature body to the lower temperature body.

If a container of hot water and an equal-sized container of cold water are mixed together in a third container, the final temperature of the mixture is somewhere between the two original temperatures, because heat was transferred from the hot water to the cold water.

The amount of temperature change that objects experience whenever heat is transferred depends upon the specific heat capacity of the objects. The standard for measuring specific heat capacity is the behavior of water. We identify the amount of heat that is required to raise the temperature of one gram of water by one degree Celsius. Unfortunately this unit was established in the early days of the investigations of thermodynamics before the equivalence of the various forms of energy was established in physics. Heat can be measured in Joules as other forms of energy are measured, but the use of the calorie as a unit is so firmly established historically that it is essentially impossible to change. The conversion from calories to Joules is given as 1 calorie = 4.19 Joules.

No doubt you have heard the term calorie used in reference to food and diets. Unfortunately the term that dietitians use as the calorie is actually 1000 times the calorie that is defined above. One way to keep this straight is to refer to the dietary term as the Calorie with a capital letter thereby reminding you that it is 1000 times as large as the physics calorie, that is to say that 1 Calorie as used by a nutritionist is the amount of heat necessary to raise the temperature of 1000 grams (1 kilogram) of water by 1^o C.

The first law of thermodynamics is really nothing more than a statement of conservation of energy specifying that if heat is added to a system the energy either goes to doing work or to increasing the internal energy of the system. In the vernacular it states that you cannot get something for nothing. Nevertheless it is amazing how many schemes are proposed each year that offer to provide something for nothing in violation of the first law of thermodynamics. An example of the increase of internal energy is the increased motion of gas molecules whenever heat is added to a closed container of a gas.

Of the three methods of heat transfer, you are probably most familiar with conduction, the process whereby heat is transferred because of direct contact between objects. A material that easily allows such heat transfer, for example a metal, is called a good conductor while a material that does not allow such heat transfer, for example Styrofoam, is called an insulator. You can demonstrate the effect of conduction for yourself by grasping one end of a metal object such as a spoon and placing the other end in contact with something at a higher temperature, for example a bowl of soup or a cup of hot coffee. After a relatively short time the end of the spoon that was not placed in direct contact with the higher temperature object will also reach a higher temperature because of heat conduction through the metal.

Heat transfer by convection requires mass movement of material. The most common example of heating by convection is the forced air heating systems used in many homes. Warm air molecules are moved throughout the house by blowers in order to transfer heat as shown in Figure 10.20 on page 193 in the text.

Heat transfer by radiation occurs even when there is no medium present and is responsible for all the energy the Earth receives from the Sun. An object that is black in color is both a better radiator of energy and a better absorber of energy.

Matter can exist in the solid, liquid, or gaseous phase. In the case of water we call these three phases ice, water, and steam respectively. The molecules have a higher average energy in the liquid phase than they do in the solid phase, and they have an even higher energy in the gaseous phase. When matter is transformed from one phase to another, as for example when ice is melted to produce liquid water, energy must be added to provide the higher molecular energy in the new phase. This energy is added at the melting temperature for the transfer from solid to liquid and at the boiling point for the transfer from liquid to gas. At the melting point a certain amount of heat must be added to achieve the phase transformation to a liquid. For water we must add 80 calories for every gram of ice that is melted. To convert water to steam we must add 540 calories for every gram of water that is boiled. Because the addition of this heat results in a phase change with no increase in temperature it is called the latent (or hidden heat). There are different values for the latent heats for different substances, and the values are listed in handbooks.