The Fireside by Don Hargrove To understand our increasing need for renewable energy, we must know that our future lies not in the hands of those who abuse, but in the hands of those who efficiently use our valuable resources. In this and future articles I will discuss heat: its definition and its use. By being aware of this basic technology, we can gain a working knowledge of heat's daily application in various systems. WHAT IS HEAT? Heat is a form of invisible energy. Only the work that heat does can be seen. For instance: when the gas in your automobile engine is ignited, the burning gases expand. This expansion causes a release of heat (energy), which in turn causes work to be done. This work then becomes mechanical energy. TEMPERATURE AND HEAT All matter consist of molecules in motion. This motion is defined as internal energy or heat. The amount of this internal energy depends upon how rapidly the atoms or molecules are moving. The faster these particles are moving, the hotter the object is and the higher amount of internal energy it contains. TEMPERATURE is an indication of an object's internal energy. A thermometer measures this in degrees; Fahrenheit and Centigrade (Celsius) being the two most common scales. The temperature of an object determines if that object will gain or lose internal energy. Heat always flows from a hotter object to a colder one. This is a temperature "hill". Like water, heat flows downhill. The greater the difference in temperature between two objects, the steeper the hill, and the faster the heat will flow between the two objects. The hotter object is giving up some of its internal energy to the colder one. Given enough time, these two objects will equalize their temperature. It is important to remember that heat and temperature are not the same thing. Temperature is an indication of the amount of internal energy and heat is the transfer of this internal energy between two objects. Heat is measured in two basic units: BTUs (British Thermal Units) and calories. One BTU is the amount of heat needed to raise one pound (approximately one pint) of water one degree Fahrenheit. One calorie will raise one gram (0.035 ounce) of water one degree Centigrade. These units are calculated at sea level atmospheric pressure (one atmosphere). Heat and temperature tell only part of what is happening. Let's look at what happens to an object when heat flows into it. As the heat raises the internal energy of the object, its molecules start moving more rapidly. The more heat an object has, the more disorderly its molecular pattern becomes. Science defines the amount of disorder in a system as entropy. Heat flowing from an object will decrease its internal energy, its amount of molecular disorder, and thus its entropy. The temperature of the object will usually change, according to the direction of heat flow, but not always. When an object changes its physical state (from solid to liquid to gas), energy, disorder, and entropy change, but the temperature will remain the same until the particular change of state is completed. INSERT ICE CUBE GRAPHIC As an example of heat content versus temperature see the following graph. It shows how many BTUs are required to raise one pound of water from - 4¡F to 212¡F at sea level (1 atmosphere). For comparison, kilocalories (1000 calories=1 kilocalorie) are also given. It takes 1 calorie to raise the temperature of 1 gram of water 1 degree Centigrade. Here we have converted it for you: 454 grams = 1 pound 454 calories = 1.8 BTUs 1.8/454 = 0.00397 BTUs in 1 calorie 454/1.8 = 252 calories in 1 BTU 252/1000 = 0.252 kilocalories in 1 BTU Note that there is no change in temperature until there is a change of state. Ice stays at 32¡F until it is completely melted. To accomplish a complete change of state from ice to water requires 144 BTUs of latent heat. Now that the ice has completed its change to water, each added BTU will cause a rise in temperature of 1¡F, until the water reaches its boiling point of 212¡F. At this point, 972 BTUs are required to complete another change of state from water to steam. The temperature, however, remains at 212¡F until all the water has become steam. Once again, the temperature will start rising 1¡F for each BTU added. There is one added requirement for the temperature of the steam to continue rising. Steam is water in its gaseous state. Were it not contained, this "water vapor" would simply expand and eventually recondense elsewhere. Therefore a high pressure vessel is needed to contain this expansion. Now, any addition of BTUs to the vessel containing the steam will cause a corresponding rise in temperature. The water molecules within this steam now have a very high internal energy and they are moving at an extremely high rate of speed. HOW HEAT TRAVELS Heat passes from one place or object to another by three methods: CONDUCTION is the movement of heat through a material without carrying that material along with the heat (that is, without changing the conducting materials physical structure). Example: Heat a copper rod on one end only. The copper molecules in the heated end will start vibrating due to the increased internal energy. These vibrating molecules will strike unheated copper molecules next to them, transferring heat. This chain reaction will continue until the entire rod is heated. Note that the copper molecules themselves have not moved. It is only their internal energy bumping against each other causing the heat transfer. CONVECTION is the transfer of heat by movement of heated material. Example: the suns rays hit the earth and heat it. The air next to the ground is heated by conduction. This heated air expands, becomes lighter, and rises. Cooler air, being denser and therefore heavier, will flow downwards to replace the lighter air. This process is called convection and the flow of heated air upwards is known as a convection current. Convection occurs in liquids as well: the bottoms of oceans, lakes, and rivers are the coldest. RADIATION--Conduction and convection transmit heat by particle vibration. Heat can also move through a vacuum which contains no matter. Heat can move as radiant energy. When this radiant energy strikes an object, the particles in that object speed up. An example of radiant heat, or infrared as scientists call it, is the heat striking the earth from the sun. Understanding basic heat definitions and the different ways heat moves will help us to better and more efficiently use it. In following issues, I will show you practical applications of the rules of heat. I will compare methods of using and saving BTUs. Look forward to reading about solar heating, thermostats, stack robbers, methods of heating living space and water, BTU content comparisons of differing materials and lots more. Until then-- stay warm, hopefully as efficiently as possible.