What is heat?

1. What is heat? Heatis spontaneous transfer of energy from a hotter body to a colder one, other than by work or transfer of matter, whenever there is a suitable physical pathway between them.

2. Kinetic Theory of Matter Kinetic Theory of Matter proposes that heat is a measure of internal kinetic energy of the molecules and atoms making up the substance

3. The precise relationship between the average molecular kinetic energy KEave and absolute temperature T is found to be KEave = (3/2) kT(Eqn 11.7) Where k, known as Boltzmann’s constant, has the value k = 1.38 x 10-23 J/K This equation holds for the molecules of all gases of their molecules.

4. Using this theory, temperature and heat differ based on the following definitions; Temperature– a measure of the average kinetic energy of a system. Heat – a measure of the total kinetic energy of a system.

5. Internal Energy and Heat How much internal energy a body has depends on its temperature, its composition, its mass, and its state (solid, liquid, gas). The temperature of the body alone, however, is what governs whether internal energy will go from it to another body it is in contact with, vice versa. Because heat is a form of energy, the unit of heat is joule. However, the kilocalorie (kcal) and British thermal unit (Btu) are the once used specifically for heat energy. To distinguish heat from other forms of energy, many use the unit kilocalorie. The kilocalorie is the amount of heat required to raise the temperature of 1 kg of water by 1C. The relationship between joule and kilocalorie are as follows: 1 J = 2.39 x 10-4 kcal 1kcal = 4185 J

6. Quantity of Heat We said that aside from temperature, heat is also affected by other factors. According to the kinetic theory of matter, heat is a measure of the total kinetic energy of a system, so the amount of matter affects heat. The amount of matter is just mass. The movement of the molecules also depends on the type of material used. Scientists refer to this property as the Specific heat of a material.

7. So with these factors in mind, the amount of heat gained or lost in a system is given by the following equation: Heat lost or gained = mass x c x Temperature change Q = mcT(Eqn. 11.8) where Q is the heat lost or gained in kcal, m is the mass in kg, c is the specific heat kcal/kg C and T is the temperature change in C. If you find that you are given c in c/calgC, this equation also works when heat is in calories, mass in grams, and temperature change in C.

8. Specific heat Specific heat is a property of a material that explains why some foods and liquids remain hotter longer that others. The specific heat (c, in cal/kg C) of a material is a number, which gives us an idea how much heat in calories (or kcal) is needed to raise the temperature of one gram (or kg, if the heat is in kcal) of the material by one degree Celsius.

9. For example, You are more likely to burn you tongue on the filling of a hot apple pie than the crust. This tells you experimentally that the apple pie filling has a greater specific heat that the crust. capplepie > ccrust Using our definition, it takes more heat to bring one gram of the apple pie to a given temperature, and you must also remove more heat to drop the temperature of the apple pie.

10. Specific heat of various substances The higher the specific heat of a substance, the more heat it can absorb and hold.

11. Example How much heat must be added to 5 kg of water to increase it temperature by 20C? The specific heat of water is 1 kcal/kgC. Answer Given mass of water m 5kg specific heat of water c 1 kcal/kgC change in temperatureT 20C Find: Amount of Heat (Q) Solution: Q = mcT = 5kg(1kcal/gC)(20C) = 100kcal

12. Thermal equilibrium Thermal Equilibrium occurs when one or more substances reach a common temperature by exchanging heat. When two substances are in thermal contact, heat will flow from the hotter substance to the colder substance until they reach the same temperature and heat will no longer flow. Heat lost = Heat gained Qlost = Qgained(Eqn 11.9) For example, if you put ice in warm water, the warm water gets cold, the ice gets warm, and melts. When the ice completely melts, they will all have the same temperature. They are in equilibrium.

13. Example A 0.5-kg aluminum block at 75C is dropped into a cup of water with a mass of 2.0 kg at 40C and comes to thermal equilibrium. What is the final temperature of the water and aluminum? The specific heat of aluminum is 0.21 kcal/kgC. Answer Given Aluminum Block mass mAl0.5-kg specific heat cAl0.21 kcal/kgC initial temperature TiAl75C Water mass mwater2.0 kg specific heat cwater1 kcal/kgC initial temperature Tiwater40C

14. Find: Final, Equilibrium Temperature (Tf) Solution The initial temperature of the aluminum block is greater than the initial temperature of water. Since heat flow is from hot to cold, then the aluminum block will lose heat, while water will gain heat. Since the aluminum block will lose heat, T = (initial temperature – final temperature) = (75C – Tf) Water will gain heat, so T = (final temperature – initial temperature) = (Tf– 40C) Applying the equilibrium condition: Heat lost by Aluminum = Heat gained by water mAl x cAl x TAl = mwaterx cwaterx Twater Substituting the values, 0.5 kg(0.21 kcal/kgC)(75C – Tf) = 2 kg(1 kcal/kgC)(Tf – 40C) 7.9 – 0.11Tf = 2Tf – 80 Grouping like terms together, 7.9 + 80 = 2Tf + 0.11Tf 88 = 2.1Tf Tf= = 42C

15. Transmission of heat Also known as the movement of heat from the hotter item to the colder item can be done in any of the following ways: Conduction, Convection, and Radiation.

16. Conduction Conduction occurs when two objects touch. When you place one end of a bar of metal in the flame of a bunsen burner, heat is conducted from the end in the flame to the end you are holding. When two materials are in direct contact, like two parts of the same metal bar, then conduction can occur. Materials that readily conduct heat are known as good conductors of heat. Metals are the best heat conductors. It is easy for atoms or molecules of metal to transfer their energy to atoms of molecules that are nearby. On the other hand, there are materials that slow down the conduction of heat, and those are called insulators. Wood, cork, wool, paper, and styrofoam are all poor conductors of heat, and therefore good insulators.

17. ConVECtion Convection occurs when the heated substance itself moves. When the layer of water along the bottom of the spaghetti pot becomes hot from the stove, this layer of water expands and these hotter molecules rise letting the cooler water above them move downward to be heated. Convection occurs in all unevenly heated liquids and gases, creating convection currents. Convection currents in our atmosphere are the main causes of different weather patterns around the world.

18. Radiation Radiation is the only type of heat transfer that can occur in empty space, such as in outer space. Any energy transferred by this method is called radiant energy.