Presentation on theme: "Everyone knows that when you heat something up it expands. Well, almost everything, except water around the freezing point and a few other weird substances."— Presentation transcript:
Everyone knows that when you heat something up it expands. Well, almost everything, except water around the freezing point and a few other weird substances. One example of something that expands when heated is the brass plate shown in the photograph below. When the brass plate is heated up by the gas burner shown in the photograph, the hole will: (1) get bigger. (2) get smaller. (3) remain the same size. Answer
A flask containing green water is resting in a styrofoam bucket of ice water as seen in the photograph at the left below. A cork seals the top of the flask so that there is no air on top of the water. Inserted into the water are a thermometer and a capillary tube, as seen in the photograph at the right. Some of the water from the flask extends into the capillary tube. If the water in the flask were to expand, the green water level would rise higher into the capillary tube. On the other hand, if the water in the flask were to contract, the green water level would fall lower in the capillary tube. The flask used is made of quartz, which has a very small coefficient of thermal expansion, so if the flask and the water change temperature the predominant effect will be due to the expansion or contraction of the water. Suppose that the flask is now removed from its ice water bath and allowed to slowly warm up. What will the water level in the capillary tube do as the water in the flask warms up? Will it: (1) rise. (2) fall. (3) rise for a bit and then fall. (4) fall for a bit and then rise. (5) remain the same.
Answer Pictures were taken at approximately three minute intervals.
CH 18: Heat and the first law of thermodynamics
We know that when work is done on a system it can change the internal energy of the system. For example friction increases the internal energy of a system, but reduces the total mechanical energy of a system. Internal Energy – All the energy of a system associated with the microscopic components when viewed from a reference frame at rest with respect to the center of mass of a system. The internal energy does not include motion of the center of mass The internal energy is associated with translation, rotation and vibration of atoms and molecules in a system. (Only related to temperature for a particular state, this relationship will be discussed later) This includes KE for each of these motions. This includes PE for the interatomic interactions. Friction adds energy to the system, which cause the molecules of the object to have a larger average translational kinetic energy (temperature of the object increases). Heat – Transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings. Heat is a transfer of energy. It is NOT energy!! Heat always flow from an object with excess energy to an object with an absence of energy. Heat flows from hot to cold. Units of heat: 1 cal – energy required to raise 1 g of H 2 O by 1 o C. 1 Cal = 1000 cal = 1kcal – Food calories 1 BTU – energy required to raise 1 lb of H 2 O by 1 o F. 1 cal = 4.186 J Convert calories to Joules
Specific Heat and Calorimetry Heat can flow between any two objects. Is the rate at which two objects accept or release heat the same for all objects? No! If a wood stick and a metal rod were placed with one end in a hot fire for a period of several minutes, which would you be willing to take out of the fire with your bare hands? The wooden stick, because it does not transfer energy along its length as quickly. We can calculate the heat required to change the temperature of a specific material. Q – Heat [J] C – Heat capacity [J/ o C] T – Change in temperature [ o C] The heat capacity is the energy required to change the temperature of an object by 1 o C. The heat capacity depends on the mass of the object and the ability of the object to resist the flow of heat into or out of the object. m – mass [kg] C – Heat capacity [J/ o C] c – Specific heat [J/kg o C] We can now rewrite the expression for the heat in terms of the specific heat. Specific heat – measure of a materials resistance to energy transfer.
A common use of the heat equation is to determine the final temperature of a mixture of two different objects at different initial temperatures. This is necessary to compensate for the different directions of heat flow. This would be the final temperature of a mixture of two materials with different mass, specific heats and initial temperatures. This expression is not valid if there is a phase change at any time during the process.