Thermodynamics Chapter 12

1. Thermodynamics Chapter 12

2. Temperature and Heat • The temperature of a hot cup of coffee left sitting on the table will fall until it reaches thermal equilibrium with the air temperature in the room. • When a soda can is taken out of the refrigerator and left on the kitchen table, its temperature will rise – rapidly at first but then more slowly – until the temperature of the soda equals that of the air in the room. At this point, the soda and the air temperature in the room are in thermal equilibrium. • The change in temperature is due to the transfer of energy between the object and the environment.

3. Temperature and Heat • Thermal energy: the total potential and kinetic energy associated with the random motion and arrangement of the particles of a material. • Heat, Q, is thermal energy that is absorbed, given up, or transferred from one body to another. • Heat is thermal energy in motion. • Heat is used when the transfer of thermal energy from one body to another body at a different temperature is involved.

4. Temperature • Temperatureis a measure of the average kinetic energy of all the particles within an object. • The temperature of a substance will increase if the averagekinetic energy of its particles is increased. • If the average kinetic energy of particles decreases, so does the temperature of the substance.

5. Specific heat Every substance gains or loses heat based on it’s identity. This physical property of the substance is called the specific heat capacity of the object. • The specific heat capacity, C,of a solid or liquid is defined as the heat required to raise a unit mass of the substance by one degree of temperature.

6. Define: Specific Heat • Amount of energy required to raise the temperature of 1kg by 1oC Heat Energy =(mass)x specific x change heat in temp OR Q = m c Dt

7. Heat Change • To determine the amount of thermal energy gained or lost by a mass: • Heat energy is gained if Q is positive. • Heat energy is lost if Q is negative.

8. Ex. A 4.0 kg sample of glass heated from 1o C to 41o C, and was found to have absorbed 32 J of energy. What is the specific heat of the glass? Q = C x m x ΔT ΔT = 41oC – 1oC = 40oC 32 J = C (4.0 kg)(40oC) 32 J = C (160 kg oC) C = 0.2 J/kgoC

9. Methods of Heat Energy Transfer • Conduction is the transfer of heat energy by • Between particles of objects in direct contact • Convection is the transfer of heat energy by • the movement of fluids(gas or liquid) • convection currents due to hot fluid rising and cold fluid sinking • Radiation is the transfer of heat energy by • electromagnetic waves • does not involve the movement of matter

10. A Conduction transfers energy as heat along the wire and into the hand B. Embers swirl upward in the convection currents that are created by the warmed air above the fire which rises C. Electromagnetic waves emitted by the hot campfire transfer energy by radiation

11. Heat Transfer by Conduction • Conduction is the transfer of thermal energy without any net movement of the material itself. • When a metal poker is put in a hot fire, the exposed end of the poker soon becomes hot as well, even though it is not directly in contact with the source of heat. We say that heat has been conducted from the hot end to the cold end.

12. Heat conduction in many materials can be visualized as the result of molecular collisions. As one end of the object is heated, the molecules there move faster and faster. As they collide with their slower-moving neighbors, they transfersome of their energy to these molecules whose speeds thus increase. These in turn transfer some of their energy by collision with molecules farther along the object. Thus the energy of thermal motion is transferred by molecular collision along the object. Good thermal conductors such as silver, copper, aluminum, and gold are also good electrical conductors.

13. Heat Transfer by Convection • Convection is the process of heat transfer through the mass motion or flow of some fluid, such as air or water. • When a pot of water is heated, convection currents are set up as the heated water at the bottom of the pot rises because of its reduced density and is replaced by cooler water from above.

14. Heat Transfer by Convection • Although liquids and gases are generally not very good conductors of heat, they can transfer heat quite rapidly by convection. Convection is the process whereby heat is transferred by the mass movement of molecules from one place to another. Whereas conduction involves molecules (and/or electrons) moving only over small distances and colliding, convection involves the movement of molecules over large distances.

15. Perhaps the first thing that most people say is "heat rises". While not wrong, what you should say is "hot air rises" or "hot water rises". Anything fluid - that is gases or liquids - will tend to change density with changes in temperature. For example, if heated, air decreases in density. The surrounding air is cooler and denser. This makes it heavier, so it falls beneath the hot air, forcing it upwards.

16. Heat Transfer by Radiation Radiation is a more rapid transfer of thermal energy in the form of electromagnetic radiation accomplished by a process that requires neither contact nor mass flow. A hot object also loses heat energy by radiation. This radiation is similar to light and can pass through empty space. The warmth you fell when you warm yourself by a fire is due to this radiation. If the object is hot enough, some of the radiation is visible and can indeed be seen.

17. What type of heat transfer is shown in the following pictures? B. Radiation A. Convection C. Conduction D. Radiation