Transcript Document
Chapter 6: Thermal Energy Section 1: Temperature and Heat Temperature is related to the average kinetic energy of the particles in a substance. Temperature Continued… SI unit for temp. is the Kelvin K = C + 273 (10C = 283K) C = K – 273 (10K = -263C) Thermal Energy – the total of all the kinetic and potential energy of all the particles in a substance. Thermal Energy Relationships As temperature increases, so does thermal energy (because the kinetic energy of the particles increased). Even if the temperature doesn’t change, the thermal energy in a more massive substance is higher (because it is a total measure of energy). Heat Heat- The flow of thermal energy from one object to another. • Heat always flows from warmer to cooler objects. Specific Heat Some things heat up or cool down faster than others. Specific Heat Continued… Specific heat is the amount of heat required to raise the temperature of 1 kg of a material by one degree (C or K). C water = 4184 J / kg C C sand = 664 J / kg C Why Does Water Have a High Specific Heat??? water metal Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds and do not need as much energy to break them. Calculating Changes In Thermal Energy Q = m x T x C Q = change in thermal energy (J) m = mass of substance (kg) T = change in temperature (C) (Tf – Ti) C = specific heat of substance (J/kgC) Lets Do an Example: The air in a living room has a mass of 60.0kg and a specific heat of 1,020.0J/(kg x C). What is the change in thermal energy of the air when it warms from 20˚C to 25˚C? Calorimeter A calorimeter is used to help measure the specific heat of a substance. Section 2: Transferring Thermal Energy Conduction Conduction is the transfer of thermal energy by collisions between particles in matter. Conduction occurs because particles in matter are in constant motion. Example: The metal stick that the marshmallows are on heats up as it is near the flames. The thermal energy is transferred up the metal stick. Convection Convection is the transfer of thermal energy in a fluid by the movement of warmer and cooler fluid from place to place. More energetic particles collide with less energetic particles and transfer thermal energy. Radiation Radiation is the transfer of energy by electromagnetic waves. These waves can travel through space even when no matter is present. Example: Sun’s radiation warms Earth Example: When you sit near a fire, radiant energy warms you. Radiation Continued… • When radiation strikes a material, some of the energy is absorbed, some is reflected, and some may be transmitted through the material. Controlling Heat Flow • Almost all living things have special features that help them control the flow of heat. • For example, the Antarctic fur seal’s thick coat helps keep it from losing heat. This helps them survive in a climate in which the temperature is often below freezing. Controlling Heat Flow Continued… In the desert the scaly skin of the desert spiny lizard has just the opposite effect. It reflects the Sun’s rays and keeps the animal from becoming too hot. Insulators A material in which heat flows slowly is an insulator. Examples: wood, some plastics, fiberglass, and air. Materials like metals that are good conductors are poor insulators. Reducing Heat Flow A thermos bottle uses a vacuum and reflective surfaces to reduce the flow of heat into and out of the bottle. The vacuum prevents heat flow by conduction and convection. The reflective surfaces reduce the heat transfer by radiation. Section 3: Using Heat Heating Systems Forced-Air Systems Radiator Systems Air heated in a furnace and blows through pipes to individual rooms to heat a home. Closed metal container that contains water or steam. Thermal energy is transferred to surroundings by conduction, then by convection. Electric Heating Systems No central furnace. Electrically heated coils placed in floors and walls. Heat moves through room by convection. Solar Heating Passive Solar Heating Radiant energy from the sun is transferred to the room through windows. Active Solar Heating Uses solar collectors to absorb solar energy which heats water in pipes. A pump circulates the water to radiators throughout the house. Thermodynamics Thermodynamics- the study of how heat, thermal energy, and work are related. First Law of Thermodynamics: the temperature of a system can be increased by adding heat to the system, doing work to the system, or both. Example: rubbing your hands together to make them warm Second Law of Thermodynamics: it is impossible for teat to flow from a cool object to a warm object unless work is done. Converting Heat to Work Heat engine- a device that converts heat to work. It is impossible to build a device that converts heat completely into work! Example: A car’s engine converts chemical energy in gasoline to heat, then the engine transforms some of the thermal energy into work by rotating the car’s wheels. Only about 25% of the heat released by burning gasoline is converted into work. The rest is transferred to the engine’s surroundings. Heat Movers A refrigerator does work on the coolant in order to transfer heat from inside the refrigerator to the warmer air outside.