HEAT Obj 5 6 a and 6 b

HEAT

• Obj. 5 -6 a and 6 b • Describe the law of conservation of energy • Investigate and demonstrate the movement of heat through solids, liquids, and gases by convection, conduction, and radiation.

Temperature & Kinetic Energy • The temperature of a substance will increase if the average kinetic energy of its particles is increased. • If the average kinetic energy of particles decreases, so does the temperature of the substance.

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

Key Words to Look For • Conduction - If the word touch or contact is used, then the method of transfer is conduction. • Convection - If the words air, gas, water, liquid, or fluid are used, it is convection. • Radiation – If the question or answers mention sun or sunlight, radiation is the method of transfer.

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

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.

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 slowermoving neighbors, they transfer some 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.

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.

• 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.

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.

Heat Transfer by 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.

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

Conductors and Insulators • Conductors readily transfer heat energy. – Gases are the poorest conductors – Solids are the best conductors • Insulators are poor conductors of heat energy. – Fiberglass batting is better than air alone because it traps air.

Law of Heat Exchange • For a closed system in which heat energy cannot enter or leave, the heat lost by objects at a higher temperature is equal to the heat gained by objects at lower temperature until thermal equilibrium is reached (at which point the final temperature of both objects is the same). • The final temperature will be somewhere between the initial low temperature and the initial high temperature.

Heats of Transformation • When energy is absorbed as heat by a solid or liquid, the temperature of the object does not necessarily rise. • The thermal energy may cause the mass to change from one phase, or state, to another.

Phase Changes

Thermal Expansion of Solids • Solids expand when heated and contract when cooled (with a few exceptions). – Heated solids increase or decrease in all dimensions (length, width, and thickness). – When a solid is heated, the increase in thermal energy increases the average distance between the atoms and molecules of the solid and it expands.

Thermal Expansion of Solids • Thermal expansion can be explained on a molecular basis. • Picture the interatomic forces in a solid as springs, as shown in the picture on the right. • Each atom vibrates about its equilibrium position. When the temperature increases, the amplitude and associated energy of the vibration also increase.

Examples of Uses of Thermal Expansion • • Dental materials used for fillings must be matched in their thermal expansion properties to those of tooth enamel, otherwise consuming hot drinks or cold ice cream would be painful. In aircraft manufacturing, rivets and other fasteners are often cooled using dry ice before insertion and then allowed to expand to a tight fit.

• You can loosen a tight metal jar lid by holding it under a stream of hot water. Both the metal of the lid and the glass of the jar expand as the hot water adds energy to their atoms. With the added energy, the atoms can move a bit farther from each other than usual, against the interatomic forces that hold every solid together. However, because the atoms in the metal move farther apart than those in the glass, the lid expands more than the jar and is loosened. • Expansions slots are often placed in bridges to accommodate roadway expansion on hot days. This prevents buckling of the roadway. Driveways and sidewalks have expansion slots for the same reason.

• • Abnormal Expansion of Water When water freezes the formation of hydrogen bonds in the ice crystals causes the ice to expand. Increase the temperature of any common liquid and it will expand. Water at the temperature of melting ice, 0 C contracts when the temperature is increased. As the water is heated and its temperature rises, it continues to contract until it reaches a temperature of 4 C. With further increase in temperature, the water then begins to expand the expansion continues all the way to the boiling point, 100 C.

Heating systems • Heating systems – transfer energy to raise the temperature. – Food energy is transferred as heat by blood throughout the body. – Air or water transfers heat in home heating systems.

Cooling systems • Cooling systems – transfer energy out of an object to lower its temperature. – Evaporation absorbs energy from surroundings(cools house) – Condensation releases energy to surroundings(warms outside)

Colors and Heat • Black absorbs all colors of the visible spectrum and will therefore heat up more. It is used at times when heat must be collected such as solar heaters, etc • White reflects all colors of the visible spectrum, items painted white will not heat up as quickly

Heat moves by conduction in solids since the particles are close together and vibrate. . . Heat convection occurs in gases and liquids. Heat convection does not occur in solids because solids are unable to — A absorb heat by vibrating B transfer heat by fluid motion C emit radiation by reflecting light D exchange heat by direct contact Solids do radiate heat to their surroundings

The primary way liquids and gases transmit heat is by the process of — F reflection G conduction H radiation J convection Fluid heat movement is convection. Fluid motion occurs in liquids and gases.