12 Weeks to TAKS

Week 4

• Obj. 5-6a and 6b

• 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 and Heat• 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 temperature of a hot cup of coffee left sitting on the table will fall until it also reaches thermal equilibrium with the air temperature in the room.

• The change in temperature is due to the transfer of energy between object and the environment.

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.

• (a) Q is negative when heat energy is transferred to the environment from the system.

• (b) Q = 0 J when the transfer of heat energy between the system and the environment is equal.

• (c) Q is positive when heat energy is transferred to a system from the environment.

Temperature• Central concept of thermodynamics is

temperature.– Our “temperature sense” is often

unreliable. The same quantity of thermal energy in different bodies does not give each the same temperature.

– On a cold winter day, an iron railing seems much colder to the touch than a wooden fence post, even though both are at the same temperature. This error in perception results because the iron removes energy from our fingers more quickly than the wood does.

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.

Temperature• Temperature is a measure of

the average kinetic energy of all the particles within an object.

• Thermometer is a device that measures temperature.– Thermometers are based on

the principle that liquids expand when their temperature increases and contract when their temperature falls.

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

A Conduction transfers energy as heat along the wire and into the handB. 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 slower-moving 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 theobject. Thus the energy ofthermal motion is transferred bymolecular collision along the object.Good thermal conductors such assilver, copper, aluminum, and goldare 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

Radiation is a more rapid transfer ofthermal energy in the form of electromagneticradiation accomplished by a process thatrequires neither contact nor mass flow. A hot object also loses heat energy byradiation. This radiation is similar to lightand can pass through empty space. Thewarmth you fell when you warm yourself bya fire is due to this radiation. If theobject is hot enough, some of the radiationis visible and can indeed be seen.

What type of heat transfer is shown in the following pictures?

A. B.

C.

D.

Convection

Radiation

Radiation

Conduction

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.

• 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.

Define: Specific Heat

•Amount of energy required to raise the temperature of 1g by 1oC

•Energy =(mass)x specific x change heat in temp

OR•Q = m c t

Specific Heat Capacity c

• To calculate the specific heat capacity c of a solid or liquid you will use this equation:

• Unit: or

if TTmQ

ΔTmQ

c

Ckg

Jo Cg

calo

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.

)T(TcmΔTcmQ if

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.

Law of Heat Exchange

• Conservation of Energy:Q lost = Q gained

• To avoid problems with signs, for • Q lost = Q gained problems,

• it is best to make T = Thi – Tlo

objecttemphighlohi

objecttemplowlohi

TTcm

TTcm

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.

• The amount of energy per unit mass that must be transferred as heat when a mass undergoes a phase change is called the heat of transformation, L.

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.

Examples of Uses of Thermal Expansion

•

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 4C.

• With further increase in temperature, the water then begins to expand and 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)