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Lots of notes and discussion today

Thermodynamics

Chapter 12

Heat, energy and temperature in our lives…

• Deciding what you are going to wear during the day and at night when you go to bed. How many covers you will put on you at night, in order to maintain the right temperature.

• Most homes, schools & work have heating and cooling systems which control the temperature so that the temperature is just right.

• Some people install fans or use portable fans in homes to keep comfortable in the summer. Also, people buy portable heaters and generators to keep warm in the winter.

• Most of us have cars that are equipped with heating and air conditioning systems; some may even have a meter that registers the indoor and outdoor temperatures.

• Many of us watch and listen to weather reports so that you can make a decision on what to wear.

• You probably remember mom or dad sticking a thermometer under your tongue (or elsewhere) to get your body temperature to see if you had a fever.

Our bodies are very sensitive to hot and cold…

• We learn very early in life that we shouldn’t touch a hot pan on the stove or a hot light bulb in a lamp.

• We also learn to be careful about tasting hot foods or drinks.

• We all know the experience of sweating, our bodies built-in mechanism of cooling us down when temperatures begin to rise.

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.

Thermal Equilibrium• When the flow of energy from one body

into another equals the flow of energy in the reverse directions, the objects are in thermal equilibrium. They both have the same temperature.

• Thermal energy always travels spontaneously from hot to cold.

Remember this???

• Thermal energy moves in a predictable pattern from warmer to cooler until all the substances attain the same temperature such as an ice cube melting.

You learned this in the 6th grade!!!

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 simply 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.

Temperature• Temperature is a measure

of the average kinetic energy of the molecules which make up a substance.

-The temperature of a substance will increase/decrease if the average kinetic energy of its particles is increased/ decreases.-The SI unit of temperature is the kelvin (K). One kelvin equals one degree Celsius. Zero degrees Celsius equals 273.15 K.

Temperature• Central concept of thermodynamics is

temperature.– Our “temperature sense” is often unreliable. – 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.

Day 2

• 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 of mass of the substance by one degree of temperature.

Specific heat

Specific Heat is the measure of how much thermal energy

is required to change the temperature of a substance.

Heat = (mass)x specific x

change Energy heat in temp

OR Q = m C T

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(TcmQ

ΔTcmQ

if

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 = m C ΔT

ΔT = 41oC – 1oC = 40oC

32 J = (4.0 kg) C (40oC)

32 J = (160 kg oC) C

C = 0.2 J/kgoC

Day 3 of note taking

• (S)More notes today then practice problems

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

Laws of Thermodynamics

First Law of Thermodynamics

• The first law of thermodynamics states that the total increase in the energy of a system is equal to the sum of the heat added to the system and the work performed on the system

• ---This is merely a restatement of the law of conservation of energy.

2nd Law of Thermodynamics

The second law of thermodynamics states that the entropy of the universe always increases.

Entropy is the measure of the amount of disorder of a system.

Physics Textbook reading

Figure 12-16

The spontaneous mixing of the food coloring and water is an example of the

2nd law of thermodynamics

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).

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

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.