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What is Thermodynamics? We can break the word thermodynamics into two parts. –Thermo – meaning heat –Dynamics – meaning change So thermodynamics is the.

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Presentation on theme: "What is Thermodynamics? We can break the word thermodynamics into two parts. –Thermo – meaning heat –Dynamics – meaning change So thermodynamics is the."— Presentation transcript:

1 What is Thermodynamics? We can break the word thermodynamics into two parts. –Thermo – meaning heat –Dynamics – meaning change So thermodynamics is the study of how heat changes. (there is no point in looking at heat that does not change) Therefore, we first must ask what is heat?

2 What is Heat? Heat is produced every time work is done. –Mechanical, biological, all work produces heat. For example, all real machines that work use more energy (Work input) than they do for you (Work out put). –Work Input > Work Output – No Machine is 100% efficient. So energy must be wasted. But we know that energy must be conserved!! –This is were heat comes in.

3 Heat is Lost Energy If a person uses 10,000J of electrical energy to run an electrical motor to lift a box, and the motor in the end only gives the box 7,000J of potential energy, what happened to the missing 3,000J? As the motor was running, what happened to the temperature of the motor? The temperature got hotter because the missing 3,000J of mechanical energy became heat. Work input =Work output + Heat

4 What is Heat Part 2 This Heat is the mechanical energy that is transformed into energy of a different type Called Thermal Energy (energy that affects temperatures. Heat is really the flow of energy. –When objects cool down they releases energy (lose heat) –When objects warm up they absorb energy (gain heat) –If the objects temperature is constant it does not lose or gain energy (No heat) In short heat is the thermal energy that moves in or out of an object.

5 Thermal Energy and Mechanical Energy As the electrical motor runs, the molecules that make up the motor begin to vibrate more and more kinetic energy. This increase in kinetic energy of the molecule is the result of the work done by friction inside the motor. As the kinetic energy of the molecules increase, the heat of the motor increases. So heat is basically the kinetic energy of vibrating molecules. Since this energy is inside the object and we can not use it we call this internal energy

6 Internal Energy Internal Energy in NOT heat. Internal energy is the sum of the kinetic energies of an objects molecules Heat is the amount of energy that flows into or out of an object changing that objects internal energy. Internal energy is the energy your molecules have. Heat is the change in that energy.

7 Sample Problem A block rests on the top of an incline, and it initially has 100 J of potential energy. It slides down the incline, but in the end only achieves 90 J of kinetic energy when it reaches the bottom. How the combine internal energy of the block and incline increase? 100 J of Mechanical Energy (E pg ) E pg EKEK U 100 J 90 J of Mechanical Energy (E K ) E pg EKEK U 90 J 10 J of energy must of be transformed from the blocks mechanical energy to the system s internal energy. 10 J

8 The Units of Heat Well, since HEAT IS A TRANSFER OF ENERGY!!! The units for heat should be the units of energy!!! However, since we deal with large amounts of energy, we have several units we can use, depending on the situation and use. Units of Heat: –Joule (J) –calorie (cal) [Science Unit] 1 cal = J –Calorie (Cal) [Dietary Calorie] 1 Cal = 4186 J –British Thermal Unit (BTU) [Engineering Unit] 1 BTU = 1055 J

9 Internal Energy and Temperature As objects molecules increase their motion (E K ), their temperature tends to increase, as things lose their motion (E K ), their temperature tends decrease. Temperature is related to the average kinetic energy of one Molecule –Internal Energy is the total sum of all the molecule's kinetic energy Temperature and Internal Energy are directly related (it temp is measured in Kelvin) But the are not the same.

10 Thermal Expansion Since the molecules that makes an object gain E K when they are heated, each molecule requires a little more space to move around in. This causes objects to expand in size when they are heated. The increases tend to be very small, but the force causing this change is huge (and sometimes disastrous)

11 Linear Expansion L TiTi TfTf LoLo LfLf L = (Original Length)(coefficient of thermal Expansion)(Change in Temp) L = (L o )( )( T)

12 Coefficient of Thermal Expansion Represents how easily a material will expand when heated (contract when cooled) Tends to be very small (10 -6 magnitude) Units: (C O ) -1 C O means a change in temperature O C means a given temperature

13 L f = (L o )+(L o )( )( T) L L Expansion of an Area A = A f - A o = [ (L o )+(L o )( )( T)] 2 - (L o ) 2 A = [ (L o ) 2 +2(L o )(L o )( )( T)+ {(L o )( )( T)} 2 ]- (L o ) 2 A = 2(L o ) 2 ( )( T)+ (L o ) 2 ( ) 2 ( T) 2 ( ) 2 is so small ( ) we round to 0 A = 2(L o ) 2 ( )( T) A = 2(A o )( )( T) AoTiAoTi AfTfAfTf A f = [ (L o )+(L o )( )( T)] 2

14 Expansion of a Volume L L L V = V f - V o = [ (L o )+(L o )( )( T)] 3 - (L o ) 3 V = [ (L o )+(L o )( )( T)] [ (L o ) 2 +2(L o )(L o )( )( T)+ {(L o )( )( T)} 2 ]- (L o ) 3 V = [ (L o )+(L o )( )( T)] [ (L o ) 2 +2(L o ) 2 ( )( T)+ (L o ) 2 ( ) 2 ( T) 2 ]- (L o ) 3 V = [ (L o ) 3 +2(L o )(L o ) 2 ( )( T)+ (L o )(L o ) 2 ( )( T)+ {(L o ) 3 ( ) 2 ( T) 2 } ]- (L o ) 3 V = [ (L o ) 3 +3(L o ) 3 ( )( T) ]- (L o ) 3 V = 3(V o )( )( T) 0 0

15 Equations of Thermal Expansion Length (1 Dimension): L = (L o )( )( T) Area (2 Dimensions): A = 2(A o )( )( T) Volume (3 Dimensions): V = 3(V o )( )( T)

16 Expansion of Fluids Since fluids have no shape of their own (only volume) they do not have a coefficient of linear expansion ( ), instead they have a confect of volume expansion ( ) For solids is approximately 3( ) V = 3(V o )( )( T) V = (V o )( )( T)

17 If Everything Expands Does Nothing Expand as Well? Let us say that we have a sheet of metal that has a hole cut out of. If we heat up that entire sheet we know the sheet will expand in size, but what will happen to the hole? Will it become larger, smaller, or be unaffected?

18 Again the molecules when heated require more room for move around. This means that the molecules want to move farther apart For the hole to become smaller the molecules of the sheet around the hole must move closer together. And they dont want to do that!!!!!

19 So Nothing Expands, Just Like Everything Else. Important note: Since the hole expands because of the molecules of the sheet of metal. The hole expands just as if it were the metal itself. The molecules of the sheet around the hole must spread apart. This makes the hole bigger.


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