Chapter 12 Thermal Energy

Heat and Temperature Light

Law of Conservation of Energy? When rub your hands together What is the Net KE? –Zero, the motion is in opposite directions. What is the change is PE? –Zero, your hands are at the same height. You did work against friction, where did the energy go?

Thermal Energy Thermal Energy: The total internal Energy Internal Energy: The sum of the kinetic and potential energies of the internal motion of particles that make up an object.

Kinetic-molecular Theory All matter is made up of molecules and atoms Molecules are in constant motion Objects in motion have Kinetic Energy.

Kinetic-molecular Theory Molecules are in constant motion Objects in motion have Kinetic Energy. When particles get hotter, they move faster (e.g. higher Kinetic Energy) Heat Heat Applet

The Nature of Matter Molecules in liquids and gases move freely. Molecules in solids simply vibrate. This means that all molecules possess their own kinetic energy (KE), the energy of motion. SolidLiquid/Gas

Temperature Molecules move at different speeds. Now let’s add some heat? –Now they are all moving faster! Temperature relates to the average kinetic energy of the molecules in a substance. The higher the temperature, the faster the molecules move.

Temperature Scales -40  32  212  -40  00 100  FF CC Celsius –Water boils at 100 o C –Water at Freezes0 o C Fahrenheit –Water boils at 212 o F –Water Freezes at32 o F Kelvin –Absolute Zero0ºK –Water boils at 373 o K –Water Freezes at273 o K

Temperature Scales conversions Fahrenheit scale freezes 32 o above Celsius

Temperature Scales conversions Celsius scale freezes 32 o below Fahrenheit

Temperature Scales There are three major temperature scales that we will deal with. –The Fahrenheit Scale (°F) –The Celsius Scale (°C) –The Kelvin Scale (°K) Conversion formulas are shown below.

Sample Temperature Conversion A beaker of water at room temperature is measured to be at 21°C. What is the Fahrenheit and Kelvin temperature?

Temperature Scales conversions Example 1 Convert 45ºF to Celsius

Temperature Scales conversions Example 1 Convert 32ºC to Fahrenheit

Homework WS1 1-5 WS 1a 1-3

Absolute Zero The final temperature to note is absolute zero, 0K. This is the lowest possible temperature. Here molecules are in a complete state of rest, which means that there is no kinetic energy. Absolute zero has never been reached. However, scientists have come within 0.1K Pressure versus Temp graphs 0K0K

Thermal Contact KE transfer through thermal contact Thermal Equilibrium: –Both objects have the same average Kinetic Energy HOT COLD KE Warm WARM

What is Heat? Heat is energy in transfer from an object of higher temperature to one of lower temperature. The quantity of energy transfer from one object to another because of a difference in temperature. ColdHot Heat Flow Warm

Thermal Energy The sum of all the kinetic energies within a material is known as thermal energy. Both full cups of coffee are at the same temperature. –Which cup contains greater thermal energy? –Which cup contains a higher average kinetic energy within the molecules? A B B Both are the same!

Heat & Kinetic Energy Both Brass blocks are at 115 o C. Which block has the higher KE? Which block has the higher average KE? Brass

Homework WS #1 4-9 WS #1 4-13

Specific Heat Video Clip

Specific Heat Heat: The energy that flows as a result of a difference in temperature Q: The symbol for heat. Measured in Joules Specific Heat (C): The amount of energy needed to raise a unit of mass one temperature unit. (J/kg K)

Heat gained Q: Heat m:mass ΔT:Change in Temperature

Sample Heat Problem How much heat is required to raise the temperature of a 2.4kg gold ingot (c = 129J/kgK) from 23°C to 45°C? 2.4kg Light

Heat Transfer Example 1 A 0.40kg block of iron is heated from 295K to 325K. How much heat is absorbed by the iron?

Heat Transfer WS3 #1 How much heat is absorbed by 60.0g of copper when it is heated from 20ºC to 80ºC

Practice Problems WS 3a –#’s 1-3Specific heat

Calorimerty Calorimeter: A device used to measure changes in thermal energy. Calorimerty Utilizes on the conservation of energy. Q gained + Q lost = Zero

Calorimerty Example A.4kg block of is placed in.5kg of Find the final temperature.

Calorimerty Example A.1kg block of is placed in.2kg of Find the final temperature.

Practice Problems Book Page 252 –#’s 9-11Calorimetery WS4 –# 7-11Calorimetery WS #3 –Problem # 1 a-d –Problem #2 a-d Quiz

Latent Heat (Enthalpy) Describe what Q=mCΔT means Is it possible to add heat without changing the temperature? A heat transfer is also required to change phase. Heat must be added to go to a more energetic phase. Heat must be removed to go to a less energetic phase. Solid Vapo r Liquid +Q -Q

Substances in Phase Transfer 40  32  212  40  00 100  FF CC

Heats of Fusion and Vaporization Heat of fusion (H f ): the amount of heat energy required change 1kg of substance in the solid state into the liquid state. The equation is as follows: The heat of vaporization (H v ) is the amount of heat energy required to change 1kg of substance in the liquid state into the vapor state. The equation is as follows:

Sample Heating Across Phase A block of ice (m = 1.2kg) at -10°C being heated to water at 80°C. How much heat is used? These problems have 1-5 steps. Ice -10°CIce 0°CWater 0°C Water 80°C HeatingMeltingHeating

The Heating Curve (H 2 O) As H 2 O is heated from ice below freezing to steam above boiling, the temperature can be plotted with respect to time. Where are the flat spots located? Temperature remains constant during phase changes. Once each phase change is complete, temperature can rise again. Heat HeatingMeltingBoiling Heat

The Cooling Curve (H 2 O) The cooling curve is very similar to the heating curve What can you conclude about temperatures when cooling between phases? SolidifyingCoolingCondensing Cooling also requires the temperature to be held constant while in the midst of a phase change. The temperature can continue to drop after all of the substance is converted into the next phase. Cool

Phase Change WS7 #1 Between A-B: The ice is warming to 0 o C Between B-C: Thermal energy melts the ice at 0 o C Between C-D: The water is warm to 100 o C Between D-E: The water boils and changes to vapor at 100 o C After E:The temperature of the vapor increases A B C E D

Practice Problems WS #7 –# 1 b-g –# 2,3

Heat of fusion WS8 #1 If 5,000J is added to ice at 0 o C, how much ice is melted? Q=mL f Q=mL v Water H f =3.34x10 5 J/kg WaterH v =2.26x10 6 J/kg

Heat of fusion WS8 #2 How much heat must be transferred to 100g of ice at 0 0 C until the ice melts and the temperature of the resulting water rises to 20 0 C? Water H f =3.34x10 5 J/kg

Homework WS #8 –#’s 3-5 Book Page 255 –# 16 Heat of fusion WS #7 (5 heats)

Thermal Energy Transfer Conduction: –Transfer of Kinetic Energy by contact Convection: –Heat transfer by the motion of a fluid (e.g. air) Radiation: –Electromagnetic waves carry energy Note: Conduction and Convection require matter

Thermal Energy Transfer video Clip

Conduction Conduction is the transfer of heat through molecular collisions. This form of heat transfer best occurs in solids where molecules are closely packed. Materials that conduct heat well are called conductors. (Eg. metals such as copper and iron) Materials that conduct heat poorly are called insulators. (Eg. foam, air, and asbestos) Magnification

Convection Convection is the transfer of heat though moving fluids. A fluid is any substance that flows, which includes all liquids and gases. Examples include convection ovens and cloud formation. Convection ?

Have you ever watched a pot of water when it is being heated, especially while boiling? What do you notice? You will notice that there are currents within the pot. The Heating Water Pot The process of convection transfers heated water from the bottom of the pot to the top, where it is exchanged for cooler water.

Convection Gulf stream current

Convection It was remarkable how weightlessness affected things: I found that if I stayed perfectly still, my own body heat built up around me like a tenuous blanket, because there was no convection to carry it away. But my slightest motion dispelled this warmth and let me cool. –Jim Lovell, Apollo 13

Radiation (not radioactivity) Radiation is the transfer of heat via electromagnetic waves. These waves include visible light, but are mostly infrared. No matter is required for this type of heat transfer. Examples include the sun’s heat and warmth felt from a flame. Open Space ? Radiation

More on Radiation All objects emit heat in the form of radiation (radiant heat). Hotter objects emit more energetic waves. Some extremely hot objects can emit visible light, a form of electromagnetic radiation. In fact, all forms of radiation travel at the speed of light. Objects in thermal equilibrium will emit the same amount of radiation that they receive from other objects.

Heat Transfer Question Consider a camp fire burning vigorously. –How is heat normally transferred while warming its viewers? Radiation –How is heat transferred when you put a hand in the smoke? Convection –How is heat transferred to a stick when it is placed in the hot coals? Conduction Some situations involve multiple heat transfer types like this.

Homework WS #4 1-6 –Thermal Energy Transfer

Homework WS #2 –Temperature Conversions

Thermal Expansion The change in length of a material due to change in temperature.

Thermal Expansion Video Clip Expansion and Contraction movie clip.

Linear Expansion Most objects expand when heated and contract when cooled The change in length of a solid is proportional to ΔT The change in length is proportional to the length of the object

Linear Expansion animation Expansion Joint

Linear Expansion animation Expansion joint versus NO joint

Linear Expansion animation Expansion in a U joint

Linear Expansion animation Expansion in an elbow joint

Linear Expansion WS9 #1 A metal bar is 2.6m long at 21 0 C. The bar is heat to 93 0 C and the length increases by 3.4mm. What is the coefficient of linear expansion? Heat

Sample Linear Expansion Problem A train rail is initially 20m long in the morning when the temperature outside is 10°C. By how much will the rail expand in the heat of the day when the temperature reaches 35°C? The coefficient of linear expansion for steel is: Now find  L. What is the final length?

Linear Expansion WS9 #2 A piece of aluminum siding 3.66m long on a C winter day is how long on a hot 39 0 C summer day?

Practice Problems WS 9Linear Expansion –#’s 3-5

The Bimetallic Strip A bimetallic strip consists of two metal strips pressed together into a single strip. A common strip consists of steel and brass. Since different materials expand at different rates, the strip will bend. Room Temp Brass Steel High Temp Brass Steel Low Temp Brass Steel

The Bimetallic Strip (Cont.) Bimetallic strips are most often used in temperature sensitive instruments as thermostats. The strip is wound into a coil. As temperature changes, the coil expands or contracts to activate a switch controlling a heating/cooling system. Heat AC I’m Hot!!! I’m Cold!!

Expansion of Water Water initially at 10 0 C that cools to 4 0 C Water initially at 4 0 C that cools to 0 0 C Water initially at 0 0 C that freezes to 0 0 C ice Water initially at 10 0 C that cools to 4 0 C

Unique Properties of Water (Cont.) In liquid water, the molecules move freely in no particular order or array. This is what allows them to flow. When water freezes, the molecules form a hexagonal pattern. Why may ice possess less density? O H H O HH O H H O H H O H H O H H O H H O H H O HH O H H O H H O H H O H H O H H O H H O H H O H H O HH Liquid Water O H H O HH O HH O H H O H H O H H Solid Ice O H H O HH O HH O H H O H H O H H O H H O HH O HH O H H O H H O H H Open Space

Applications of Linear Expansion Linear expansion must be taken into consideration for engineers designing something that will experience a range of temperatures. Here are just a few select examples: RailroadsBridgesMetal Roofs

Practice Problems WS # Thermal Expansion

End Unit on Heat

E

40  32  212  40  00 100  FF CC

Change of State States of mater –Solid –Liquid –Gas

examples Examples –Temperature scales Examples –Convert Celsius to Fahrenheit –Convert Fahrenheit to Celsius Heat transfer types Calorimeter and specific heat Thermal expansion Examples –Conservation of energy transfer

40  32  212  40  00 100  FF CC

Thermal Energy

COLD HOT