HEAT

TEMPERATURE is a measure of the average kinetic energy per molecule TEMPERATURE is a measure of the average kinetic energy per molecule. The infrared radiation coming from the air canal in the ear passes through the optical system of the thermometer and is converted to an electrical signal that gives a digital reading of body temperature.

Temperature Temperature is related to the kinetic activity of the molecules, whereas expansion and phase changes of substances are more related to potential energy. Although not true in all cases, a good beginning is to define temperature as the average kinetic energy per molecule.

Temperature vs. Internal Energy The large pitcher and the small one have the same temperature, but they do not have the same thermal energy. A larger quantity of hot water melts more of the ice.

Temperature Equilibrium Heat is defined as the transfer of thermal energy that is due to a difference in temperature. Thermal Equilibrium Hot Coals Insulated Container Two objects are in thermal equilibrium if and only if they have the same temperature. Cool Water Same Temperature

Thermometer A thermometer is any device which, through marked scales, can give an indication of its own temperature. T = kX X is thermometric property: Expansion, electric resistance, light wavelength, etc.

Limitations of Relative Scales The most serious problem with the Celsius and Fahrenheit scales is the existence of negative temperatures. -250C ? Clearly, the average kinetic energy per molecule is NOT zero at either 00C or 00F! T = kX = 0 ?

Comparison of Four Scales 1 C0 = 1 K 1000C 00C -2730C Celsius C 273 K 373 K Kelvin 0 K K Fahrenheit 320F -4600F 2120F F Rankine 0 R 460 R 672 R R ice steam 5 C0 = 9 F Absolute zero TK = tC + 2730

Expansion is the same in all directions (L, W, and H), thus: Volume Expansion Expansion is the same in all directions (L, W, and H), thus: The constant b is the coefficient of volume expansion.

Photo © Vol. 05 Photodisk/Getty FOUNDRY: It requires about 289 Joules of heat to melt one gram of steel. In this PowerPoint, we will define the quantity of heat to raise the temperature and to change the phase of a substance.

Heat Defined as Energy Heat is not something an object has, but rather energy that it absorbs or gives up. The heat lost by the hot coals is equal to that gained by the water. Hot coals Cool water Thermal Equilibrium

Units of Heat One calorie (1 cal) is the quantity of heat required to raise the temperature of 1 g of water by 1 C0. 10 calories of heat will raise the temperature of 10 g of water by 10 C0. Example

Units of Heat (Cont.) One British Thermal Unit (1 Btu) is the quantity of heat required to raise the temperature of 1 lb of water by 1 F0. 10 Btu of heat will raise the temperature of 10 lb of water by 10 F0. Example

Comparisons of Heat Units: The SI Unit of Heat Since heat is energy, the joule is the preferred unit. Then, mechanical energy and heat are measured in the same fundamental unit. Comparisons of Heat Units: 1 cal = 4.186 J 1 Btu = 778 ft lb 1 kcal = 4186 J 1 Btu = 252 cal 1 Btu = 1055 J

Temperature and Quantity of Heat The effect of heat on temp- erature depends on the quantity of matter heated. 200 g 600 g 200C 220C 300C The same quantity of heat is applied to each mass of water in the figure. The larger mass experiences a smaller increase in temperature.

Quiz 1 Two objects are made of the same material, but have different masses and temperatures. If the objects are brought into thermal contact, which one will have the greater temperature change? (A) the one with the higher initial temperature (B) the one with the lower initial temperature (C) the one with the greater mass (D) the one with the smaller mass (E) the one with the higher specific heat Pre-Lecture Quiz 14

Heat Capacity Lead Glass Al Copper Iron 37 s 52 s 60 s 83 s 90 s The heat capacity of a substance is the heat required to raise the temperature a unit degree. Lead Glass Al Copper Iron 1000C 1000C 1000C 1000C 1000C 37 s 52 s 60 s 83 s 90 s Heat capacities based on time to heat from zero to 1000C. Which has the greatest heat capacity?

Conservation of Energy Whenever there is a transfer of heat within a system, the heat lost by the warmer bodies must equal the heat gained by the cooler bodies:  (Heat Losses) =  (Heat Gained) Hot iron Cool water Thermal Equilibrium

Change of Phase When a change of phase occurs, there is only a change in potential energy of the molecules. The temperature is constant during the change. Solid Liquid Gas Q = mLf Q = mLv fusion Vaporization Terms: Fusion, vaporization, condensation, latent heats, evaporation, freezing point, melting point.

Change of Phase The latent heat of fusion (Lf) of a substance is the heat per unit mass required to change the substance from the solid to the liquid phase of its melting temperature. For Water: Lf = 80 cal/g = 333,000 J/kg The latent heat of vaporization (Lv) of a substance is the heat per unit mass required to change the substance from a liquid to a vapor at its boiling temperature. For Water: Lv = 540 cal/g = 2,256,000 J/kg

First, let’s review the process graphically as shown: Example 3: How much heat is needed to convert 10 g of ice at -200C to steam at 1000C? First, let’s review the process graphically as shown: temperature t ice steam steam and water 540 cal/g steam only 1000C water only 1 cal/gC0 ice and water 80 cal/g 00C ice cice= 0.5 cal/gC0 -200C Q

Q1 = (10 g)(0.5 cal/gC0)[0 - (-200C)] Example 3 (Cont.): Step one is Q1 to convert 10 g of ice at -200C to ice at 00C (no water yet). -200C 00C Q1 to raise ice to 00C: Q1 = mcDt t Q1 = (10 g)(0.5 cal/gC0)[0 - (-200C)] 1000C Q1 = (10 g)(0.5 cal/gC0)(20 C0) Q1 = 100 cal 00C ice cice= 0.5 cal/gC0 -200C Q

Example 3 (Cont.): Step two is Q2 to convert 10 g of ice at 00C to water at 00C. Melting Q2 to melt 10 g of ice at 00C: Q2 = mLf t Q2 = (10 g)(80 cal/g) = 800 cal 1000C Q2 = 800 cal Add this to Q1 = 100 cal: 900 cal used to this point. 80 cal/g ice and water 00C -200C Q

Step three is Q3 to change 10 g of water at 00C to water at 1000C. 00C to 1000C Q3 to raise water at 00C to 1000C. Q3 = mcDt ; cw= 1 cal/gC0 t Q3 = (10 g)(1 cal/gC0)(1000C - 00C) 1000C water only 1 cal/gC0 Q3 = 1000 cal Total = Q1 + Q2 + Q3 = 100 +900 + 1000 = 1900 cal 00C -200C Q

Step four is Q4 to convert 10 g of water to steam at 1000C? (Q4 = mLv) vaporization Q4 to convert all water at 1000C to steam at 1000C. (Q = mLv) Q4 = (10 g)(540 cal/g) = 5400 cal 1000C 5400 cal Total Heat: 800 cal 1000 cal 100 cal 7300 cal steam and water 00C ice and water water only -200C ice Q

Quiz 2 1 kg of water at 100 oC is poured into a bucket that contains 4 kg of water at 0 oC. Find the equilibrium temperature (neglect the influence of the bucket). (A) 0 oC (B) 20 oC (C) 50 oC (D) 80 oC (E) 100 oC Pre-Lecture Quiz 14

TRANSFER OF HEAT is minimized by multiple layers of beta cloth TRANSFER OF HEAT is minimized by multiple layers of beta cloth. These and other insulating materials protect spacecraft from hostile environmental conditions. (NASA)

Heat Transfer by Conduction Conduction is the process by which heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move. Conduction Direction From hot to cold.

Quiz 3 Given your experience of what feels colder when you walk on it, which of the surfaces would have the highest thermal conductivity? (A) a rug (B) a steel surface (C) a concrete floor (D) has nothing to do with thermal conductivity Pre-Lecture Quiz 14

Heat Transfer by Convection Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid. Heated fluid rises and is then replaced by cooler fluid, producing convection currents. Convection Convection is significantly affected by geometry of heated surfaces. (wall, ceiling, floor)

Heat Transfer by Radiation Radiation is the process by which heat energy is transferred by electromagnetic waves. Radiation Sun Atomic No medium is required !

Summary: Heat Transfer Conduction: Heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move. Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid. Radiation is the process by which heat energy is transferred by electromagnetic waves.

Examples of Thermal Conductivity Comparison of Heat Currents for Similar Conditions: L = 1 cm (0.39 in.); A = 1 m2 (10.8 ft2); Dt = 100 C0 2050 kJ/s 4980 Btu/h Aluminum: 3850 kJ/s 9360 Btu/h Copper: Concrete or Glass: 8.00 kJ/s 19.4 Btu/h 0.400 kJ/s 9.72 Btu/h Corkboard:

THERMODYNAMICS Thermodynamics is the study of energy relationships that involve heat, mechanical work, and other aspects of energy and heat transfer. Central Heating

Zeroth Law of Thermodynamics The Zeroth Law of Thermodynamics: If two objects A and B are separately in equilibrium with a third object C, then objects A and B are in thermal equilibrium with each other. A Object C A B Thermal Equilibrium Same Temperature B Object C

A THERMODYNAMIC SYSTEM A system is a closed environment in which heat transfer can take place. (For example, the gas, walls, and cylinder of an automobile engine.) Work done on gas or work done by gas

INTERNAL ENERGY OF SYSTEM The internal energy U of a system is the total of all kinds of energy possessed by the particles that make up the system. Usually the internal energy consists of the sum of the potential and kinetic energies of the working gas molecules.

P.E.=mgh Joule = newton/meter Quiz 4 The water flowing over Niagara Falls drops a distance of 50 m. Assuming that all the gravitational energy is converted to thermal energy, by what temperature does the water rise? (A) 0.10 C° (B) 0.12 C° (C) 0.37 C° (D) 0.42 C° P.E.=mgh Joule = newton/meter Heat 14 (13 of 42)

TWO WAYS TO INCREASE THE INTERNAL ENERGY, U. WORK DONE ON A GAS (Positive) HEAT PUT INTO A SYSTEM (Positive)

TWO WAYS TO DECREASE THE INTERNAL ENERGY, U. Wout hot HEAT LEAVES A SYSTEM Q is negative Qout hot -U Decrease WORK DONE BY EXPANDING GAS: W is positive

THE FIRST LAW OF THERMODYAMICS: The net heat put into a system is equal to the change in internal energy of the system plus the work done BY the system. Q = U + W final - initial) Conversely, the work done ON a system is equal to the change in internal energy plus the heat lost in the process.

HEAT ENGINES Qhot Wout Qcold Absorbs heat Qhot Performs work Wout A heat engine is any device which through a cyclic process: Cold Res. TC Engine Hot Res. TH Qhot Wout Qcold Absorbs heat Qhot Performs work Wout Rejects heat Qcold

THE SECOND LAW OF THERMODYNAMICS Wout Cold Res. TC Engine Hot Res. TH Qhot Qcold It is impossible to construct an engine that, operating in a cycle, produces no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work. Not only can you not win (1st law); you can’t even break even (2nd law)!

Heat will flow spontaneously from a hot object to a cold object. The Second Law of Thermodynamics The second law of thermodynamics is a statement about which processes occur and which do not. There are many ways to state the second law; here is one: Heat will flow spontaneously from a hot object to a cold object. It will not flow spontaneously from a cold object to a hot object.

EFFICIENCY OF AN ENGINE The efficiency of a heat engine is the ratio of the net work done W to the heat input QH. Cold Res. TC Engine Hot Res. TH QH W QC e = = W QH QH- QC e = 1 - QC QH

REFRIGERATORS Qhot Win Win + Qcold = Qhot Qcold WIN = Qhot - Qcold A refrigerator is an engine operating in reverse: Work is done on gas extracting heat from cold reservoir and depositing heat into hot reservoir. Cold Res. TC Engine Hot Res. TH Qhot Qcold Win Win + Qcold = Qhot WIN = Qhot - Qcold

Entropy

a state of greater disorder. Entropy Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law: Natural processes tend to move toward a state of greater disorder. Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet. No amount of stirring will get the milk and sugar to come back out of solution.

Entropy Another example: when a tornado hits a building, there is major damage. You never see a tornado approach a pile of rubble and leave a building behind when it passes. Another consequence of the second law: In any natural process, some energy becomes unavailable to do useful work.