Chapter 11. Heat & Temperature

Transcription

1 Chapter 11 Heat & Temperature

2 Heat & Temperature Both words are used interchangeably in day - to day conversation, but they have different scientific definitions. Temperature is a state or a number that decides the direction of heat flow. Heat is the energy transferred from one object to another because of a temperature difference between them.

3 Thermometers Used to measure the temperature of an object or a system Make use of physical properties that change with temperature Many physical properties can be used Volume of a liquid Length of a solid Pressure of a gas held at constant volume Volume of a gas held at constant pressure Electric resistance of a conductor Color of a very hot object

4 Thermometers, cont A mercury thermometer is an example of a common thermometer The level of the mercury rises due to thermal expansion Temperature can be defined by the height of the mercury column

5 Temperature Scales Thermometers can be calibrated by placing them in thermal contact with an environment that remains at constant temperature Environment could be mixture of ice and water in thermal equilibrium Also commonly used is water and steam in thermal equilibrium

6 Celsius Scale Temperature of an ice-water mixture is defined as 0º C This is the freezing point of water Temperature of a water-steam mixture is defined as 100º C This is the boiling point of water Distance between these points is divided into 100 segments or degrees

7 Kelvin Scale When the pressure of a gas goes to zero, its temperature is º C This temperature is called absolute zero This is the zero point of the Kelvin scale º C = 0 K To convert: T C = T The size of the degree in the Kelvin scale is the same as the size of a Celsius degree

8 Fahrenheit Scales Most common scale used in the US Temperature of the freezing point is 32º Temperature of the boiling point is 212º 180 divisions between the points

9 Comparing Temperature Scales

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11 Temperature Scales Fahrenheit Celsius Kelvin K C F In order to covert temperatures T 1.8T F C 32 T T K C 273

12 Questions And In class problems Convert the following Fahrenheit temp. to Kelvin : 120⁰ F -456 ⁰ F At what temperature is the Celsius and Fahrenheit value the same?

13 The Atomic Basis of Temperature Temperature is related to the random motion of atoms and molecules in a substance. Temperature is actually a measure of the magnitude of the average speed of atoms and molecules. Temperature is directly proportional with average translational kinetic energy of random molecular motion.

14 The Atomic Basis of Temperature Molecules may also rotate or vibrate, with associated rotational or vibrational kinetic energy but these motions are not translational and don t define temperature.

15 Check Point : True or False? Temperature is a measure of the total kinetic energy in a substance.

16 Answer: False, Temperature is a measure of the average translational kinetic energy of molecules in a substance. If you have 1L of boiling water A 2L of boiling water B Total kinetic energy in B=2 total kinetic energy in A They have the same temperature.

17 Thermal Expansion The length of a solid changes as temperature changes. The change in length is proportional to the change in temperature. L2 L1(1 T ) L 2 the length of the material at temp. T 2 L 1 the length of the material at temp. T 1 Coefficient of Linear Expansion

18 Example The coefficient of expansion for steel is per degree c. A particular bridge is 100 meters long when the temp. is 0 ⁰ C How long will that bridge be in the summer when the temp. climbs to 40 ⁰ C?

19 Solution L 2 L 1 (1 T ) L 2 100m C 40 C L m

20 Application In times past, railroad tracks were laid in 39- foot segments connected by joint bars, with gaps for thermal expansion. In summer months, the tracks expanded and the gaps were narrow. In winter, the gap widened.

21 Application Even in routine daily activities such as cooking, thermal stress is often is a serious consideration. Pyrex glass, for instance, is now often used to make dishes that can be placed directly into ovens because this material expands only about 1/3 as much as regular glass. It is therefore much less likely to crack when subjected to thermal stress inside the oven.

22 Applications of Thermal Expansion Bimetallic Strip Thermostats Use a bimetallic strip Two metals expand differently Since they have different coefficients of expansion

23 Application : Bimetal strip

24 More Applications of Thermal Expansion Pyrex Glass Thermal stresses are smaller than for ordinary glass Sea levels Warming the oceans will increase the volume of the oceans

25 Unusual Behavior of Water Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink. The liquid contracts as it is cooled; because the molecules are moving slower they are less able to overcome the attractive intermolecular forces drawing them closer to each other. Then the freezing temperature is reached, and the substance solidifies, which causes it to contract some more because crystalline solids are usually tightly packed.

26 Unusual Behavior of Water Water is one of the few exceptions to this behavior. When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%. This unusual behavior has its origin in the structure of the water molecule. There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position). The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very "open", as shown in the following picture:

27 Unusual Behavior of Water It is this open solid structure that causes ice to be less dense than liquid water. That is why ice floats on water, for which we should all be thankful because if water behaved "normally" many bodies of water would freeze solid in the winter, killing all the life within them. Water's "density maximum" is a product of the same phenomenon. Close to the freezing point, the water molecules start to arrange locally into ice-like structures. This creates some "openness" in the liquid water, which tends to decrease its density. This is opposed by the normal tendency for cooling to increase the density; it is at approximately 4 degrees Celsius that these opposing tendencies are balanced, producing the density maximum.

28 Unusual Behavior of Water As the temperature of water increases from 0ºC to 4 ºC, it contracts and its density increases Above 4 ºC, water exhibits the expected expansion with increasing temperature Maximum density of water is 1000 kg/m 3 at 4 ºC

29 Heat & Internal Energy Matter does not contain heat. Heat is energy transferred when there is a difference in temperature. Internal energy is the grand total of energies inside the substance; 1. Translational Kinetic Energy. 2. Rotational Kinetic Energy. 3. Kinetic Energy due to internal movement of atoms within molecules. 4. Potential energy due to the forces between molecules.

30 Heat & Internal Energy When a substance absorbs or gives off heat, the internal energy of the substance increases or decreases. When ice is melting, the added heat does not increase molecular kinetic energy, therefore no increase in temperature.

31 Heat & Internal Energy For two things in thermal contact, heat flows from the higher temperature substances to the lower temperature substances. This is not necessarily a flow from more internal to less internal

32 How much heat flows depends on Temperature difference T. Amount of material m. The substances used. Q m c T

33 Heat Units Jules(J) Calorie (c )(small c) :Amount of heat required to change the temperature of 1 gram of water by 1 Celsius degree.1 calorie=4.184 J Calorie (kilocalorie in fact)( C) energy rating : amount of heat required to change temperature of 1 kg of water by 1 Celsius degree.

34 Heat in Customary units (USA) The British thermal unit (BTU or Btu) is a traditional unit of energy equal to about 1.06 kilojoules. It is approximately the amount of energy needed to heat one pound of water one degree Fahrenheit. It is used in the power, steam generation, heating and air conditioning industries. A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63 F to 64 F 1 btu = joules

35 Check Point An iron thumbtack and a big bolt are removed from a hot oven. Both are red-hot and have the same temperature.when dropped into identical containers of water of equal temperature.which one raises the water temperature more?

36 Check your answer! The big iron bolt has more internal energy to impart to the water and warms it more than the thumbtack. Although they have the same initial temperature (the same average kinetic energy per molecule), the more massive bolt has more molecules and therefore more total energy-internal energy. This example underscores the difference between temperature and internal energy.

37 Specific Heat Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1 C The specific heat, c, of a substance is a measure of this amount c Q m T

38 Units of Specific Heat SI units J / kg C Historical units cal / g C

39 Heat and Specific Heat Q = m c ΔT ΔT is always the final temperature minus the initial temperature When the temperature increases, ΔT and ΔQ are considered to be positive and energy flows into the system When the temperature decreases, ΔT and ΔQ are considered to be negative and energy flows out of the system

40 A Consequence of Different Specific Heats Water has a high specific heat compared to land On a hot day, the air above the land warms faster The warmer air flows upward and cooler air moves toward the beach

41 Calorimeter One technique for determining the specific heat of a substance A calorimeter is a vessel that is a good insulator which allows a thermal equilibrium to be achieved between substances without any energy loss to the environment

42 Calorimetry Analysis performed using a calorimeter Conservation of energy applies to the isolated system The energy that leaves the warmer substance equals the energy that enters the water Q cold = -Q hot Negative sign keeps consistency in the sign convention of ΔT

43 Phase Changes A phase change occurs when the physical characteristics of the substance change from one form to another Common phases changes are Solid to liquid melting Liquid to gas Evaporation Phases changes involve a change in the internal energy, but no change in temperature

44 Latent Heat During a phase change, the amount of heat is given as Q = ±m L L is the latent heat of the substance Latent means hidden L depends on the substance and the nature of the phase change Choose a positive sign if you are adding energy to the system and a negative sign if energy is being removed from the system

45 Latent Heat, cont. SI units of latent heat are J / kg Latent heat of fusion, L f, is used for melting or freezing Latent heat of vaporization, L v, is used for boiling or condensing

46 Sublimation Some substances will go directly from solid to gaseous phase Without passing through the liquid phase This process is called sublimation There will be a latent heat of sublimation associated with this phase change

47 Graph of Ice to Steam

48 Conduction The transfer can be viewed on an atomic scale It is an exchange of energy between microscopic particles by collisions Less energetic particles gain energy during collisions with more energetic particles Rate of conduction depends upon the characteristics of the substance

49 Conduction example The molecules vibrate about their equilibrium positions Particles near the stove coil vibrate with larger amplitudes These collide with adjacent molecules and transfer some energy Eventually, the energy travels entirely through the pan and its handle

50 Conduction, cont. In general, metals are good conductors They contain large numbers of electrons that are relatively free to move through the metal They can transport energy from one region to another Conduction can occur only if there is a difference in temperature between two parts of the conducting medium

51 Convection Energy transferred by the movement of a substance When the movement results from differences in density, it is called natural conduction When the movement is forced by a fan or a pump, it is called forced convection

52 Convection example Air directly above the flame is warmed and expands The density of the air decreases, and it rises The mass of air warms the hand as it moves by

53 Convection applications Boiling water Radiators Upwelling Cooling automobile engines Algal blooms in ponds and lakes

54 Radiation Radiation does not require physical contact All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules

55 Radiation example The electromagnetic waves carry the energy from the fire to the hands No physical contact is necessary Cannot be accounted for by conduction or convection

56 Ideal Absorbers An ideal absorber is defined as an object that absorbs all of the energy incident on it This type of object is called a black body An ideal absorber is also an ideal radiator of energy

57 Ideal Reflector An ideal reflector absorbs none of the energy incident on it

58 Applications of Radiation Clothing Black fabric acts as a good absorber White fabric is a better reflector Thermography The amount of energy radiated by an object can be measured with a thermograph Body temperature Radiation thermometer measures the intensity of the infrared radiation from the eardrum

59 Resisting Energy Transfer Dewar flask/thermos bottle Designed to minimize energy transfer to surroundings Space between walls is evacuated to minimize conduction and convection Silvered surface minimizes radiation Neck size is reduced

60 Global Warming Greenhouse example Visible light is absorbed and re-emitted as infrared radiation Convection currents are inhibited by the glass Earth s atmosphere is also a good transmitter of visible light and a good absorber of infrared radiation