Lecture 22. Temperature and Heat

Transcription

1 Lecture 22 Temperature and Heat

2 Today s Topics: 0 th Law of Thermodynamics Temperature Scales Thermometers Thermal Expansion Heat, Internal Energy and Work Heat Transfer

3 Temperature and the Zeroth Law of Thermodynamics Definition of heat: Heat is the energy transferred between objects because of a temperature difference. Objects are in thermal contact if heat can flow between them. When the transfer of heat between objects in thermal contact ceases, they are in thermal equilibrium.

4 The Zeroth Law of Thermodynamics The zeroth law of thermodynamics: If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with object B, then objects A and C will be in thermal equilibrium if brought into thermal contact. Temperature is the only factor that determines whether two objects in thermal contact are in thermal equilibrium or not.

5 Temperature Scales Temperatures are reported in degrees Celsius or degrees Fahrenheit.! 1 C = 9 5 F!

6 The Kelvin Scale Kelvin temperature T = T c Absolute Zero (K) = o C

7 How do you get that and what is the significance of 0K? A constant-volume gas thermometer. Different low-density gases extrapolate back to C

8 Thermometers Thermometers make use of the change in some physical property with temperature. A property that changes with temperature is called a thermometric property. The voltage depends upon the difference in temperature at the two junctions.

9 Thermal Expansion Yet other thermometers make use of the thermal expansion of solids or liquids with temperature D L a L o T coefficient of linear expansion 1 -! Common unit for the Coefficient of Linear Expansion: ( ) 1 C! = C

10 Linear Thermal Expansion D L = a L o D T

11 Example: Thermal Expansion The coefficient of linear expansion of steel is 12 x 10 6 /C. A railroad track is made of individual rails of steel 1.0 km in length. By what length would these rails change between a cold day when the temperature is 10 C and a hot day at 30 C? In the design of structures. The thermal expansion of materials must be considered! Thermal expansion can introduce stresses and lead to structural failure

12 Linear Thermal Expansion D L = a L o D T THE BIMETALLIC STRIP DEMO: Bimetallic strip

13 Volume Thermal Expansion The volume of an object changes when its temperature changes: DV = b VoDT coefficient of volume expansion 1 -! Common Unit for the Coefficient of Volume Expansion: ( ) 1 C! = C For most solids, β 3α

14 The unusual (and critical) properties of water Water also expands when it is heated, except when it is close to freezing; it actually expands when cooling from 4 C to 0 C. This is why ice floats.

15 Heat and Internal Energy Heat is energy that flows from a highertemperature object to a lower-temperature object because of a difference in temperatures. SI Unit of Heat: joule (J) The heat that flows from hot to cold originates in the internal energy of the hot substance. The internal energy, in turn, is related to the kinetic energy of the molecules of the substance

16 Heat and Mechanical Work One kilocalorie (kcal) is defined as the amount of heat needed to raise the temperature of 1 kg of water from 14.5 C to 15.5 C. Through experiments, it was possible to find the mechanical equivalent: This is the energy required to heat 1 lb of water from 63 F to 64 F.

17 Convection Convection is the process in which heat is carried from one place to another by the bulk movement of a fluid. Heating water at the bottom of the pot decreases its density causing it to rise and be replaced by colder water. Heating air at the floor of the room decreases its density causing it to rise and be replaced by colder air.

18 Conduction Conduction is the process whereby heat is transferred directly through a material. One mechanism for conduction occurs when the atoms or molecules in a hotter part of the material vibrate or move with greater energy than those in a cooler part. By means of collisions, the more energetic molecules pass on some of their energy to their less energetic neighbors. Materials that conduct heat well are called thermal conductors, and those that conduct heat poorly are called thermal insulators.

19 The heat Q conducted per unit time, Δt, through a bar of length L and cross-sectional area A is Q Dt = ( kadt ) L thermal conductivity SI Units of Thermal Conductivity: J/(s m C o )

20 Example At what rate is heat lost through a 1.0 m 1.5 m rectangular glass windowpane that is 0.5 cm thick when the inside temperature is 20 C and the outside temperature is 5 C? The thermal conductivity for glass is 0.80 W/(m C ). (a) 18 W (c) 720 W (e) 7200 W (b) 36 W (d) 3600 W

21 Let s get creative about materials Materials with dead air spaces are usually excellent thermal insulators. Why are double glazed windows more insulating than single glazed windows? Why dress in layers?

22 Radiation Radiation is the process in which energy is transferred by means of electromagnetic waves. No medium of transfer is necessary. A material that is a good absorber is also a good emitter. A material that absorbs completely is called a perfect blackbody.

23 THE STEFAN-BOLTZMANN LAW OF RADIATION The radiant energy Q, emitted in a time t by an object that has a Kelvin temperature T, a surface area A, and an emissivity e, is given by Q = es T 4 At Stefan-Boltzmann constant s = ( 2 4 s m K ) J The emissivity e is a dimensionless number between zero and one. It is the ratio of what an object radiates to what the object would radiate if it were a perfect emitter.

24 Example Assuming a filament in a 100 W light bulb acts like a perfect blackbody, what is the temperature of the hottest portion of the filament if it has a surface area of 6.3 x 10 5 m 2? The Stefan-Boltzmann constant is 5.67 x 10 8 W/(m 2 K 2 ).

25 Example: Thermos bottle The space between the inner walls of a thermos bottle is evacuated to minimize heat transfer by (a) radiation. (d) conduction and radiation. (b) conduction. (e) conduction, convection, and radiation. (c) conduction and convection. The space is evacuated to minimize heat transfer by conduction and convection

26 Example: Thermos bottle Complete the following statement: The interior of a thermos bottle is silvered to minimize heat transfer due to (a) radiation. (b) conduction. (c) conduction and convection. (d) conduction and radiation. (e) conduction, convection, and radiation.