TEMPERATURE is a measure of the average kinetic

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 thermometer and is converted to an electrical signal that gives a digital reading of body temperature.

Objectives: After finishing this unit, you should be able to: • Work with Celsius, Kelvin, and Fahrenheit temperature scales for both specific temperatures and temperature intervals. • Write and apply formulas for linear, area, and volume expansion.

Thermal Energy Thermal energy is the total internal energy of an object: the sum of its molecular kinetic and potential energies. Thermal energy = U + K Internal energy -- spring analogies are helpful: U = ½kx 2 K = ½mv 2

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

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

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. Object C A Thermal Equilibrium A Object C B B Same Temperature

Temperature Scales The lower fixed point is the ice point, the temperature at which ice and water coexist at 1 atm of pressure: 1000 C 2120 F 320 F 00 C or 320 F The upper fixed point is the steam point, the temperature at which steam and water coexist at 1 atm of pressure: 1000 C or 2120 F

Comparison of Temperature Intervals: 1000 C 2120 F 5 C 0 = 9 F 0 100 C 0 180 F 0 If the temperature changes from 790 F to 700 F, it means a decrease of 5 C 0. t. C t. F 00 C 320 F 100 C 0 = 180 F 0

Temperature Labels If an object has a specific temperature, we place the degree symbol 0 before the scale (0 C or 0 F). t = 600 C We say: “The temperature is sixty degrees Celsius. ”

Temperature Labels (Cont. ) If an object undergoes a change of temperature, we place the degree symbol 0 after the scale (C 0 or F 0) to indicate the interval of temperature. ti = 600 C tf = 200 C t = 600 C – 200 C t = 40 C 0 We say: “The temperature decreases by forty Celsius degrees. ”

Specific Temperatures Same temperatures have different numbers: 0 C 0 F 1000 C 2120 F 100 C 0 180 F 0 t. C t. F 00 C 320 F

Example 1: A plate of food cools from 1600 F to 650 F. What was the initial temperature in degrees Celsius? What is the change in temperature in Celsius degrees? Convert 1600 F to 0 C from formula: t. C = 71. 10 C 9 F 0 = 5 C 0 t = 52. 8 C 0

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

Constant Volume Thermometer Absolute pressure Valve Constant volume of a gas. (Air, for example) A search for a true zero of temperature can be done with a constantvolume thermometer. For constant volume: T = k. P The pressure varies with temperature.

Absolute Zero of Temperature P 1 P 2 T 1 T 2 Absolute Zero T -2730 C 00 C P 1000 C Plot points (P 1, 00 C) and (P 2, 1000 C); then extrapolate to zero. Absolute Zero = -2730 C

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

Linear Expansion to t Copper: = 1. 7 x 10 -5/C 0 Iron: = 1. 2 x 10 -5/C 0 Lo L L Concrete: = 0. 9 x 10 -5/C 0 Aluminum: = 2. 4 x 10 -5/C 0

Example 2: A copper pipe is 90 m long at 200 C. What is its new length when steam passes through the pipe at 1000 C? t = 1000 C - 200 C = 80 C 0 Lo = 90 m, t 0= 200 C L = a. Lo. Dt = (1. 7 x 10 -5/C 0)(90 m)(80 C 0) L = 0. 122 m L = L o + L L = 90 m + 0. 122 m L = 90. 12 m

Applications of Expansion Brass Iron Expansion Joints Bimetallic Strip Expansion joints are necessary to allow concrete to expand, and bimetallic strips can be used for thermostats or to open and close circuits.

Area Expansion on heating. A 0 A Area expansion is analogous to the enlargement of a photograph. Example shows heated nut that shrinks to a tight fit after cooling down.

Calculating Area Expansion A 0 = L 0 W 0 A = LW L = L 0 + a. L 0 Dt W = W 0 + a. W 0 Dt DW W DL Wo L = L 0(1 + a. Dt ) W = W 0(1 + a. Dt A = LW = L 0 W 0(1 + a. Dt)2 Lo L A = A 0(1 + 2 t) Area Expansion: DA = 2 a. A 0 Dt

Summary Thermal energy is the total internal energy of an object: the sum of its molecular kinetic and potential energies. Thermal energy = U + K 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. Object C A B Thermal Equilibrium A B

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

Summary: Expansion Linear Expansion: to t L L Area Expansion: Expansion A 0 Lo A DA = 2 a. A 0 Dt