Chapter 16. Temperature and Expansion A PowerPoint Presentation by Paul E. Tippens, Professor of Physics Southern Polytechnic State University © 2007

Internal energy -- spring analogies are helpful: 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 = ½kx2 K = ½mv2

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

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

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

Temperature Labels Dt = 600C – 200C Dt = 40 C0 If an object undergoes a change of temperature, we place the degree symbol 0 after the scale (C0 or F0) to indicate the interval of temperature. ti = 600C tf = 200C Dt = 600C – 200C Dt = 40 C0 We say: “The temperature decreases by forty Celsius degrees.”

Specific Temperatures Same temperatures have different numbers: 0C 0F 2120F 320F 1000C 00C 180 F0 100 C0 tC tF

Convert 1600F to 0C from formula: Example 1: A plate of food cools from 1600F to 650F. What was the initial temperature in degrees Celsius? What is the change in temperature in Celsius degrees? Convert 1600F to 0C from formula: tC = 71.10C 9 F0 = 5 C0 Dt = 52.8 C0

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

Linear Expansion to t Copper:  = 1.7 x 10-5/C0 Lo L to t Copper:  = 1.7 x 10-5/C0 Concrete:  = 0.9 x 10-5/C0 Iron:  = 1.2 x 10-5/C0 Aluminum:  = 2.4 x 10-5/C0

DL = aLoDt = (1.7 x 10-5/C0)(90 m)(80 C0) Example 2: A copper pipe is 90 m long at 200C. What is its new length when steam passes through the pipe at 1000C? Lo = 90 m, t0= 200C Dt = 1000C - 200C = 80 C0 DL = aLoDt = (1.7 x 10-5/C0)(90 m)(80 C0) DL = 0.122 m L = Lo + DL L = 90 m + 0.122 m L = 90.12 m

Applications of Expansion Expansion Joints Bimetallic Strip Brass Iron 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 Expansion on heating. A0 A Area expansion is analogous to the enlargement of a photograph. Example shows heated nut that shrinks to a tight fit after cooling down.

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. A B Thermal Equilibrium A Object C B

Summary: Expansion Linear Expansion: to t Area Expansion: DA = 2aA0 Dt Lo L to t DA = 2aA0 Dt Area Expansion: Expansion A0 A