1. Phys141 Principles of Physical Science Chapter 5 Temperature and Heat Instructor: Li Ma Office: NBC 126 Phone: (713) 313-7028 Email: malx@tsu.edumalx@tsu.edu Webpage: http://itscience.tsu.edu/ma Department of Computer Science & Physics Texas Southern University, Houston Sept. 27, 2004

2. Topics To Be Discussed Temperature Heat Specific Heat and Latent Heat Heat Transfer Phases of Matter

3. Heat &Temperature Commonly used terms Well-known phenomena related to them: cold when holding a piece of ice, hot near to the fire, warm when rubbing your hands or standing in sun shine, how is the weather for everyday, etc. Very important concepts in our daily life They have different & distinct meanings

4. Molecules of Substance Most substances are made of very small particles called molecules, which are chemical combinations of atoms Molecules are the smallest unit for substance

5. Temperature It is a relative measure or indication of hotness or coldness It is a measure of the average kinetic energy of the molecules of a substance The temperature perception in our sense of touch is not reliable Quantitatively measure the temperature: thermometer

6. Temperature (cont) Thermometer: a instrument that utilizes the physical properties of materials for the purpose of accurately determining temperature Thermal expansion: temperature- dependent property used to measure temperature Liquid-in-glass thermometer

7. Temperature (cont) Thermometers are calibrated so that numerical values can be assigned to different temperature Two reference (fixed) points: –Freezing (ice) point & boiling (steam) point A choice of unit: –Fahrenheit scale: 32°F/212°F/180 equal units –Celsius scale: 0°C/100°C/100 equal units

8. Temperature (cont) Absolute zero: the lower limit of temperature –-273°C, -460°F Another temperature scale: Kelvin scale –Unit is kelvin (K) –0 K is at the absolute limit –No negative temperature –The kelvin and the degree Celsius are equal intervals

9. Temperature (cont) Conversion between different temperature scales: –T K = T C + 273 –T F = 1.8T C + 32 or T C = (T F – 32)/1.8 Examples: –T F = 98.6°F (normal human body temperature), what is it on Celsius scale? –T C = (T F – 32)/1.8 = (98.6 – 32)/1.8 = 37°C

10. Heat Heat is a form of energy Molecules of a substance have energy –Vibrate, rotate, move – kinetic –The molecular bond, vibration and/or rotation of atoms within molecules – potential Internal energy: –Total energy (kinetic + potential) within an object

11. Heat (cont) Heat is net energy transferred from one object to another because of a temperature difference Unit in SI: joule (J) Common and traditional unit: calorie –The amount of heat necessary to raise one gram of pure water by one Celsius degree at normal atmospheric pressure –1 cal = 4.186 J (≈ 4.2 J), 1 kcal = 1000 cal

12. Heat (cont) One effect of heating a material is expansion –Nearly all matter expands when heated and contracted when cooled Exception of this rule –When water is frozen, it expands –Ice at 0°C occupies a larger volume than the same mass of water at 0°C

13. Heat (cont) Heat expansion characteristics plays a major role in many aspects in our life: –Cracks in highway: the concrete will not buckle in summer –Expansion joint designed into bridges –The first steam motor

14. Specific Heat Heat is added to material, temperature may change Same amount of heat is added to equal mass of different materials, the temperature increased may be different: –Same amount of heat is added, same amount of internal energy is gained. –The internal forces of materials are different –The more internal energy goes into internal kinetic energy, the higher temperature will increase

15. Specific Heat (cont) The specific heat of a substance is the amount of heat necessary to raise the temperature of one kilogram of the substance one Celsius degree Unit for specific heat is J/kg·°C or kcal/kg·°C Water has a specific heat of 1.00 kcal/kg·°C (c = 1.00 kcal/kg·°C) –Table 5.1 on page 95

16. Specific Heat (cont) Water has one of the highest specific heats and so can store more heat energy for a given temperature change –Use water to store solar energy collected during the day time amount of heat to change temperature = mass x specific heat x temperature change H = mcΔT or

17. Latent Heat Three phases of matter: solid, liquid and gas –The molecules of a gas are relatively father apart than the molecules in a liquid or solid Heat is added to material, temperature may not change, but phase changes During a phase change, the heat energy goes into the work of separating the molecules, instead of increasing the molecular kinetic energy

18. Latent Heat (cont) The latent heat is the heat associated with phase The latent heat of fusion of the substance is the amount of heat necessary to change one kilogram of a solid into a liquid at the same temperature –This temperature is melting point (solid => liquid) or freezing point (liquid => solid)

19. Latent Heat (cont) The latent heat of vaporization of the substance is the amount of heat necessary to change one kilogram of a liquid into a gas at the same temperature –This temperature is boiling point (liquid => gas) Sublimation & deposition

20. Latent Heat (cont) amount of heat to melt a substance = mass x latent heat of fusion H = mL f or amount of heat to boil a substance = mass x latent heat of vaporization H = mL v or

21. Latent Heat (cont) For water –L f = 80 kcal/kg = 3.35 x 10 5 J/kg –L v = 540 kcal/kg = 2.26 x 10 6 J/kg Example: –Calculate the amount of heat necessary to change 0.20 kg of ice at 0°C into water at 10°C –H = H melt ice + H increase temperature –H = mL f + mcΔT = 0.20 kg x (80 kcal/kg + 1.00 kcal/kg·°C x 10 °C) = 18.0 kcal

22. Heat transfer Three methods of heat transfer: –Conduction: transfer of heat by molecular collision thermal conductor, thermal insulator –Convection: transfer of heat by the movement of a substance from one position to another, heated gas or water –Radiation: transferring energy by means of electromagnetic waves, heat got from Sun, no need of material medium

23. Phases of Matter Three common phases of matter: solid, liquid & gas –Figure 5.5 on page 99 –A solid has definite shape and volume –A liquid has a definite volume but no definite shape –A gas has no definite volume or shape Sometimes they are also referred to as states of matter