Heat and Temperature

1. Heat and Temperature Physics 101

2. Common Temperature Scales Name Absolute 0 FP water BP Water • Fahrenheit -459.67 32 212 • Rankine 0 491.67 671.64 • Centigrade -273.15 0 100 • Kelvin 0 273.15 373.13 Conversion between Celsius and Fahrenheit Between K and Celsius Kelvin (K) = oC + 273

3. Common Temperatures Photosphere of Sun is 6000 oC or 11,000oF Sun Spots are cooler at 4000oC or 7,000oF 16 to 20 million oC at the center of the Sun which sustains nuclear fusion Dry Ice -78.5 oC Liquid N2 77.2 K Kelvin (K) = oC + 273 What is Liquid N2 in oC Absolute 0 is 0 K What is Absolute 0 in oC Room Temperature is 20 oC

4. At what temperature is the Fahrenheit and Celsius temperature the same? Question for Thought X = -40o C or -40o F

5. Temperature which sustains life 37 oC Outer Space 3 K Dwarf Star 35,500 K surface 100 billion K is the temperature of a supernova See http://en.wikipedia.org/wiki/Supernova for more information on a supernova Temperature of ice is 0 oC Water boils at 100 oC Temperatures continued

6. Types of Thermometers A Thermometer is a device for measuring temperature • Galileo thermometer • Mercury thermometer • Thermistor • Thermocouple • Transistor (a solid state device) • Bi-Metalic Strip The higher an object’s temperature the faster its atoms or molecules move Temperature is not a measure of the total kinetic energy of atoms and molecules in a substance Atoms can move into a state from which it is impossible to extract more energy—this is Absolute Zero

7. Thermal Expansion • The tendency for a heated object to expand is expressed in terms of the coefficient of linear expansion  Note the units of  Water is an anomaly since it actually shrinks slightly when its temperature is raised between 0 and 4o C. This property of water may have been very important in the evolution of life.

8. If a 50 m steel footbridge experiences extreme temperatures between -15 C and 45 C, what is the range in size of the bridge if it measures exactly 50 m at 20 C (Steel has a coefficient of linear expansion of 0.000011/0C -15 C – 20 C 50(1+0.000011 * -35) = 49.981 m 50(1 + 0.000011*25) = 50.014 20 C – 45 C Range is 50.014 to 49.981 m

9. Heat and Internal (thermal) energy • Thermal energy resides in an object because of the motion and interactions of its molecules and atoms • Heat is reserved for energy that is transferred from one body or substance to another due to a temperature difference • Under special circumstances absorbed heat may break inter-atomic bonds while the temperature remains constant (change of phase) • Heat is measured in a unit called the calorie (defined as the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. Heat (calories) is like joules in the m.k.s. system, in fact 1 calorie is = 4.186 joules. • In the English system the unit of heat is the BTU (British Thermal Unit) and is defined as the amount of heat required to raise one pound of water, 1 degree Fahrenheit. • Do not confuse calorie with a Food Calorie (C). One food Calorie = 1000 calories.

10. Specific Heat • Heat capacity is a measure of how much heat an object can absorb. A large mass has more heat capacity than a small mass. • Specific heat is a measure of the ability of a material to absorb heat. It is defined as the quantity of heat required to raise the temperature of one gram of that material by 1 degree Celsius. Water displays the highest specific heat of any familiar substance. (examine question #9, “If water had a lower specific heat, would your chances of enjoying a long hot bath be greater or less?”

11. Change of Phase • Specific Heat gives us a good indication of how well any substance absorbs heat. However a change of phase to another is a different situation. • The amount of heat needed to rearrange 1 gram of molecules from their conversion in a solid to a configuration in a liquid is called the latent heat of fusion. This value is 80 calories for each gram of ice.

12. Latent Heat of Vaporization • The amount of heat required to convert one gram of liquid to a gas is called the latent heat of vaporization. • It is 540 calories per gram for water. • Other substances have different latent heats of vaporization and latent heats of fusion.

13. Thought question for the day? • Many fruit growers even in southern climates face the dangers of frost harming fruit trees and crops. An “old wives tale” is to sprinkle your crops with water (especially in the wee hours of the morning) to protect them from freezing. A similar tale has you putting soaked newspapers between the flower and plant rows. Does this have any scientific validity?

14. Forms of Heat Transfer • Heat transfer occurs by three mechanisms, conduction convection and radiation. • Insulation acts as to slow the conduction of heat. (insulation is measured by R values) • Conduction requires atoms or molecules that vibrate and collide with each other. • Convection requires atoms or molecules of bulk fluid • Radiation can occur in a vacuum like outer space. (Radiation is transfer by electromagnetic radiation)

15. Copied from Nick Strobel “the sun and stellar structor”, www. Astronomynotes.com Our Sun

16. Design of a solar home