"Chapter 15 TEMPERATURE, HEAT, AND EXPANSION Temperature"
Chapter 15 TEMPERATURE, HEAT, AND EXPANSION • Temperature is a measure of the average translational kinetic energy of molecules • Tells how warm or cold object is with respect to some standard. • Tells us which way heat will flow (from hot to cold) Galloway-PHYS 1310 Chapter 15 Elaboration Temperature What’s the difference between a beaker of cold water and a beaker of hot water? cold hot Galloway-PHYS 1310 Chapter 15 1 TEMPERATURE SCALES • FAHRENHEIT (32 – 212 oF) • CELSIUS (0 – 100 oC) • KELVIN (Absolute with Celsius sized increments) (273 K -373 K) • 00 Celsius = 273 Kelvin Galloway-PHYS 1310 Chapter 15 HEAT • The energy transferred from one object to another due to temp difference. • CALORIE (a unit of energy) • 1 CALORIE = Amount of heat needed to raise 1 gram of water by 1 degree Celsius. Galloway-PHYS 1310 Chapter 15 2 Engage Mixing Water If you mix some cold water and some hot water, what’s the final temperature? Can you predict what it would be? Suppose you put the same amount of heat into 1 liter of water as you put into 2 liters of water? Galloway-PHYS 1310 Chapter 15 Explanation Heat Transfer As the hot water cools down, the cool water warms up.. Heat gained = Heat lost As an example, if 20 calories of heat are lost by 5 g of water, then each gram of water loses 4 calories and the temperature of the 5 g of water will decrease 4°C in temperature.Galloway-PHYS 1310 Chapter 15 3 Elaborate Heat Transfer Examples Suppose you have 5 g of water at 20°C and you add 10 cal of heat, what is the final temperature? What if you add 20 cal of heat? There would be 10 cal for 5 g of water or 2 cal for each g. The water would heat up 2 °C and the final temp would be 22°C. Suppose you have 10 g of water and you want to raise the temperature by 7°C. How much heat must be added? You would need to add 7 cal for each g of water. Since there are 10 g, this would mean adding 70 calories. Galloway-PHYS 1310 Chapter 15 SPECIFIC HEAT CAPACITY • Heat required to raise the temp of 1 gm substance by 1 degree Celsius. Heat = (mass) x (temp change) x (specific heat) Heat lost = Heat gained Heat is a form of energy Energy is conserved. Galloway-PHYS 1310 Chapter 15 4 Explanation Specific Heat Specific Heats aluminum = 0.22 cal/ g·°C copper = 0.09 cal/g·°C glass = 0.20 cal/g·°C water = 1.0 cal/g·°C ice = 0.53 cal/g·°C vegetables = 0.9 cal/g·°C fat = 0.5 cal/g·°C Different substances have different specific heats? What are some of the effects of this observed in everyday life? Galloway-PHYS 1310 Chapter 15 Explanation Heat transfer in substance Heat = mass x temp change x specific heat Suppose you add 20 cal to 10 g of Aluminum, what is the temperature change? 20 cal = 10 g x (temp change) x 0.22 cal/g·°C Temp change = 20 cal/(10 g x 0.22 cal/g·°C) Temp change = 9.1°C Suppose you add 20 cal to 10 g of water, what is the temperature change? 20 cal = 10 g x (temp change) x 1 cal/g·°C Temp change = 20 cal/(10 g x 1 cal/g·°C) Temp change = 2°C Galloway-PHYS 1310 Chapter 15 5 Examples of heat capacity Q. Which will stay cooler after you take them out of the refrigerator on a hot day… a watermelon or a sandwich? A. The watermelon will because it has much more water than the sandwich and water has a greater specific heat than the non-watery parts of the sandwich. *http://wise.fau.edu/~jordanrg/push-ups_13/answers_13.htm, Dr. R.G. Jordan, Physics, Florida Atlantic University Galloway-PHYS 1310 Chapter 15 Engagement Examples of heat capacity Q. In the ‘good ol’ days’ on a cold night it was common to take a hot object to be with you(!). Would you be better to keep warm through the night with a 10 kg iron brick or a 10 kg container of water? A. The brick would lose heat more rapidly than the water because iron has a much smaller specific heat than water. As a result, the heat given off is much less than the same mass of water. *http://wise.fau.edu/~jordanrg/push-ups_13/answers_13.htm, Dr. R.G. Jordan, Physics, Florida Atlantic University Galloway-PHYS 1310 Chapter 15 6 THERMAL EXPANSION • Most materials expand when heated. • Different materials expand at different rates. • Road joints on bridges are necessary to keep the road from buckling. • Demo with bimetallic strip Galloway-PHYS 1310 Chapter 15 STRANGE CASE OF WATER • WATER IS MOST DENSE AT 4O C. VE 14-9(Thermostat Model), 10(Pin Breaker), 13(Thermal Expansion of Water), 14(Negative Expansion Coefficient of Water) Picture from Hewitt, Conceptual Physics, p. 300 Galloway-PHYS 1310 Chapter 15 7 Heat and Energy Heat is a form of energy. Discuss where the energy given off by the light in a light bulb arises. Can you trace this energy back to its source? Galloway-PHYS 1310 Chapter 15 8