Physics 102, Class 13 Thermal Expansion, Temperature, Heat, Heat Capacity, and Water 10/12/2005

Transcription

1 Physics 102, Class 13 Thermal Expansion, Temperature, Heat, Heat Capacity, and Water 10/12/2005

2 Thermal Expansion Thermal Expansion in Gases, which expand against external constraint 4E Boyle's Law Apparatus PV=const if you reduce particle density, you reduce pressure 4E Ideal Gas Law temp goes up, pressure goes up, gas wants to expand 4B Thermal Convection in Gasses at constant pressure, the heated gas expands influence of water on climate convection in ponds

3 Thermal Expansion Thermal Expansion in Liquids and Solids, which have conserve volume It s another effect of the Interatomic Potential

4 Thermal Expansion Thermal Expansion in Liquids and Solids, which conserve volume 4A Water Thermometer 4A Bi-metal Strip expansion of solids is less: how can you see it? 4A Balls and Rings does the hole in the ring get larger or smaller? Examples: bridges

5 What s funny about this picture? Thermal Expansion From Hewitt: Figure Thermal expansion. The extreme heat of a July day in Asbury Park, New Jersey, caused the buckling of these railroad tracks. (Wide World Photos)

6 Thermal Expansion How do you measure thermal expansion? Thermal Expansion Coefficient (How much it lengthens per degree)/(how long it is) Units (length/degree)/length=1/degree Small effect, usually measured in units of 10-6 /degree C called parts per million per degree C data and more info at Aluminum 23 Brass 19 Concrete 12 Copper 17 Diamond 1 Glass 8.5 Glass, Pyrex 3.3 Gold 14 Iron or steel 12 lead 29 nickel 13 Mercury 60 Platinum 9 Quartz, fused 0.59 Silicon 3 Carbon Steel 10.8 Stainless Steel 17.3 Tungsten 4.5

7 Thermal Expansion The complete story on thermal expansion is more complicated, and there are actually a small number of materials which contract when they are heated: expansion

8 CPS Question When normal liquids and solids are heated, their atoms A: move closer together, on average B: move farther apart, on average C: stay the same distance apart, on average

9 CPS Question When normal liquids and solids are heated, their atoms A: move closer together, on average B: move farther apart, on average C: stay the same distance apart, on average They move farther apart because of the asymmetric shape of the potential energy curve. The curve is asymmetrical because it is much harder to mash two atoms together than it is to pull them apart.

10 Thermometers Thermometers Quantify Human Sense and Observation heat stroke hypothermia first one by Galileo in 1602 Liquids and Solids 4A Galileo's Thermometer expansion of water, its density reduces, objects with matching densities rise and fall really good thermometer 4A Water Thermometer 4A Bi-metal Strip 4A Thermometry

11 Thermometers Thermometers What material would make a good thermometer? on board: straight line on potential curve versus avg KE Could you make a thermometer out of a bridge? drawing on board Would you want to make a gas thermometer? drawing on board

12 Temperature Scales Fahrenheit first accurate temperature scale what does accurate mean here? not based on water water freezes at 32F, boils 212F Celsius metric based on water water freezes at 0C, boils 100C Kelvin: best! zero is in the right place same size degrees as Celsius water freezes at 273K, boils 373K

13 Temperature Scales Fahrenheit: water freezes at 32F, boils at 212F Celsius water freezes at 0C, boils at 100C There are =180 F degrees to cover the same range of temperature which is covered by only 100 C degrees C degrees are 180/100=9/5 the size of F degrees Conversions degrees C=(degrees F-32) * 5/9 degrees F=(degrees C * 9/5) + 32 degrees K=degrees C + 273

14 CPS Question What would happen if the glass expands more than the liquid inside when the temperature changes? A: nothing, it works fine B: the scale inverts C: the distance between the degrees will decrease somewhat

15 CPS Question What would happen if the glass expands more than the liquid inside when the temperature changes? A: nothing, it works fine B: the scale inverts C: the distance between the degrees will decrease somewhat If you heat the thermometer and the marks on the glass move up more than the level of the mercury moves up, then the marks for hotter temperatures must be closer to the bulb than marks for colder temperatures. So the scale would be inverted.

16 Heat Heat when you put something that feels hotter together with something that feels colder, the hotter one cools down and the cooler one warms up until they feel the same is it a (conserved) fluid, caloric? no, it can t be

17 Heat Heat is energy being transferred provisional description: temperature is proportional to translational kinetic energy per molecule, and energy flows from one body to another until the translational kinetic energy per molecule is equal in the two bodies Thermal Equilibrium after sufficient time has passed, any bodies in contact will have reached the same temperature, and heat will no longer flow

18 calories and Calories How do you measure heat? sensible: Joules! older units: BTU (British Thermal Unit): energy to raise 1 pound of water by 1 degree F calorie or gm-calorie: energy to raise 1 gm water by 1 degree C at 15C=4.184J

19 calories and Calories How do you measure heat? sensible: Joules! older units: BTU (British Thermal Unit): energy to raise 1 pound of water by 1 degree F calorie or gm-calorie: energy to raise 1 gm water by 1 degree C at 15C=4.184J Calorie or kilocalorie: energy to raise 1 kg water by 1 degree C at 15C=4184J calories and Calories

20 CPS Question Two pieces of iron, one massing 5 grams and one massing 50 grams, are heated in an oven to the same temperature 450F. They are each taken out and quickly put into two separate 1 gallon jugs of water at room temperature. Which piece raises the water temperature more? A: the 50 gram piece B: the water temperature is the same at the end C: the 5 gram piece

21 CPS Question For a given material (iron in this example) the amount of heat required to raise the temperature is proportional to the mass. (You already know this because you know that it takes much longer to heat up a full pot of water than it takes to heat up a pot that has only a little water in the bottom.) Since there is more heat stored in the larger iron sample, there is more heat available to raise the temperature of the water in the jug, so the Two pieces of iron, one massing 5 grams and one massing 50 grams, are heated in an oven to the same temperature 450F. They are each taken out and quickly put into two separate 1 gallon jugs of water at room temperature. Which piece raises the water final temperature is higher. temperature more? A: the 50 gram piece B: the water temperature is the same at the end C: the 5 gram piece

22 Heat Capacity Heat Capacity the energy transfer required to change the temperature of a body is proportional to the mass of the body and the size of the temperature change: Δ T = ΔQ cm the constant of proportionality c is the specific heat (J/kg K), which depends on the material and the temperature Hewitt doesn t say: specific heat per atom is more fundamental than specific heat per mass

23 Heat Capacity Heat Capacity depends on the degrees of freedom which can store energy in a material translational only: monatomic gas more: molecular gases more: liquids and solids In solids and liquids the Heat Capacities generally get larger quickly as temperature increases

24 CPS Question Two samples of identical mass 50 grams are prepared. One sample has specific heat of 1 J/kgK, the other has specific heat of 2 J/kgK. Both samples are heated in an oven to the same temperature 450F. They are each taken out and quickly put into two separate 1 gallon jugs of water at room temperature. Which piece raises the water temperature more? A: the sample with lower specific heat B: the water temperature is the same at the end C: the sample with higher specific heat

25 CPS Question This is very similar to the last CPS question, except now we Two samples of identical mass 50 grams are are thinking of the same mass of two different materials, prepared. instead of two One different sample masses has specific of the same heat material. of 1 The J/kgK, specific the heat other is how has much specific energy heat it takes of to 2 raise J/kgK. 1 kg of a Both material samples by 1 degree. are heated If two in samples an oven of the to same the same mass are temperature 450F. They are each taken out and quickly put into two separate 1 gallon jugs of heat stored in the sample with higher specific heat, there is water at room temperature. Which piece raises the water temperature more? raised from room temperature to 450F, the sample with higher specific heat has more energy put into it. Since there is more more heat available to raise the temperature of the water in the jug, so the final temperature of the water will be higher for the sample A: the with sample higher with specific lower heat. specific heat B: the water temperature is the same at the end C: the sample with higher specific heat

26 Water Water is Special for Heat Capacity and Expansion Highest heat capacity of common materials inversion of thermal expansion curve: very unusual among all materials The high specific heat has a huge effect on climate discussion

27 Global Sea Surface Temperatures ocean temperature variations are small compared to land temperature variations, both seasonally and daily. This reflects the large specific heat of water onshore/offshore breezes

28 Water The inversion of the thermal expansion coefficient is vital for animals and plants that live in lakes discussion

29 CPS Question Which would you expect to have bigger temperature variations over the course of a year? A: Albuquerque B: Hawaii C: They should be about the same, because they re both pretty far south.

30 CPS Question Which would you expect to have bigger temperature variations over the course of a year? A: Albuquerque B: Hawaii C: They should be about the same, because they re both pretty far south. Monthly highs and lows for Kahului, Maui, showing very little variation over 1 full year. Graphic from Albuquerque has much bigger temperature variations because it is farther north and it is very far from any large bodies of water. Hawaii is closer to the equator and surrounded by ocean, so its temperature variation is tiny by comparison, both day-night and over the course of a year. See Hewitt s description of the effects of the oceans on world climate also.

31 CPS Question Lake Superior is a deep lake that ices over in the winters. What s the temperature at the bottom of Lake Superior in the middle of the winter? A: below 0 degrees C B: 0 degrees C C: between 0 degrees C and 4 degrees C D: 4 degrees C E: above 4 degrees C

32 CPS Question Lake Superior is a deep lake that ices over in the winters. What s the temperature at the bottom of Lake Superior in the middle of the winter? Water has a maximum density at 4C. When the lake is warmer than 4C and the air is colder than the water, the water at the top of the lake is cooled and becomes denser, so it sinks to the bottom. The warmer water is forced upward, so convective mixing occurs, as we discussed in class, so the lake is always presenting warm water to the cold air, so heat transfer is high and the lake cools efficiently. However, when the lake cools to 4C, the convective mixing stops, because now the colder water at the top is less dense than the 4C water further down in the lake. The lake becomes convectively stable, as we discussed in class, and the water on the top of the lake insulates the water farther down from the cold air. Further cooling of the lake will be much slower, and for deep lakes like Lake Superior the water at the bottom never cools below 4C. See also Hewitt s explanation in the book. A: below 0 degrees C B: 0 degrees C C: between 0 degrees C and 4 degrees C D: 4 degrees C E: above 4 degrees C

33 Assignments Reading: Chapters 16 and 17 Homework: WebAssign HW #12