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Thermal Physics IB Physics

Thermal Physics IB Physics . Topic 3 & Option C. Thermodynamics. Understanding the words Temperature Heat Heat capacity The 0, 1, 2 laws of thermodynamics (one of) Kelvin’s legacy’s. WilliamThompson (Lord Kelvin). I feel hot. What is Heat?.

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Thermal Physics IB Physics

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  1. Thermal PhysicsIB Physics Topic 3 & Option C

  2. Thermodynamics • Understanding the words • Temperature • Heat • Heat capacity • The 0, 1, 2 laws of thermodynamics • (one of) Kelvin’s legacy’s WilliamThompson (Lord Kelvin)

  3. I feel hot What is Heat? • Perception as to hot and cold defined relative to our own body temperature, i.e. object is hotter or colder than oneself • Objective measurement of temperature • Macroscopic, display of temperature gauge • Microscopic behaviour of atoms and molecules

  4. Measuring temperature • Properties of materials change with temperature • Length • Volume • Resistance

  5. Hotter things become longer • All(?) solids get bigger when they get hot • A 1 metre long bar heated by 1 degree gets bigger by • Steel ≈0.01 mm • Glass ≈ 0.001 mm • Zerodur ≈ 0.0001mm Rails expand and may buckle on a hot summer day

  6. A bimetallic strip • Join two metals with different coefficient of thermal expansion e.g. fire alarm

  7. Hotter things take up more volume -1 • Most materials get bigger when they get hot (but not water 0°C -> 4°C gets smaller!) • Thermometer relies on a thermal expansion of a liquid (e.g.mercury) Thin tube (Gives big length change for small increase in volume) Large volume of reservoir

  8. Hotter things take up more volume -2 • Gases (as we will see) can behave near perfectly Hotter

  9. Hotter things change their resistance • All hotter metals have a higher electrical resistance or conductivity • Digital thermometer • All hotter semiconductors have a lower electrical resistance • key definition between to distinguish metals and insulators!

  10. Example You have a (glass) jar and you can’t get the metal lid off. What should you do: a) ask your girlfriend b) run the jar & lid under cold water c) run the jar & lid under hot water

  11. Solution: a) ask your girlfriend b) run the jar & lid under cold water c) run the jar & lid under hot water Because the metal has a substantially higher coefficient of thermal expansion than the glass, heating them will make both of them bigger, but the metal will be more bigger.

  12. How long do you have to leave a thermometer in your mouth? • Hot things stay hot if you insulate them, e.g. • coffee in a vacuum flask (keeps things cold too) • an explorer in a fur coat • The mercury in the thermometer must reach the same temperature as you –Thermal Equilibrium!!

  13. Insulation • Example of good (thermal) insulators • A vacuum, polystyrene, fibreglass, plastic, wood, brick • (low density/foam structure, poor electrical conductors) • Examples of poor insulators, i.e. good conductors • Most metals (but stainless steel better than copper) e.g. gold contact used within IC chips to prevent heating • Gases, liquids • (high density, “mobile”, good electrical conductors)

  14. Ask a friend if it’s cool enough to eat • Your friend eats the “hot” loaf and says it cool enough to eat (i.e it is “close” enough to their own temperature that it does not burn) • Is it safe for you to eat too • If it is safe for him, it’s safe for you!

  15. The 0th law of thermodynamics • If A and B are each in thermal equilibrium with C then A and B are in thermal equilibrium with each other • If Alfred and the Bread are the same temperature as Cliff then Alf is the same temperature as the Bread. =Temp =Temp? =Temp Cliff Alf

  16. Temperature and scales • Temperature scales (melting & boiling of water) • Degrees Celsius (MP 0°C 100°C) • Degrees Kelvin (MP 273.15 K BP 373.15 K) • Degree Fahrenheit (MP 32° F BP 212°F) • Explain how a temperature scale is constructed.

  17. Fahrenheit & Celsius Celsius & Fahrenheit The Common Temperature Scales

  18. Absolute zero • Ideal gas has zero volume • Resistance of metal drops to zero (actually superconductivity cuts in above 0K) • Brownian motion ceases (kinetic energy due to thermal excitation) • But lowest temperature attained is ≈ 10-9K

  19. Absolute zero, 0K

  20. Lord Kelvin • William Thompson, born Belfast 1824 • Student in Natural Philosophy • Professor at 22! • Baron Kelvin of Largs in 1897 • A giant • Thermodynamics, Foams, Age of the Earth, Patents galore!

  21. Converting between scales • Kelvin to Celsius • K = C + 273.15 • C = K - 273.15 • Fahrenheit to Celsius (Not IB) • F = C x (9/5) + 32 • C = (F - 32) x (5/9) State the relation between Kelvin & Celsius Scales.

  22. Convert the following temperatures into °F and K Boiling water, 100°C Freezing water, 0°C Absolute zero, -273.15°C Example 212°F, 373.15K 32°F, 273.15K -460°F, 0K

  23. Type of thermometer • Change in electrical resistance (convenient but not very linear) • Change in length of a bar (bimetallic strip) • Change in volume of a liquid • Change in volume of gas (very accurate but slow and bulky)

  24. Heat and internal energy • Can you describe the difference between the terms. • Temperature • Heat • Internal Energy

  25. Temperature & Absolute Temperature • Temperature is a property that determines the direction of thermal energy transfer between two bodies in thermal contact. • Absolute temperature is a measure of the average kinetic energy of the molecules of a substance. • Average kinetic energy is proportional to absolute temperature in Kelvin.

  26. What is the kinetic energy of an oxygen molecule at room temperature ( 21˚C)? (k = 1.38x10-23J/K) Since we know the kinetic energy, how is it travelling? This is called the root mean squared speed or rms speed. Example KE = 3/2 kT = 3/2(1.38x10-23 x 294) = 6.09 x 10-21 Joules We could equate KE = 1/2 mv2 = KE = 3/2 kT and get v2 = 3kT/m mass must be in kg!!!! Not u.

  27. Heat (Energy) • Is the flow of energy in or out of a system. • Heat (energy) flows because of temperature difference • Bigger temperature difference bigger heat flow • Less insulation give more heat flow for the same temperature difference • Heat will not flow between two bodies of the same temperature

  28. Equilibrium • Two objects of different temperature when placed in contact will reach the same temperature + = Cold milk Light brown coffee Hot black coffee

  29. Heat transfer = energy transfer • Energy measured in Joules but heat often measured in Calories • One cal raises one gram of water from 14.5°C to 15.5°C • 1 cal = 4.186J • Doing work on something usually makes it hot • Joules Experiment! • 1st law of thermodynamics heat and work are both forms of energy

  30. Sir James Joule • James Prescott Joule 1818-1889 • Stirring water made it warm • Change in temperature proportional to work done • Showing equivalence of heat and mechanical energy • Also that electrical current flow through a resistor causes heating

  31. Joules Experiment

  32. Internal Energy • Is the total potential and kinetic energy of the molecules in a substance. • Potential energy is associated with intermolecular forces. • Kinetic energy includes both translational and rotational motion.

  33. Three Phases – Atomic Model • Three States of Ordinary Matter Solid ® liquid ® gas

  34. Atomic Model of Matter • Comparing Molecular Forces • Solid – Largest molecular forces • Liquid • Gas – Weakest molecular forces • When the kinetic energy of the molecules become comparable to the energy required for separation the molecules change there position and separate (PE increase). This is a phase transition • Melting or vaporizing

  35. Avogadro constant • One mole a any substance is that quantity of the substance whose mass in grams is numerically equal to the substances molar mass, μ. • EX: The moloar mass of O2 is 32 g mol-1 • NA = 6.02 x 1023 molecules mol-1

  36. How many grams are there in a quantity of oxygen gas containing 1.2 x 1025 molecules? Example The number of moles is (1.20 x 1025)/6.02 x 1023 = 19.93 mol Since the molar mass is 32 g mol-1 The mass is 19.93 x 32 = 638 g or 0.638 kg

  37. So, how fast is that O2 molecule traveling? O2 = 32 g/mole Example v2 = 3kT/m (rms speed of a molecule) m = 0.032/(6.023 x 1023) = 5.3x10-26 kg v2 = 3(1.38 x 10-23J/K)(294)/(5.3x10-26 kg) v = 479 m/sec

  38. Transferring heat energy • 3 mechanisms • Conduction • Heat transfer through material • Convection • Heat transfer by movement of hot material • Radiation • Heat transfer by light

  39. Conduction of heat • Conduction in solids • Heat energy causes atoms to vibrate, a vibrating atom passes this vibration to the next • Conduction in metal • Heat energy causes electrons to gain energy, electrons travel through metal (conduction) and carry heat energy with them • Metals are good conductors of both heat and electricity

  40. Conduction of heat • The atoms at the bottom are at a higher temperature and will oscillating more strongly than those at the top.

  41. Rate of heat flow • Heat flow (H) is energy transfer per unit time, depends on • Temperature difference • Thermal conductivity (k) • k (copper) = 385 W/(m K) • k (glass) = 0.8 W/(m K) • k (air) =0.02 W/(m K) A TH TC L

  42. Two rods of the same cross-sectional area are joined together. The right rod is a better conductor of heat than the rod at left. The ends are kept at fixed temperatures. In which rod is the rate of heat transfer the largest? Is the temperature at the joining point lower are higher than 54 ˚C? Example Heat entering the joint must equal the heat leaving the joint. (Conservation of Energy). Hence, the rate of heat transfer is the same. Since the second conductor is poor a much larger temperature gradient can be maintained. Thus, the temperature at the junction will be larger.

  43. Thermal conduction vs thermal resistance • Also can use thermal resistance, cf • Can make equation of heat flow more general

  44. Convection of heat • “Hot air rises” (and takes its heat with it!) • Radiators

  45. Convection of heat • “Hot air rises” (and takes its heat with it!) • Cumulus clouds

  46. Figure 16-11Alternating Land and Sea Breezes

  47. Convection of heat • Hurricanes • Plate tectonics

  48. Radiation of heat • Don’t confuse with radioactivity • Instead realize that light carries heat (e.g. the sun heats the earth) • Anything above absolute zero radiates heat • P a AT4 Stefan-Boltzmann law.

  49. Radiation of heat • involves the generation and absorption of photons. Unlike conduction or convection, however, radiation requires no intervening medium to transport the heat. • All objects radiate energy continuously in the form of electromagnetic waves • The hotter an object the more power it radiate sand the shorter the wavelength of the peak emission wavelength

  50. Not all things emit heat the same • Heat emission from an object area A • P = AesT4 • s = Stafan’s constant = 5.6x10-8 W/(m2 K4) • e = emissivity of a body, 0 -1 • ecopper = 0.3 • ecarcoal ≈ 1

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