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Thermal Physics

Thermal Physics. Heat flows from hot to cold. Net energy flow stops when their temperatures are the same i.e. They are in thermal equilibrium. The Kelvin temperature scale. T c = T k - 273 T k = T c +273 How many times hotter is 100 ºC than 33ºC?

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Thermal Physics

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  1. Thermal Physics

  2. Heat flows from hot to cold Net energy flow stops when their temperatures are the same i.e. They are in thermal equilibrium

  3. The Kelvin temperature scale • Tc = Tk - 273 • Tk = Tc +273 • How many times hotter is 100ºC than 33ºC? • How many times hotter is 10ºC than 0ºC? • What is your temperature in 0C and 0K

  4. Kinetic theory • Why is it that you can put out a candle flame with moist fingers without hurting yourself although it is at 750 oC but it is very painful to put your fingers into a cup of hot water at 80 oC?

  5. Heat energy and molecular movement • What happens to the molecules when their temperature increases? • What is the difference between internal “heat” energy and temperature? • What types of energy do the molecules have?

  6. Heat energy and temperature 100mL at 200C 400mL at 200C Which has the “most heat” energy? Which has the higher temperature? 1mL at 2000C 400mL at 800C

  7. Heat capacity Click to play Why are the temperatures different?

  8. Temperature

  9. The internal energy of a substance is…… The potential energy of the molecules is…. The thermal energy of a substance is…..

  10. Gas Pressure Gas pressure is basically due to gas molecules hitting the sides of the container they are in: Hyperlink

  11. Kinetic Theory of Gases • Let’s make some assumptions: • Brownian motion suggests that a pure gas consists of identical molecules in constant random motion. • Molecules never stop so collisions between molecules are, on average, elastic (no kinetic energy is lost). • Because gases can be compressed easily the volume of the individual molecules is negligible compared to the volume they occupy when moving around. • Therefore the molecules are further apart, so we assume that there are no forces on the molecules except for when they collide.

  12. Internal Energy Basically, the internal energy of a substance is the sum of the molecular kinetic and potential energies. Consider a particle: This particle can store energy by moving in a number of ways: 1) Moving in three dimensions 2) Vibrating 3) Rotating These are called “degrees of freedom”, and each one can store energy.

  13. The Mole and Molar mass One mole of green atoms contains 6x1023 atoms One mole of carbon contains 6x1023 atoms One mole of “anything” contains 6x1023 atoms (or molecules) One mole of carbon weighs 12g Define the mole

  14. Molar mass • One mole of carbon contains 6x1023 atoms • It weighs 12 grams • How much does one mole of iron weigh? • How many atoms are there in 2 moles of tungsten? • How many atoms are the in 0.5 moles of water? • Define Avagadro’s constant • Define the Mole

  15. Examples • Calculate the number of moles of oxygen molecules contained in 64.0 g of oxygen gas, O2. • Calculate the number of oxygen molecules in part of this example. • The number of molecules present in 0.5 mol SO3 is • Calculate the mass in 0.75 mol of carbon dioxide gas.

  16. Specific Heat Capacity This can be thought of as “the capacity of an object to store heat”. Consider some water: If we heat this beaker up it’s fairly clear that the amount of energy it gains depends on how much water there is and how hot it gets… Energy αmass x temperature rise Energy = mass x s.h.c x temp E = mcΔT

  17. Specific Heat Capacity A V 12V How can we do this experimentally? E = VIt and E = mcΔT Possible errors with this experiment: • Temperature throughout the liquid should be the same • Solution: • Heat is lost to the surroundings • Solution(s):

  18. Another way… 12V A V E A metal

  19. SHC 12V A V E Temperature Q=mcΔT Q=Pt, and P=IV ΔT=IVt/mc Time

  20. Thermal Physics Questions • Specific heat capacities: • copper400Jkg-1K-1 iron460Jkg-1K-1 water4200Jkg-1K-1 ice2100Jkg-1K-1 •  Specific latent heat of fusion of ice = 3·3×105Jkg-1 • Molar heat capacities of a diatomic ideal gas:Cv = 12·5J(molK)-1 and Cp = 20·8J(molK)-1

  21. Question 1 A piece of metal of mass 0·2kg is heated to a temperature of 200°C. It is then put into 0·2kg of water at 20°C in a container of negligible heat capacity. The "final" temperature, after stirring, is 40°C. Calculate the specific heat capacity of the metal. Question 2 A piece of metal of mass 100g, has a temperature of 100°C. It is put into 100g of water at 20°C in a container of negligible heat capacity. After stirring, the maximum temperature of the "mixture" (metal and water) is 27·5°C. Calculate the specific heat capacity of the metal. Question 3 The specific heat capacity of water is very high. What effect does this have on the weather conditions experienced by people living on islands? Question 4 How long will it take to change the temperature of 200kg of water from 15°C to 40°C, using a heater of power 3kW. Assume that all the thermal energy remains in the water.

  22. 3 Phases of matter Solids –fixed positions, many strong bonds (potential energy) Liquids – Changing positions, many temporary bonds Gases – Free moving, no bonds

  23. Changing state Hyperlink

  24. Change of state (Melting and boiling) Heat energy is going to potential energy only, therefore there is no rise in temperature. Heat energy is going to internal EK

  25. Heating ice This flat line shows where energy is being used to break the temporary bonds for boiling. The amount of energy needed to turn 1kg of a liquid into a gas is called the Specific Latent Heat of Vaporisation L. This flat line shows where energy is being used to break bonds – this has to be done during melting. The amount of energy needed to turn 1kg of a solid into a liquid is called the Specific Latent Heat of Fusion L. Temp/OC Time/s

  26. Latent heat (changes in potential energy)

  27. Melting Q’s • Question 1 • A piece of ice at -20°C is put into a copper calorimeter of mass 0·2kg which contains 0·15kg of water at 20°C. The water is stirred until all the ice has melted. At this time the temperature of the water (and calorimeter) is 15°C. Calculate the mass of the piece of ice. (8 grammes). • Question 2 • A refrigerator is capable of removing 50J of heat per second from a container of water. How long will it take to change 2kg of water at 10°C into ice at -5°C? Assume that the rate of removal of heat remains constant and that the container has negligible heat capacity. Are these assumptions likely to be valid in practice?

  28. Latent Heat of Fusion A V 12V From the previous slide we can say that the energy needed to melt water is given by… Energy = mass x specific latent heat of fusion To work out L experimentally you could… VIt = mL

  29. Tsokos page 171 q’s 1-11.

  30. Evaporation Average speed decreases • High energy molecules escape when they reach the surface. • The average kinetic energy of the remaining particles drops • The temperature of the remaining liquid is lowered

  31. Boiling Hyperlink

  32. Evaporation and Boiling • Boiling occurs at a fixed temperature • Evaporation occurs at any temperature • Boiling happens throughout the body of the liquid • Evaporation only happens at the surface of the liquid.

  33. Pressure F Pressure (in N/m2) = Force (in N) Area (in m2) P A OR in cm2 and N/cm2 • Pressure depends on two things: • How much force is applied, and • How big (or small) the area on which this force is applied is. • Pressure can be calculated using the equation:

  34. Some pressure questions Pressure = force area P = F A • Calculate the pressure exerted by a 1000N elephant when standing on the floor if his feet have a total area of 1m2. • A brick is rested on a surface. The brick has an area of 20cm2. Its weight is 10N. Calculate the pressure. • A woman exerts a pressure of 100N/cm2 when standing on the floor. If her weight is 500N what is the area of the floor she is standing on? • (Hard!) The pressure due to the atmosphere is 100,000N/m2. If 10 Newtons are equivalent to 1kg how much mass is pressing down on every square centimetre of our body?

  35. Kinetic theory • Particles collide with the walls of the container • They change their momentum • They exert a force on the wall • This creates “pressure”. The collisions of the molecules with the container (not with each other) create the pressure.

  36. Assumptions of the kinetic theory • Molecules behave as if they were hard, smooth, elastic spheres. (i.e. the collisions are perfectly elastic) • Molecules are in continuous rapid, random motion. • The average kinetic energy of the molecules is proportional to the absolute temperature of the gas. • The molecules do not exert any appreciable attraction on each other. • The volume of the molecules is infinitesimal when compared with the volume of the gas. • The time spent in collisions is small compared with the time between collisions.

  37. Macroscopic behaviour of a gas • Why does the volume of a gas expand when heated? • Why does the pressure of a gas increase when heated? • How does the temperature relate to the molecular behaviour?

  38. Tsokos page 171 q’s 1-10.

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