WELCOME TO PERIOD 5: THERMAL ENERGY, THE MICROSCOPIC PICTURE. Homework #4 is due today at the beginning of class.

Transcription

1 WELCOME TO PERIOD 5: THERMAL ENERGY, THE MICROSCOPIC PICTURE Homework #4 is due today at the beginning of class.

2 PHYSICS 1104 PERIOD 5 How are temperatures measured? How do atoms and molecules act at different temperatures? What causes phase changes?

3 Temperature vs. thermal energy Temperature: a measure of the AVERAGE kinetic energy of the atoms and molecules of a substance. Thermal Energy: a measure of the TOTAL internal energy of the atoms and molecules of a substance. Heat is thermal energy in transit from one location to another. Thermal conductivity is a measure of how quickly a material transports thermal energy. The larger the thermal conductivity constant K, the more rapidly heat is conducted through the material.

4 Kinetic energy and temperature Higher temperature substances have faster average velocity molecules and greater kinetic energy of the molecules. E kin = ½ M v 2 with E kin = kinetic energy (joules) M = mass (kilograms) v = velocity (meters/sec)

5 Graph of speed of gas molecules The graph shows the distribution of gas molecule speeds that corresponds to a gas temperature of 295 kelvin.

6 Gas molecules at different temperatures This graph shows the distribution of molecule speeds for samples of oxygen gas at two temperatures: 295 kelvin and 1,000 kelvin.

7 Transfer of thermal energy: conduction Molecules in a higher temperature object have a higher average energy than the molecules of a lower temperature object. Energy of the material at the higher temperature will be transferred to the molecules of the material at the lower temperature. The average energy of the molecules of the higher temperature object will decrease and the average energy of the molecules of the lower temperature object will increase. The temperature of the hotter object will decrease, and the temperature of the colder object will increase.

8 Transfer of thermal energy: convection Thermal energy is transferred due to the motion of the substance (liquid or gas). The molecules of a higher temperature portion of a material will move so as to mix with the molecules of a lower temperature portion of the material. In this way, the average molecular energy of the cold portion of the material increases, while the average molecular energy of the warm portion of the material decreases. Thermal energy has been transferred.

9 Transfer of thermal energy: radiation Electromagnetic radiation can transfer thermal energy usually as infrared radiation. Packets of radiant energy called photons are emitted by electrons.

10 Electrons emit radiant energy Infrared Photon Electron Visible Light Photon Nucleus An electron drops one energy level and emits a lower energy infrared photon with a longer wavelength An electron drops two energy levels and emits a higher energy visible light photon with a shorter wavelength.

11 Brownian motion of molecules The atoms and molecules of a substance move at different speeds and with random motions known as Brownian motion. Diffusion is the mixing of molecules due to this random motion. The higher the temperature of an object, the faster the average speed of the molecules and the greater the diffusion.

12 Measuring temperature: thermometers Each type of thermometer measures temperature with a change in measuring substance. What is the changing property in the thermometers? 1) alcohol thermometers 2) bimetallic strip thermometers 3) liquid crystal thermometers 4) infrared thermometer 5) Galileo s thermometer

13 Temperature scales Three temperature scales are commonly used: Fahrenheit, Celsius, and Kelvin o Fahrenheit o Celsius Kelvin Water boils Water freezes Nitrogen boils Absolute zero

14 Fahrenheit vs. Celsius Water boils: F C Water freezes: 32 0 F 0 0 C

15 Converting temperature scales convert Fahrenheit to Celsius: T C = 5/9 (T F 32) convert Celsius to Kelvin: T K = T C + 273

16 States (phases) of matter In Solids, molecules or atoms are held in a fixed position by electromagnetic bonds between the electrons and protons. Solids have a fixed shape and volume. In Fluids, molecules or atoms feel an attraction to one another, but there are no fixed bonds between them. Fluids have a fixed volume, but not a fixed shape. In Gases, molecules move independently of one another. Gases have no fixed volume or shape. In Plasma, electrons are stripped from the atom s nucleus, leaving charged particles.

17 Evaporative cooling 1) The molecules with the most kinetic energy are most likely to leave (evaporate) from the surface of a substance. These are the fastest moving molecules. 2) When the molecules with the most energy leave, the average kinetic energy of the remaining liquid molecules decreases. 3) A decrease in the average kinetic energy of the molecules means the liquid s temperature decreases.

18 Why does the Dippy Duck dip? Water evaporates from the duck s head, cooling it. Cooling the freon gas inside the duck s head causes the gas molecules to condense from gas into liquid. This reduces the volume of the gas and lowers the gas pressure in the head. As a result, liquid freon flows up the tube from the region of higher pressure in the duck s tail to the region of lower pressure in the head. As the freon moves up, the center of mass is raised until the duck topples over into the cup of water. While the duck s head is in the water, freon flows from the head into the tail, and the duck stands up again. The process repeats.

19 Temperature and volume Thermal expansion: Most materials expand when heated and contract when cooled. Water is an exception. Frozen water (ice) occupies a greater volume than liquid water. Check the water level of the beaker of ice and water on your table. Did the water level increase or decrease since the beginning of class?

20 Phase changes Phase changes occur when matter changes from one state of matter to another state. solid liquid solid liquid gas gas (sublimation) Phase changes occur with a change in. temperature pressure both temperature and pressure.

21 BEFORE THE NEXT CLASS Read textbook chapter 6. Complete Homework Exercise 5. Print out Activity Sheet 6.