Thermal Energy and Heat Transfer

Transcription

1 Thermal Energy and Heat Transfer Thermal Energy - Kinetic Molecular Theory Heat is the thermal energy of a substance. Thermal energy is really the kinetic energy of the individual molecules as they vibrate, move from one place to another (not in a solid) or rotate within a substance. All matter is made up of small particles (atoms and molecules) that are in constant motion. Atoms and molecules exert forces on one another to keep them a certain distance apart. Moving closer causes for repulsion, moving farther apart causes attraction. But the distances between molecules and the strength of the force between them is responsible for the three physical states of matter: solid, liquid, and gas. If an object heats up the particles have more kinetic energy and therefore move faster. Energy transfers from one particle to another by collisions. (See diagram below) 1

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3 Thermal Energy and Temperature Temperature: a system of measuring the average kinetic energy of all of the particles in a quantity of matter. Thermometer: a device for measuring temperature that was originally based on the thermal expansion (increase in the volume of a substance when it is heated) of substances. Thermometers must be calibrated with two reference points. Fahrenheit: a temperature scale used in the old imperial system of measure. (freezing point of water 32 F, boiling point 212 F) Celsius: the metric measurement for temperature. (freezing point of water 0 C, boiling point 100 C. Kelvin: the scale designed with the lowest possible temperature being 0 K (absolute zero) where it is believed that all molecular motion stops. 3

4 Thermometers may also be engineered in different ways. Heat vs. Temperature Which has a greater temperature, the cup or the kettle? Which has the greatest heat? 4

5 Thermal Energy Transfer There are three ways that heat (thermal energy) is transferred. (A) Conduction, (B) Convection, (C) Radiation (A) Conduction Conductor is a material that allows heat to transfers easily through it. Metals are good conductors. A Heat insulator does not allow easy heat transfer. Heat Conduction is the process of transferring heat by particle collision. (See the diagram and table below) Here heat is conducted from the hot end to the cold end by molecular collisions. 5

6 To Conductivity is a relative scale that describes how well heat can be conducted in a substance. The greater the conductivity the easier heat is conducted. Copper is almost twice the conductivity of aluminum which is why it is used on the bottom of many cooking pots. (B) Convection Convection is the process of transferring heat by a circulating path of fluid. The circulating path of fluid is called of convection current. Any part of the fluid that heats up becomes less dense, leaving cooler more dense fluid to take its place. Baseboard heaters use convection currents to heat a room. 6

7 Sea Breezes and land breezes are really caused by convection currents. (C) Radiation Radiation transfers heat energy through a wave form of electromagnetic radiant energy. Wave length is called infrared radiation. Warm substances radiate infrared radiation where the energy is either absorbed, reflected from, or transmitted through the material that receives this energy. Infrared radiation (heat radiation) is part of the electromagnetic spectrum. 7

8 Controlling Heat Transfer: To control heat transfer all three forms of heat transfer must be considered. The idea is to reduce heat transfer by all three forms. This is done by: For Conduction a better insulator is used between the hot and cold areas. (Example: Use of styrofoam) For Convection all fluid (water or air) is removed from the area by either covering or surrounding the hot substance. (Example: Double glazed windows with air removed between them) For Radiation a reflector is used to re-reflect radiate heat energy back towards the hot substance. (Example: The use of aluminum foil in cooking to reflect radiant heat) All three examples can be shown in one product, the thermos bottle. Diagram of Thermos Bottle A thermos is a glass envelope holding a vacuum. Inside a thermos is glass, and around the glass is a vacuum. The glass envelope is fragile, so it is encased in a plastic or metal case. A thermos then goes one step further. The glass is silvered (like a mirror) to reduce infrared radiation loss by re-reflecting it inside. The combination of a vacuum and the silvering greatly reduces heat transfer by convection, conduction and radiation. 8

9 Activity: Look at pages Make point form notes (with diagrams) to describe how heat transfer is effected in windows, mittens/gloves and with the Greenhouse effect. 9

10 Heat Exchange and Changes of State When heat is added to a substance (like a pot of water on the stove) the temperature will rise. However there is a difference in the way two different substances (like cooking oil or water) will respond to heat being added. The heat transfer depends on the temperature difference, the mass of the substance being heated and the type of substance. How does each one of these three things effect the amount of heat transferred? The greater the temperature difference the more/less heat is transferred. The greater the mass of the substance being heated the more/less heat is transferred. But what about the type of substance? This has to do with what is called the heat capacity of a substance. In general, the greater the heat capacity (whatever that is) the faster/slower a substance will heat up or cool down. Specific Heat Capacity When a substance is being heated there is no direct way that we can measure the amount of heat being transferred. Instead, heat transfer is measured in an indirect way: specific heat capacity (c)= energy transferred (mass)(temperature change) c= E m T 10

11 The units for c, the specific heat capacity, are: J (kg)( O C) So the specific heat capacity is: the amount of heat transferred when the temperature of 1.0 kg of a substance changes by 1.0 O C. In general this means that substances with high specific heat capacities take a long time to heat up or cool down and take a lot of heat transfer to cause a temperature change. Some sample specific heat capacities are listed below: Substance Specific Heat Capacity [J/(kg O C)] Water 4200 Concrete 3000 Ethyl Alcohol 2500 Ethylene Glycol (antifreeze) 2200 Vegetable oil 2000 Air 995 Aluminum 920 Glass 840 Sand 790 Iron 450 Copper 390 Brass 380 Silver 240 Lead 130 Heat can be calculated then by using the specific heat capacity formula that has been rearranged for E, energy. E= mc T 11

12 Principle of Heat Transfer/Heat Exchange: When two different substances at different temperatures are mixed, the heat released by the hotter substance equals the heat gained by the colder substance. m E h c h H t lost h = E or = H gained t ( c c c m c ) 12

13 Changes of State and Latent Heat When heat is transferred into or out of a substance the result could be one of two things: (A) A temperature change (E=mc t) (B) A change in state There are 5 changes of state 1. Fusion or melting (solid to liquid) 2. Vaporization liquid to gas...boiling is a fast form of this. 3. Condensation or Liquification (gas to liquid) 4. Solidification or Freezing (solid to liquid) 5. Sublimation (either solid to gas or gas to solid directly) The following diagram shows the changes of state. The diagram is an ideal heating/cooling curve for a pure substance such as water. Heat is constantly being added to the system yet at certain points there is no rise in temperature. The heat flow into the system is not causing a temperature change but rather a change in state. 13

14 Latent Heats of Fusion and Vaporization The Latent heat of fusion (L F ) is the amount of energy needed to melt 1.0 kg of ice. The word, latent, means to lie hidden. This refers to the fact that during melting the added heat does not cause a rise in temperature. EH LF = m Q LF = m Where E is the heat in Joules and m is the mass in kilograms. Latent heat is measured in joules per kilogram (J/kg) The Latent heat of vapourization (L v ) is the heat required to vaporize a liquid. EH LV = m Q = m like L F, L V is measured in joules per kilogram (J/kg). LV 14

15 Latent Heats of Fusion and Vaporization Substance L F (J/kg) L V (J/kg) Aluminum 9.0 x x 10 7 Ethyl Alcohol 1.1 x x 10 5 Iron 2.5 x x 10 6 Lead 2.3 x x 10 5 Silver 1.1 x x 10 6 Water 3.3 x x 10 6 If the latent heats for a substance are known (tabulated) then they can be used to calculate the heat needed to melt or boil away a certain mass of the substance. E H = L F m E H = L V m 15