Energy Transfers. Heat Transfer. Internal Energy. Going With The Flow! 5/12/17. à Thermal Energy. Q = mcdt Q =DU. U = N(1/2mv 2 )

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1 Energy Transfers What Type of Energy Can be transformed into Thermal Energy? Heat Transfer à Thermal Energy Internal Energy What happens to the energy when it absorbed by the object? U = N(1/2mv 2 ) Where: U = Internal Energy N = Total number of molecules v = average velocity Going With The Flow! If two objects interact with different temperatures, heat will flow according to the following equation: If no work is done, the amount of heat that is transferred equals the change in an object s internal energy. Q =DU Q = mcdt 1

2 Methods Of Heat Transfer If heat is the transfer of energy, how is that energy transferred? Conduction Convection Radiation What Is It? Conduction Which Experiment(s) Demonstrates this method of heat transfer? In what direction does heat flow? Always moves high energy to low energy. Moves from a hot object to a cold object. Quantifying Conduction The rate of heat flow by conduction is dependent on the temperature difference, size and shape of the object, and the material from which the object is made. = ka T 1-T 2 l Where: A = Cross-sectional area l = distance between temperatures k = thermal conductivity Rate of Conduction Example A Styrofoam box used to keep drinks cold at a picnic has a cross-sectional area of 0.80 m 2 and a wall thickness of 2.0 cm. It is filled with ice, water, and cans of cola at 0 C. What is the rate of heat flow by conduction if the sand the cooler sits on makes the outside wall of the cooler 30 C? The thermal conductivity of Styrofoam is 0.01 W/m K. Info: A = 0.80 m 2 l = 0.02 m T 1 = 30 C T 2 = 0 C k = 0.01 W/m k = ka T 1-T 2 l = (0.01 W/m K)(0.80 m 2 ) = 12 Watts (303K-273K) 0.02 m 2

3 What Is It? Convection Everyday Phenomena Examples of convection are prevalent in the Earth Science. Which Experiment(s) Demonstrates this method of heat transfer? What Is It? Radiation Radiation All objects radiate energy. Produce a mixture of different wavelengths. Peak Wavelength Which Experiment(s) Demonstrates this method of heat transfer? l peak = x 10-3 mk T What s your peak wavelength? 98.6 F = 310 K l peak = 9.3x10-6 m 3

4 Living things are typically in the infrared region. Infrared and More So is the Earth Almost 100% radiated as Infrared. Not everything is infrared on Earth though Radioactive decay is also a form of radiation. How Do You Warm Up with Radiation? The amount you absorb is more than what you emit. Opposite --> Cool off Same --> No change What color and finish would you want an object to be to be a good absorber? Flat Black! What color would you want for an object to be a good emitter? Black! What would be the worst absorber/emitter? Reflective White or silver The best absorbers are also the best emitters! Quantifying Radiation The rate of heat transfer by radiation is dependent on the object s area, surface temperature, and of the object, its temperature, and the nature of its surface. = esat 4 Where: A = Area of the object s = 5.67 x 10-8 W/m 2 K 4 e = emissivity The emissivity is characteristic of a material and is a dimensionless number between 0 and 1. Shiny surfaces have an e close to 0. Black and dark objects have an e close to 1. You Do The Math! When an object emits radiation, it is also interacting with its surroundings. If the surrounding temperature is T 2, then the net flow of heat energy by radiation can be expressed as the following: = esa(t 14 - T 24 ) The human body has an average total surface area of 1.20 m 2 and a surface temperature of 30 C. What is the rate of heat loss if the emissivity of the human body is essentially 1? = (1)(1.20)(5.67 x 10-8 )(303 K) 4 If the surroundings are 20 C, what is the net rate heat loss? = 574 W = 72 W 4

5 Which Is More Effective? Let s Model Two Solids Conduction Convection Radiation Can your model explain why these feel different two you even though they are the same temperature? Conductors and Insulators How Will A Gas and Liquid Compare? Some materials transfer heat by conduction better than others. Which type of transfer? Thermal Conductivity (k) Conductors have a large k value. Metal is a good conductor! Insulators have a small k value. Wood is a good insulator! 5

6 Can Our Model Explain It? We Add 100 J of thermal energy to both Object A and Object B which both have a mass of 100-grams. Object A increases its temperature by 10 degrees while Object B increases its temperature by on 5 degrees. Q = mcδt Object A has a lower c than Object B! Why? Practice and Homework! Section Practice: 3, 21 Homework: 1, 6, 22, 23 A Practical Application Space Shuttle Thermal Protection System Good insulators that do not break down. Ceramic tiles HRSI can withstand temperatures between 1200 F F. LRSI can withstand temperatures between 700 F F. 6