Slide 1

Chapter 5: Heat and Heat Transfer Slide 2 Introduction Terminology Heat Energy vs. Thermal Energy These two terms are essentially the same thing. They can be used interchangeably. Slide 3 Introduction What causes heat? Where does it come from and why? For over a century, scientists hotly debated the answers to these questions as they worked to develop modern heat theory. http://www.brainpop.com/science/energy/heat/ Slide 4 Introduction In a pre-match society, starting a fire was not the simple task it is today. In many early societies, people who could start fires easily were admired, often revered. If it was your job to keep the fire going, letting it go out was considered a grievous wrong. Slide 5 5.1 The Nature of Heat Friction Theory when two surfaces are rubbed together, the parts that touch resist movement This resistance is friction Count Rumford used observations about friction to change the way scientists look at heat Slide 6 5.1 The Nature of Heat Benjamin Thomson was an engineer and scientist who lived in the Thirteen Colonies at the time of the American Revolution. Because he stayed loyal to Britain, many Americans considered him a traitor. British loyalists, however, considered him a hero. Thomson was given the title of Count Rumford to reward him for his loyalty to Britain. Slide 7 5.1 The Nature of Heat Rumford was an engineer/scientist who was hired to manufacture cannons in Munich, Germany. During this project, one worker carelessly touched a rod being used to bore a hold through a piece of metal. His hand was seriously burned. In the late 1700s, Count Rumford observed that heat was created when metal cut metal. This heat results from friction! Slide 8 5.1 The Nature of Heat Have you ever watched popcorn popping in an air popper? The random, dancing motion of the kernels is easy to observe. But what causes this motion? Slide 9 5.1 The Nature of Heat A similar type of motion is occurring in a glass of water. Slide 10 5.1 The Nature of Heat Robert Brown the first to realize this similarity During the 1800s he was using a microscope to observe pollen grains in a drop of water. He noticed that although the microscope was quite still, the pollen grains bounced around. When he increased the temperature of the water, the motion increased. This motion has become known as Brownian Motion http://www.youtube.com/watch?v=hy-clLi8gHg Slide 11 5.1 The Nature of Heat Making the observation was easy. But how can we explain it? Slide 12 5.1 The Nature of Heat At first, Brown thought that the pollen grains were alive. Later he reasoned that water must be composed of tiny unseen particles. These particles are in constant, vibrating motion. The motion of the pollen grains must be caused by collisions between the pollen grains and the other unseen particles. (Brown was unsure of what the particles were) Later, his evidence helped develop the kinetic molecular theory. Slide 13 Kinetic Molecular Theory http://www.youtube.com/watch?v=_rsqBNhFG1Y Slide 14 5.1 The Nature of Heat The concept of heat is commonplace; what causes heat is fairly abstract. Technically, heat is defined as the transfer of energy from one substance to another and is identified by a difference in temperature. An object does not possess heat. Rather, an object possesses thermal energy and can lose that energy in the process of heat loss. Slide 15 5.1 The Nature of Heat It is a common misconception to think of heat as a thing rather than a process. This probably stems from the original caloric definition of heat. Scientists thought that something measurable left an object when it got colder. Recall that Rumford was able to gain a scientific understanding of heat through observations of friction. Slide 16 5.1 The Nature of Heat Before Rumford, scientists thought heat was a fluid they called caloric. Rumford did not replace the caloric theory. Rather, he demonstrated an inconsistency in the theory. Two later British scientists Sir Humphry Davy and James Prescott Joule took Rumfords ideas to the next step. Slide 17 5.1 The Nature of Heat Davy, an English chemist, designed demonstrations to disprove the caloric theory. He rubbed ice and other solids with low melting points together to show that they would melt with heat from friction. Slide 18 5.1 The Nature of Heat Joule determined the mechanical equivalent of heat by measuring the change in temperature produced by friction. Working in imperial measure, he found that, on average, a weight of 772 pounds falling through a distance of one foot would raise the temperature of one pound of water by 1F. In addition, Joules investigations showed that heat is produced by motion, contradicting the caloric theory. Slide 19 5.1 The Nature of Heat We can relate this to the calorie, which is related to food energy. The calorie is a unit of energy. One calorie is the amount of energy needed to raise 1 gram of water by 1C. Other units of energy are the joule named after James Prescott Joule and the British Thermal Unit (BTU). The BTU is used in Canada to rate thermal output of stoves, ovens, and barbecues. Slide 20 5.1 The Nature of Heat Mysterious Motion Lab P. 84 Materials: 2 glasses, food colouring, hot/cold water Slide 21 5.1 The Nature of Heat Slide 22 Practice! Check Your Understanding p. 85 #1-3 Slide 23 5.2 Heat and Temperature The modern theory of heat began with Robert Brown He was the first to suggest that the energy that came with heat thermal energy was related to the motion of unseen particles of a substance. But, what are these unseen particles? What causes the motion? Slide 24 5.2 Heat and Temperature Recall the Particle Theory of Matter: All matter is composed of tiny, unseen particles These unseen particles are in constant, random motion. Slide 25 5.2 Heat and Temperature Kinetic means movement Kinetic Art art that moves (i.e. Mobiles) Kinetic Energy a form of energy associated with motion It is a measure of the amount of motion particles have. We can use kinetic energy to explain the difference between heat and temperature Slide 26 5.2 Heat and Temperature A molecule can have three different forms of movement: Vibrational molecules vibrate back and forth Rotational molecules spin and rotate Translational molecules bump and move around Slide 27 5.2 Heat and Temperature The motion of particles can be compared to bumper cars! Slide 28 5.2 Heat and Temperature Like bumper cars, atoms and molecules collide with each other at different speeds. All particles have different kinetic energies. Slide 29 5.2 Heat and Temperature So what is the difference between heat and temperature? Temperature the average of ALL kinetic energies of all particles in an object Heat the sum of all kinetic energies of all particles in an object Slide 30 5.2 Heat and Temperature Example: 25 mL Temperature: 30C Less kinetic energy 100 mL Temperature: 30C More kinetic energy Slide 31 5.2 Heat and Temperature Because temperature is a measure of how fast molecules are, and because molecules slow as they get colder, the coldest possible temperature is finite! Particles truly stop moving at absolute zero. Absolute zero is -273.15C The lowest artificial temperature achieved to date is 0.003K. The highest is estimated at 100 000 000K and was from a nuclear blast. Slide 32 5.2 Heat and Temperature There are more temperature scales than Celsius and Fahrenheit. The Kelvin scale (K) starts at absolute zero and rises in degrees equal in magnitude to Celsius degrees. Other scales include the Rankine scale and the International Temperature Scale. Slide 33 Slide 34 5.2 Heat and Temperature Eureka Videos (links on powerschool) Eureka Videos Practice! Check Your Understanding p. 87 #1-4 Slide 35 5.3 Transfer of Heat There have been many predictions about when Earth will end. Fear of Y2K computer problems brought excitement to New Years Eve 2000. As we study heat transfer, we will learn about another proposed end to the universe Slide 36 5.3 Transfer of Heat Forms of Heat Transfer Heat flows from hot to cold. The flow continues until both objects are at the same temperature. But what is really happening? How does thermal energy transfer from one object to another? Slide 37 5.3 Transfer of Heat Conduction Molecules placed on a hot burner will vibrate quickly. They have more kinetic energy than the molecules in a cooler location (a cool pot). Contact between the pot and the burner causes the molecules of the hot burner to collide with the slower molecules of the cool pot. These collisions result in a transfer of kinetic energy. Slide 38 5.3 Transfer of Heat The molecules of the cooler pot start to vibrate faster, gaining kinetic energy. The molecules of the hot burner vibrate slower, losing kinetic energy. This transfer of heat by contact is called conduction. Slide 39 5.3 Transfer of Heat Thermal conductivity and electrical conductivity are related. Substances that conduct electricity will also tend to be good thermal conductors. Metal is a good example of this. Conversely, glass and wood are poor thermal and poor electrical conductors. Slide 40 5.3 Transfer of Heat Convection If you hold your hand above a hot burner of a stove, you will feel a warm current of air. Why? Heat is transferred, by conduction, from the hot burner to the air molecules touching the burner. These air molecules gain kinetic energy, vibrate faster, and get farther apart. Slide 41 5.3 Transfer of Heat The warmer molecules are farther apart than the cool air molecules, so the warm air is less dense than the cool air around it. This causes the warm air to rise, creating the warm current that we feel. Cool, denser air rushes to take the place of the warm air. Slide 42 5.3 Transfer of Heat Because of continuous air flow, all the air in the room will become warmer. This transfer of heat by movement is called convection. Slide 43 Convection is why firefighters crawl on the floor.. Slide 44 5.3 Transfer of Heat Radiation Picturing a hand that is close to the side of a burner, but not above the burner can help us explain radiation. The front of the hand is not being heated by conduction or convection. There is no contact with the burner and convection currents of warm air would rise away from the hand. Slide 45 Radiation Examples Heat is transmitted through an empty space. Slide 46 5.3 Transfer of Heat The front the hand is being heated by radiation. Radiation is produced by vibrating electrons, which are tiny particles present in all atoms. This vibration makes a wave called an electromagnetic wave or infrared radiation wave. These waves are similar to the waves your hand can create when it vibrates in calm water. Waves or ripples run away from your moving hand. Slide 47 5.3 Transfer of Heat Infrared radiation waves travel from the burner. They strike the hand and transfer heat energy to the molecules in the hand. This causes molecules in the hand to vibrate faster. Slide 48 5.3 Transfer of Heat On a sunny day, most of the heat that you feel is the result of infrared radiation from the Sun. Infrared radiation is one form of electromagnetic radiation. Other forms include ultraviolet radiation, radio waves, visible light waves, and X rays. Electromagnetic radiation travels in waves. Each type of electromagnetic radiation has a specific wavelength and moves through the vacuum of space. Slide 49 5.3 Transfer of Heat Slide 50 Did You Know? Hot objects warm cool ones until their temperatures are the same. Some people believe this will happen to the universe. According to the theory, the temperature of the entire universe will eventually be the same. When this happens, heat will no longer transfer. Without a source of energy, life will be impossible! This prediction is called the Heat Death of the Universe. Slide 51 5.3 Transfer of Heat Practice! Check Your Understanding p. 91 #1-4 Slide 52 5.4 Heat Transfer in Nature Convection causes many weather phenomena, such as winds. Many cottages are on the shores of lakes, where convection currents keep the cottages cool in the daytime and warmer at night. In the evening, the air over the water cools more slowly than the air over the land. Warm air from over the water rises and moves toward the land, keeping it warm. During the day, cool air from the lake moves towards the warm land. Slide 53 Convection!! Slide 54 5.4 Heat Transfer in Nature Sea and Land Breezes Recall: warm air rises, and cool air sinks The circular movement that results is called convection. All winds start with convection currents. Slide 55 5.4 Heat Transfer in Nature Land and sea breezes are convection currents of air that occur near a shoreline. They are both created by differences in temperature near the surface of the Earth. Slide 56 5.4 Heat Transfer in Nature On a sunny day, radiant heat from the Sun strikes Earths surface. Heat is absorbed by land and water. But land and water heat at different rates. Land heats quickly, but also cools quickly. Water heats slowly and takes longer to cool. Slide 57 5.4 Heat Transfer in Nature Slide 58 Day Time Sea Breeze Slide 59 Night Time Land Breeze Slide 60 5.4 Heat Transfer in Nature How Oceans Help to Moderate Climates Oceans are capable of storing large amounts of thermal energy. This prevents the area around them from having extreme temperature changes. Oceans moderate the climate of land areas near them. This means they prevent the area from becoming too hot or too cold. But how??? Slide 61 5.4 Heat Transfer in Nature In cool weather, an ocean can release great amounts of heat without cooling much itself. Even during very hot or very cold days, and as seasons change, the temperature of oceans remains constant. As the sun warms the air above the ocean, heat flows from the air into the water, cooling the air. When the temperature of the air above the ocean cools, heat flows from ocean to air and warms the air. Slide 62 5.4 Heat Transfer in Nature We could say that oceans prevent land near them from getting very warm or very cold. Because of this, coastal cities (like Vancouver) have moderate climates. Slide 63 Circulation of Ocean Surface Water Warm currents are noted in the color red and cold currents are noted in the color blue. Slide 64 Compare the climate in Canada to the climate in England.. Slide 65 Slide 66 5.4 Heat Transfer in Nature Slide 67 Chinooks Warm West Winds Slide 68 If hot air rises, why is it cold in the mountains? Slide 69 5.4 Heat Transfer in Nature Ocean Currents and Climate https://www.youtube.com/watch?v=HgoANl_97kM Bill Nye - Currents https://www.youtube.com/watch?v=KuSB6HNRT2s Practice! Check Your Understanding p. 97 #1-3 Slide 70 5.5 Heat Transfer and Technologies Many household technologies either transfer or prevent the transfer of heat through conduction, convection or radiation. That is why metal cooking pots have hard plastic or wooden handles. What other techniques are used to handle hot food? Slide 71 5.5 Heat Transfer and Technologies Stoves Most cooks want to control how food is heated. Consider the following: heating soup on a stove top. The pot is heated by conduction through contact with the burner. The soup at the bottom of the pot heats up first through conduction. Slide 72 5.5 Heat Transfer and Technologies Heated soup rises to the top, because it is less dense. Colder soup near the top of the pot flows to the bottom because it is more dense. This causes a circular motion (aka convection currents) and allows the entire pot of soup to heat to a uniform temperature. Slide 73 5.5 Heat Transfer and Technologies Ovens When an oven is turned on, convection currents move heat inside the oven. Air at the bottom of the oven is heated by burning gas or an electric element. The heated air becomes less dense and rises to the top of the oven. The cooler air sinks to the bottom. Slide 74 5.5 Heat Transfer and Technologies The hot air heats the oven walls. These walls then radiate heat in all directions. Food in an oven then becomes cooked by both convection and radiation. There is also conduction if a baking pan is used. Slide 75 5.5 Heat Transfer and Technologies Did you know? Light-coloured surfaces reflect more heat than dark surfaces. This is why people in hot countries often wear light- coloured clothing. Slide 76 5.5 Heat Transfer and Technologies Getting Rid of the Heat Combustion of fuel inside an engine produces a large quantity of thermal energy. If this energy were not removed, the engine would overheat and be damaged. How is it protected? Slide 77 5.5 Heat Transfer and Technologies The engines cooling system contains a liquid coolant (most likely antifreeze). The coolant is pumped through the engine block to the radiator. A radiator is a honeycomb made of a metal alloy. The metal alloy is a good conductor. Heat from the coolant is conducted through this alloy to air. Slide 78 5.5 Heat Transfer and Technologies Either a fan or the motion of the vehicle forces this air through the radiator. Heat is transferred to the air that rushes through the radiator. These three techniques use conduction to protect engines from heat damage. Slide 79 5.5 Heat Transfer and Technologies Keeping it Cool When you put a little water on the back of your hand, your hand becomes cool. That is because thermal energy transfers from your hand to the water through conduction. Water absorbs heat from your hand and evaporates. Evaporation removes thermal energy as the water molecules leave the water droplets and move into the air. Your hand feels cooler. Slide 80 5.5 Heat Transfer and Technologies This form of cooling is usually called cooling by evaporation. The evaporation caused the cooling, but the cooling action started with conduction of heat away from your hand. Slide 81 5.5 Heat Transfer and Technologies Heat Transfer in a Refrigerator A. A fluid called a coolant circulates through the pipes. B. Heat from the food transfers to the cooler air surrounding it. Thermal energy then transfers from the air to the coolant. C. The coolant evaporates as it gets warmer. It is pumped to the compressor. D. When it reaches the compressor, pressure is applied to change it back into a liquid. E. The liquid coolant is pumped to these coils. Thermal energy is released into the room. The cycle starts again. Slide 82 Slide 83 5.5 Heat Transfer and Technologies Air conditioners work in a similar manner. In this case, the back of the unit is outside the room or house. To keep the inside of the room cool, heat is dispersed to the outside air. Slide 84 Although useful, coolant technology has an environmental cost. For the past 50 years, the liquid coolant used in refrigerators and air conditioners has been liquid chlorofluorocarbons, or CFCs. CFCs are responsible for atmospheric ozone depletion. As of January 2000, worldwide production of the most dangerous CFCs was to be replaced by alternative liquid coolants. Research is underway to produce environmentally safer but effective coolants. Slide 85 CFC Cycle Slide 86 5.5 Heat Transfer and Technologies Practice! Check Your Understanding p. 101 #1-4 Slide 87 REVIEW Chapter 5 Review p. 102 #1 12 Hand-in Assignment #5 Slide 88 Review Complete review questions on pg. 126 Hand-in Assignment #6