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Thermal Energy Thermal Energy By the Law of Conservation of Mechanical Energy, the work we put into a system is equal to the work we get out.If a ball is dropped from a height of 2 m, does it bounce back to its original height? No! The difference between the height dropped and the height of the bounce is the energy lost. Where did the energy go? Energy to compress the ball. Energy to overcome air resistance. And, the temperature of the ball increased (energy lost as heat) because of friction of the ball coming into contact with the floor.. Thermodynamics - The Study of Heat Heat is energy. More specific, heat is the flow of thermal energy between two objects. The unit of heat flow is the joule. If 1 g of water is raised 1 oC, 4.18 joules of heat is used. Or, if 1 kg (1 liter) of water is raised 1 oC, 4180 joules of heat energy is used. Another, more familiar unit for energy is the calorie. 4.18 joules = 1 calorie. For this course we will use 4180 j/kg oC for the specific heat capacity of water. Problem: How much heat energy (in joules) is used to raise the temperature of 25.0 g of water 10.0 oC? o Heat used (joules) = (mass H2O) x (specific heat of water) x (change in temp. C) # joules = (0.0250 kg) x (4180 j/kg oC) x (10.0 oC) = 1050 joules The English system uses British Thermal Units (BTU). The amount of energy needed to change the temperature of 1 pound of water by 1oF. 1 BTU = 252 calories. Air conditioners and furnaces are rated in the number of BTU's. Larger air conditioners can remove larger amounts of heat, thus it has a larger BTU rating. Thermal Equilibrium Heat is the flow of thermal energy. If two objects of different temperatures are placed in contact with each other, energy ALWAYS flows from the hotter to the colder object. A cup of hot coffee becomes cool because the heat from the coffee flows out into the surrounding cooler air. A glass of cold water becomes hot because the heat from the surrounding air flows into the cold water. Heat will not flow if the temperatures of the two are equal. This is thermal equilibrium. Lets put on our molecular goggles and look at the particles of two substances at different temperatures. The hotter substance has a greater thermal energy because its particles are moving at higher velocities. When fast moving particles collide with slow moving particles, the fast ones slow down while the slow ones speed up. This exchange of velocities continues until all of the particles have the same velocities (on average). Thus they have equal kinetic energies (on average). This is thermal equilibrium. Heat flow continues until the temperatures of the two substances are equal. Temperature Take a gallon of boiling water and a cup of boiling water. Which is at a higher temperature? They are equal (100oC)! Which has a greater amount of heat? The gallon of boiling water! Heat is the AMOUNT of heat flow. The gallon of boiling water can transfer MORE heat because it has MORE mass. Temperature is related to the kinetic energies of the particles of the substances. At the boiling point, the energies of the particles are equal. They have the same velocities. However, there are more particles in the gallon of water. There are more particles able to transfer energy to another object. There is greater energy flow or heat flow. There are three temperature scales currently being used. Fahrenheit Scale, where the freezing point of water is 32o and boiling point is 212o. Celsius Scale, where freezing point of water is 0o and boiling is 100o . Kelvin Scale, where the freezing point of water is 273 K and boiling point of water is 373 K. The Kelvin scale is also called the absolute scale because 0 K is defined as having NO HEAT ENERGY. This is the coldest possible temperature. Notice, that the Kelvin scale is obtained by adding 273 to any temperature in the Celsius scale. Example: We have 2 containers, one with twice as much water as the other. We add 200 joules of heat to the smaller container and the temperature rises 2 oC. If we add the same amount of heat to the larger container, what is the temperature rise? Since we have twice as much water, then it takes twice as much energy to increase its temperature the same amount. Therefore, since we have double the water, we get half the change in temperature. We have twice as many particles, thus they get half the energy of the smaller container. Conduction, Convection, and Radiation Hold a metal object over a flame. The end in the flame becomes hot, while the part you hold is still cool. Eventually the part you are holding becomes hotter. Heat is transferred through the object by conduction. Heat is transferred by the atoms of the metal colliding with each other. This is a transfer of kinetic energy from one particle to another. There is no movement of particles from one spot to another. The rate at which objects conduct heat is dependent upon the substance. Particles in a solid are closer together. They collide more often thus they can transfer kinetic energy faster. Particles in a liquid are further apart. They collide less often than solids, thus they conduct heat slower than solids. Particles in a gas are further apart than solids or liquids. They collide less often than solids and liquids, thus they conduct heat slower than solids or liquids. Objects that transfer heat readily are thermal conductors. Objects that transfer heat poorly are thermal insulators. In liquids and gases, heat can be transferred by a movement of the substance from a hotter region to cooler regions. This is convection. Convection of heat is dependent on the change in density of the substance when it becomes hotter. We have seen that hot air rises. It rises because it is less dense. Density = Mass / Volume, or D = M/V As we heat a substance the particles increase their velocities and the spaces between particles increases. Therefore it occupies a greater space. In other words, its volume increases. Notice, that as the volume of the substance increases while its mass remains constant, the density decreases. Example: 1 g of a gas occupies 1 liter = 1000 cm3 (10 cm x 10 cm x 10 cm) Its density = 1 g / l. If we heat it and it expands to 2 liters, its new density is now 0.5 g/l. Because it has a lower density it will float (rise) in the surrounding cooler air. Less dense objects float on top of more dense objects. Thermal energy can also be transferred via electromagnetic waves. These can travel through a vacuum such as outer space. This is radiation. The sun's thermal energy is converted to electromagnetic radiation. This radiation travels through space and is converted back into thermal radiation when it reaches the earth. Radiation of energy requires heat to flow through another substance without it affecting its temperature. The Greenhouse Effect The earth and atmosphere are warmed by radiant energy from the sun. The earth in turn emits terrestrial radiation lost to outer space. Changes in average temperature of the earth is dependent on how much radiant energy enters and leaves the earth. During the last 50,000 years the average temperature has fluctuated between 19oC and 27oC. The earth's temperature is currently around 27oC. Light of short wavelengths from the sun enter and pass through the atmosphere rather easily. Light of long wavelengths leave the earth as terrestrial radiation. However, much of the radiation is reradiated back to earth by the atmosphere as heat. This keeps the earth warm. Molecules such as carbon dioxide in the atmosphere are responsible for the reradiation process. The more CO2, the more energy radiated back to earth. The more energy reradiated, the warmer the earth. This is a problem. .
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