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Transcript of Thermodynamics
Third Law To understand these laws, you need: This is the study of how heat and temperature affects the behavior of particles. There are two main concepts in Thermal Physics. Thermal Contact
Thermal Equilibrium Thermal Contact is when two objects can have energy exchanged between them. Thermal Equilibrium is when two objects are in thermal contact but there is no net exchange of energy There is also a completely different concept of Temperature: When studying thermodynamics, all temperature must be measured in the Kelvin Scale. The Kelvin scale is 273.15 above the degree in the Celsius Scale. Therefore 0 degrees Celsius is 273.15 Kelvin. The Kelvin scale was developed after the discovery of absolute zero, the temperature at which there is no heat (energy), thus being the coldest temperature theoretically possible. There is also the concept of Specific Heat: Specific heat is the amount of heat required to raise the internal temperature of a substance up one degree Kelvin. The specific heat is what determines whether or not a substance is a good conductor of energy. Which is a better ice cube container... the metal? or the wood? Answer: Wood This is because the specific heat of metal is lower than the specific heat of wood. Because less energy is required to increase the temperature of the metal, it would get warm faster than the wood. The equation to calculate the amount of energy absorbed is: If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other. -College Physics, Page 323 A good example of this law is an ice cube put in a bowl of hot soup. At first, the ice cube is cold and the soup is warm. However, after the ice cube melts, the soup gets cooler. In the end, everything (the melted ice cube and the soup) are the same temperature. This means that they reached a state of equilibrium. But what makes this law important??? This law is what helps us define temperature the way we do. Each temperature scale is based off something different. That is because each thermometer can be calibrated to something different. In fact, you could even make your own temperature scale and name it whatever you want to name it. I could make my own temperature scale named the Nel. However, it makes more sense when combined with the first law of thermodynamics. If a system undergoes a change from an initial state to a final state where Q is the energy transferred to the system by heat and W is the work done on the system, the change in the internal energy, U, of the system is given by: -College Physics. Page 388 This law basically means that not all the energy can be transfered, some energy must be converted into work. This law also shows conservation of energy. Both Work (W) and Internal Energy (U) are forms of energy, showing how energy was not destroyed, only converted. Work is the energy created by force and displacement. This force and displacement is shown through the expanding of the specific substance. This is why croissants expand while in the oven. This is also why the metre stick is never 100% accurate. There are Three Equations to measure expansion: Length
Volume Thermometers Each thermometer is callibrated to different things. That is why the Kelvin Scale, the Celsius Scale, and the Fahrenheit Scale are so different. The simplest scale is probably the Celsius Scale. It is callibrated to the phase changed of water. Water freezes at this exact temperature. Why don't we make that 0 degrees? Water boils at this temperature, why don't we make it 100 degrees. After setting it up like that, they can test the temperatures in between. Oh! The mercury rose to 75% of 100 degrees, let's make the 75 degrees. No heat engine operating in a cycle can absorb energy from a reservoir and use it entirely for the performance of an equal amount of work. -College Physics. Page 404 What is a heat engine? What happens is, the hot reservoir would give off thermal energy to the could reservoir. In that process, an engine is put in between the path and would ideally convert all the energy into work so that the cold reservoir gets no energy. Likewise, the opposite is true as well. Work will be supplied to force energy to go from the cold source to the hot source, thus reducing the amount of heat in the cold source. There are examples of the second law around us everyday. The steam engine is a very good example. By burning coal, heat is generated. This can act as the heat reservoir. An engine is placed so that the engine can help convert some of this energy into work. The refrigerator is another good example. By supplying work to the refrigerator, thermal energy is taken from the cold reservoir (the inside of the refrigerator) and moved to the hot reservoir, thus keeping our refrigerators cold at all times. To Understand the Third Law, First you must know a bit about: Entropy Consider the following model: To find the amount of work the engine was able to get from heat use the equation: Because it is impossible for the engine to convert all the heat into work, we need to find efficiency. That can be found with: After the wall between the two different particles is removed, the particles mix and lessen their degree of order. Entropy is their degree of disorder. By mixing together, the degree of order decreased, the degree of disorder increased, thus, entropy increased. The total amount of disorder-the total entropy-of a system plus its surroundings will never decrease. -Cracking the SAT Physics Subject Test. Page 190 This means that the entropy of the universe is forever increasing, never decreasing. Nothing will ever reach absolute zero. This is because at absolute zero, there is an absense of energy. Energy is required for particles to move. If there is no energy, particles won't move. If particles don't move, there would be no entropy. Entropy in the universe is forever increasing, therefore, it is impossible for there to be no entropy. That is why it is impossible to reach absolute zero. As a substance approaches absolute zero, eventually it will start taking in entropy and energy from its surroundings making it impossible to reach absolute zero. This is how much is out there... This is how much we know Fin Sources: