PHYS 1010 - Week 6 Notes
PHYS 1010 - Week 6 Notes PHYS 1010-01
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This 3 page Class Notes was uploaded by HaleyG on Friday February 19, 2016. The Class Notes belongs to PHYS 1010-01 at Tulane University taught by Timothy Schuler in Fall 2016. Since its upload, it has received 24 views. For similar materials see Great Ideas in Science & Tech in Physics 2 at Tulane University.
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Date Created: 02/19/16
PHYS 1010 Notes Week 6 Feb 1519 Thermal energy Based on experiments with friction Two surfaces moving in contact with one another get hotter Motion causes heat Kinetic energy can be turned into heat and vice versa Heat: change in temperature that occurs when thermal energy is transferred between two objects in contact Heat is thermal energy (internal energy), which is a form of physical energy Heat is positive when energy is transferred to an object's thermal energy from the environment, and negative when the object loses thermal energy to its environment Heat is always transferred from the hotter object to the colder object Heat is related to random kinetic energy of particles within a substance Bigger vibrations means more thermal energy Thermal expansion: adding kinetic energy to a solid lattice causes atomic bonds to stretch and leads to an expanding of the entire lattice Thermal expansion is the way we measure temperature Overstretching > turns from solid to liquid to gas Transfer of heat Radiation: heat from light Conduction: heat transferred because a hotter object is touching a cooler object Convection: heat transfer by motion of fluid (air or water) when the heated fluid moves away from the object, carrying heat with it Absorption of heat Heat capacity: tells us how much energy will be required to heat up an object Larger heat capacity means a larger temperature change for the same amount of energy; heat capacity is different for different substances Specific heat: heat capacity per unit mass of a substance More mass means more energy is required to change the temperature of the object Metal conducts heat very well; insulating materials like fiberglass and Styrofoam trap air and hold it in place to keep things cool Air almost doesn't conduct heat Temperature: used to describe a measure of heat SI unit: Kelvin Lowest possible limit: 0 K (when atoms stop vibrating); no upper limit Matter can't exist below 0 K Celsius scale is defined in relation to Kelvin scale Fahrenheit scale is defined in terms of Celsius scale Work done on a system by an external force External forces can do work on a closed system by transferring energy to or from a system Work done against friction (force times distance) becomes thermal energy (heat) Work done against the force of friction is the loss of mechanical energy to thermal energy Conservation of Energy The total amount of energy in a closed system can never change Energy may change forms but sum of all forms stay constant Earth is not a closed system Sun gives constant new energy Universe is a closed system Laws of Thermodynamics 0th Law: if objects A and B are each in thermal equilibrium with a third object C, then objects A and B are in thermal equilibrium with each other If something is in one part of a system, then it is in all parts of the system 2nd Law: the entropy of a closed system can never decrease 1st Law: the difference between the transferred thermal energy and change in mechanical energy determines the change in the internal energy of the system 3rd Law: in order to cool something to 0K, you need something colder than 0K, which can't exist as matter. Therefore nothing can be cooled to 0K. Entropy: randomness/disorder Many processes that occur in nature are irreversible (not symmetric) Problematic because conservation of energy is symmetric and doesn't suggest any kind of direction The entropy of a system, which is not conserved, sets the direction of irreversible processes If an irreversible process occurs in a closed system, the amount of entropy always increases (it can never decrease) Change in entropy is thought to be a factor in our concept of the "arrow of time" Time moving backwards would violate the concept of entropy Heat engines and refrigeration Thermal energy can be turned into work (mechanical energy) Put hot object in contact with a cold object > thermal energy moves from hot object to cold object Then, we can siphon off the energy that's moving between the objects and use it to drive the engine Q Hot Q Cold+ Work Efficiency = (Q Hot Cold / Q Hot The efficiency can never be 100% because some heat must flow to the cold object; however, the energy being transferred to the cold object doesn't do us any good Refrigerators go from hot to cold fridges are just reversing the previous flow of energy. We need a heat engine as well as the cooling system Can never be 100% efficient
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