In the game of 52 pickup, the prankster tosses an entire deck of playing cards onto the floor, and you get to pick them up. Have you ever played the card game 52 pickup? If so, you have been on the receiving end of a practical joke and, in the process, learned a valuable lesson about the nature of the universe as described by the second law of thermodynamics. Using temperatures from another, nonabsolute scale, such as Fahrenheit or Celsius, will give the wrong answer. The Kelvin scale is an absolute temperature scale that is measured in terms of the number of degrees above absolute zero. The equation for the change in entropy, Δ S Δ S, isĪbsolute temperature is the temperature measured in Kelvins. The unavailability of energy is important in thermodynamics in fact, the field originated from efforts to convert heat to work, as is done by engines. Consequently, not all energy transferred by heat can be converted into work, and some of it is lost in the form of waste heat-that is, heat that does not go toward doing work. Hence, entropy always tends to increase.Īlthough all forms of energy can be used to do work, it is not possible to use the entire available energy for work. The flow of any energy is always from high to low. It measures how much energy has been dispersed in a process. Entropy can be thought of as a measure of the dispersal of energy. When a hot object is placed in the room, it quickly spreads heat energy in all directions. When water in a dish is set on a counter, it eventually evaporates, the individual molecules spreading out in the surrounding air. For instance, if a car tire is punctured, air disperses in all directions. However, we see examples of entropy in our everyday lives. Alternatively, it is the scaling metric by which heat input/output cause the system or the surroundings to increase/decrease the number of available micro states.Ĥ) Molecules can have translational, rotational, and vibrational modes.The meaning of entropy is difficult to grasp, as it may seem like an abstract concept. Work creates friction heat, and that heat cannot be converted back to the same amount of work.ģ) It is how we define the entropy change during a process. The answer for friction is addressed using the Kelvin form of the second law. We can presume that heat is transferred across the boundary during a reversible process, but only because reversible processes are presumptions anyway. Therefore, heat transfer in the real world only occurs along an irreversible path. In this case, the system and surroundings are not at thermal equilibrium. It is that we always calculate $\Delta S$ for the system or the surroundings separately, we always use reversible processes to do so, and we only know whether the process is irreversible when we add the two values (system + surroundings).Ģ) In the real world, heat is only transported when we have a temperature difference. I've been using $ΔS=\dfrac Q /T$ must be used only in reversible processes (and never in irreversible processes). Questions 3 and 4 were just ideas that I suddenly thought about. Thermal energy does not seem reversible too, for the same reason. Heat transfer does not seem reversible because it is not quasi-static (reversible). Without the process being reversible (quasi-static), there will not be a defined and clear way to calculate the temperatures (state variable), so change in entropy cannot be calculated using the formula.Ģ. Here's my attempt to answer these questions.ġ. Other than adding thermal energy increasing the average kinetic energy of the molecules (and distributing it over a wider range of speeds for a higher number of microstates), are there any other ways for added thermal energy to increase the entropy by increasing the number of microstates? Why does the temperature at which thermal energy is added affect the change in entropy?Ĥ. Is heat transfer a reversible process? Is an object slowing down due to friction a reversible process?ģ. Why must this formula be used in reversible processes and not irreversible processes?Ģ. Where S is entropy, Q is thermal energy, T is absolute temperatureġ. One possible definition is that it is the logarithm of number of microstates of particles.įor a reversible (quasi-static + frictionless) process in a closed system (cannot transfer mass but can transfer thermal energy), I've recently started learning about entropy.
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