Thermodynamics

64 | ThermodynamicsThat is, the total work is obtained by following the process path and addingthe differential amounts of work (dW) done along the way. The integral ofdW is not W 2 W 1 (i.e., the work at state 2 minus work at state 1), which ismeaningless since work is not a property and systems do not possess workat a state.EXAMPLE 2–3Burning of a Candle in an Insulated RoomRoom(Insulation)FIGURE 2–20Schematic for Example 2–3.A candle is burning in a well-insulated room. Taking the room (the air plusthe candle) as the system, determine (a) if there is any heat transfer duringthis burning process and (b) if there is any change in the internal energy ofthe system.Solution A candle burning in a well-insulated room is considered. It is tobe determined whether there is any heat transfer and any change in internalenergy.Analysis (a) The interior surfaces of the room form the system boundary, asindicated by the dashed lines in Fig. 2–20. As pointed out earlier, heat isrecognized as it crosses the boundaries. Since the room is well insulated, wehave an adiabatic system and no heat will pass through the boundaries.Therefore, Q 0 for this process.(b) The internal energy involves energies that exist in various forms (sensible,latent, chemical, nuclear). During the process just described, part of thechemical energy is converted to sensible energy. Since there is no increaseor decrease in the total internal energy of the system, U 0 for thisprocess.(Insulation)OVENPOTATO25°C200°CFIGURE 2–21Schematic for Example 2–4.HeatEXAMPLE 2–4Heating of a Potato in an OvenA potato initially at room temperature (25°C) is being baked in an oven thatis maintained at 200°C, as shown in Fig. 2–21. Is there any heat transferduring this baking process?Solution A potato is being baked in an oven. It is to be determinedwhether there is any heat transfer during this process.Analysis This is not a well-defined problem since the system is not specified.Let us assume that we are observing the potato, which will be our system.Then the skin of the potato can be viewed as the system boundary. Partof the energy in the oven will pass through the skin to the potato. Since thedriving force for this energy transfer is a temperature difference, this is aheat transfer process.EXAMPLE 2–5Heating of an Oven by Work TransferA well-insulated electric oven is being heated through its heating element. Ifthe entire oven, including the heating element, is taken to be the system,determine whether this is a heat or work interaction.