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TODAS LS PREGUNTAS DE LOS TESTS: 1ª) Which is the difference between intensive and extensive thermodynamic variables? Extensive variables are additive, while intensive are not. That means that for extensive variables the value of the variable in a system is the addition of the values of the different parts in which you can divide the system. That is not true for intensive variables. 2ª) Write the principle of mass conservation in an open system. dm_v/dt = sum(dm_i/dt over all inflows) - sum(dm_o/dt over all outflows) 3ª) Define briefly the Andrews' bell It is the region of the phase diagram of a pure substance where liquid and gas can coexist. 4ª) Define the flow work It is the work done by the environment to maintain the inflow in a system, or the work done by the system to maintain the outflow. It is equal to pv (pressure times specific volume). 5ª) A system containing an ideal gas expands isothermally from a volume V to a volume 2V. How does it change its pressure? Does it absorb heat from or release heat to the environment? In an isothermal process in an ideal gas p·v=ct. Thus if the volume doubles the pressure must be divided by 2. Along the process the system must absorb heat from the environment to maintain constant its temperature while it expands. 6ª) Draw schematically a crossflow heat exchanger and write the equation of energy conservation in this system. dm_1/dt·Δh_1 = - dm_2/dt·Δh_2 where dm_1/dt·Δh_1 and dm_2/dt·Δh_2 are the enthalpy changes of both fluids respectively 7ª) Write the Mayer's relationship for an ideal gas. Define the adiabatic exponent C_p = C_v + R. γ=C_p/C_v 8ª) A closed system does a mechanical work of 600 J on the environment while it releases 600 J on the environment. Compute the change in its internal energy. Q = ΔU + W. Q=-600 J, W=600 J, then ΔU = -1200 J 9ª) Is it possible to establish a relationship (equality or inequality) between the specific heat at constant pressure and at constant volume that is satisfied in any material? The specific heat at constant pressure is never lower than the specific heat at constant volume. 10ª) A closed system performs a process where it does a mechanical work of 50 J on the environment while it absorbs 50 J from the environment. Compute the change in its internal energy along the process. Q = ΔU + W. Q=50 J, W=50 J, then ΔU = 0

11ª) A mixing chamber has two inputs (A and B) and one output (C). Overheated vapour comes through input A, compressed liquid water comes through input B and at the output we obtain a mixture liquid-vapour. Write the energy balance at the chamber, and the relationship between the pressures A, B and C. dm_A/dt·h_A + dm_B/dt·h_B = dm_C/dt·h_C. P_A = P_B >= P_C 12ª) Which relationship exists between the evaporation temperature and the evaporation pressure of a pure substance (if the evaporation temperature increases, the evaporation pressure increases, remains constant or decreases)? The evaporation pressure increases when the evaporation temperature increases. 13ª) What is a quasiestatic process? Why is it useful to study quasyestatic processes? Quasiestatic processes are processes were the system moves through equilibrium states infinitely close, so the state variables are always defined and satisfy the equation of state. Quasiestatic processes are useful because they are ideal representations of actual processes, and all features can be computed. Quasiestatic cycles have the highest performance or efficiency. 14ª) In an ideal gas, the relationship ΔH=m·Cp·ΔT, where Cp is the average specific heat in the ΔT interval, is only valid in processes at constant pressure or is valid in any process? Is valid in any process. 15ª) Write the principle of corresponding states. The state of two pure substances is equivalent if they have the same reduced values of temperature and pressure. Their compressibility is the same. 16ª) What is a process with steady flow? Is a process in an open system where the inflows and outflows are time independent. 17ª) Which common property have two systems that are in mechanical equilibrium? They have the same pressure. 18ª) What physical significance does it have the area below the curve of a process in the T-v graph? None. 19ª) 1 kg/s of air (R=0.287 kJ/kg/K, Cp=1.005 kJ/kg/K) enters a turbine at 1200ºC and 20 MPa and exits at 352ºC and 1 MPa. Compute the mechanical power produced by the turbine. La resposta correcta és: dw/dt = -dm/dt·Δh = 850 kW 20ª) Which is the difference between the critical and triple points of a pure substance? The critical point is the state of highest temperature where a pure substance can be in liquid phase. The triple point is the state where a pure substance can be simultaneously in solid, liquid and gas phase. 21ª) Define the massic and the volumetric flow through a pipe, and state the relation between them. The massic flow is the fluid mass crossing a surface normal to the pipe by unit time. The volumic flow is the fluid volume crossing a surface normal to the pipe by unit time. The massic flow is the volumic flow times the density of the fluid.

22ª) A thermodynamic system performs an adiabatic process while its volume increases. Write the First Law of Thermodynamics in this case, and indicate the sign of the change in internal energy in the process. Q = ΔU + W. Here Q=0 and W>0 because the system expands, then ΔU < 0 23ª) Write an equation that gives the mechanical power of a electric engine as a function of all the relevant physical processes in the engine when running. I·V = W_mechanical + W_friction + I^2·R 24ª) Write the state equation of an ideal gas in two ways; one as a function of the number of mols, and other as a function of the mass. Relate the constants appearing in them both. p·V = n·R_U·T, p·V = m·R_G·T, m·R_G = n·R_U 25ª) Write the equation that gives the pressure p as a function of the depth h in a lake p=ρ_H_2O·g·h 26ª) 0.2 kg/s of air (R=0.287 kJ/kg/K, Cp=1.005 kJ/kg/K) enter a compressor at 27ºC and 1 atm. If the power consumption of the compressor is 10 kW, which is the temperature of the outflow air? ~ 77ºC 27ª) Define the absolute scale of Temperature. It a scale measured in Kelvin degrees (K). At 0K the pressure of an ideal gas would be 0, and the temperature of the triple point of water is 273.16 K. 28ª) Is it possible to build a open system that performs pressure-volume work in steady regime? Not, because when an open system is in steady regime its volume is constant, and pressurevolume work can only be done by changing the volume of the system. 29ª) How much mechanical work is done by a system along a process where the volume is held constant? Justify your answer briefly. None. Mechanical work is given by integral(p_ext·dV), and thus is null if there is no volume change. 30ª) Which condition (or conditions) must be satisfied to ensure that the equation of state of the ideal gas gives a good description of the behaviour of a real gas? Either temperature must be over twice the critical temperature or pressure must be much lower than the critical pressure. 31ª) Estimate the pressure at the top of the Everest by using the isothermal atmosphere approximation. p=p_0·exp(-h/(8150 m)) with p_0 = 1 atm = 101 kPa. The Everest is 8848 m high, so p = p_0/e = 35 kPa 32ª) Someone says that the absolute pressure in a liquid doubles when the depth is doubled. Is it true? Justify your answer briefly. No. Manometric pressure doubles when the depth is doubled.

33ª) Write the equation of conservation of energy for an open system dU_VC/dt = dQ/dt - dW/dt + Δ(dh/dt +de_k/dt + de_p/dt) 34ª) The absolute pressure of a tyre is 200 kPa. What is its manometric pressure? 100 kPa 35ª) Consider the relationship ΔH = m·Cp·ΔT, where Cp is the average specific heat at constant pressure in the ΔT interval for an ideal gas. Is it valid only for processes at constant pressure, or is it valid for any process? It is valid for any process in an ideal gas. 36ª) Write the Postulate of Pascal. Pressure is a scalar variable. It can be compute as force divided by surface, regardless the orientation of the surface (provided it is small enough). 37ª) What physical significance does it have the area below the curve of a process in the p-v graph? It is equal to the mechanical work done by the system along the process. 38ª) Indicate which of these magnitudes are thermodynamic variables: Temperature, specific volume, enthalpy, specific heat. Temperature, specific volume, enthalpy. 39ª) An air compressor operates in steady regime with a pressure ratio rp. Write the relationship between the volumetric inflow and outflow. dV_o/dt·ρ_o = dV_i/dt·ρ_i·r_p, where dV_i/dt and dV_o/dt are the inflow and outflow respectively. 40ª) Define the compressibility factor. Z=(p·V)/(R·T) 41ª) A rigid container is filled with air at a manometric pressure of 200 kPa and a temperature of 300 K. Heat is transferred from the environment until the manometric pressure reaches 400 kPa. Which is the final temperature of the air? Assume that atmospheric pressure is 100 kPa. 500 K 42ª) Is the quality of a saturated mixture equal to the volume of the vapour phase divided by the total volume? No. The quality of a saturated mixture is the mass of the vapour phase divided by the total mass. The specific volume of each phase is always different, and this prevents the use of volumes in the definition of the quality of the mixture. 43ª) In which system and under which conditions the equation ΔU = n·cv·ΔT is valid? In an ideal gas, and not large temperature intervals, so as the specific heat can be taken as constant. 44ª) Define the reduced temperature and the reduced pressure of a pure substance. The reduced temperature and pressure are the temperature and pressure divided by the critical temperature or pressure of the substance respectively.

45ª) We use a thermometer (T) to measure the temperature of a thermodynamic system S. Being the thermometer another thermodynamic system, is there thermal equilibrium between S and T in any moment? Yes. When the temperature of T is stable is because S and T are in thermal equilibrium. 46ª) Draw schematically the p-V graph of a cycle where an ideal gas follows an isothermal expansion, a polytrophic expansion of polytrophic index 2 and an adiabatic compression. Please, look in your not es. 47ª) Write the Zero-th Law of Thermodynamics. Two systems that are not in direct contact but are in thermal equilibrium with a third system are also in thermal equilibrium between them. 48ª) Someone is boiling vegetables in a casserole which is a) open, b) closed with a light cap, and c) closed with a heavy cap. In which case the boiling time will be shorter? Why? The heavy cap will induce an increase in the internal pressure, and when the evaporation pressure of a substance increases the evaporation temperature also increases. Thus, the temperature in the casserole will be higher and the vegetables will cook faster. 49ª) Which process requires more energy: to completely evaporate 1 kg of liquid water at a pressure of 1 atm, or at a pressure of 8 atm? To evaporate the water at a pressure of 1 atm. The enthalpy of the phase change reduces when the pressure is increased. 50ª) Write de definition of the mechanical work done by a closed thermodynamic system along a process. W = integral(p_ext·dV) 51ª) Write the equation relating pressure and volume in a polytrophic process of an ideal gas p·V^gamma = ct. 52ª) A diffuser is a device that reduces the kinetic energy of a fluid. Where does the lost energy go? The lost kinetic energy is transformed into internal energy. 53ª) When a liquid evaporates does it absorb or release heat? When a solid melt does it absorb or release heat? Both absorb heat. The gas state has higher internal energy than the liquid, and the liquid state has higher internal energy than the solid. 54ª) Is it possible to compress isothermally an ideal gas in an adiabatic cylinder-piston system? Not. According to the first law of thermodynamics, Q=ΔU+W. In an adiabatic system, the internal energy (and consequently the temperature) must increase when mechanical work is done on the system. 55ª) The amount of heat absorbed when 1 kg of saturated liquid water boils at 100ºC must be equal to the amount of heat released when 1 kg of saturated water vapour condensates at 100ºC? Yes. They are exactly opposite processes.

56ª) Which is the difference between saturated vapour and overheated vapour? Saturated vapour is in a state of pressure and temperature such as it starts to condensate if heat is taken from it. Overheated vapour is in a state such as it won't start condensing to liquid neither if cooled nor if compressed. 57ª) An system containing an ideal gas expands to a given volume either at constant pressure or at constant temperature. In which of these processes will the system do more mechanical work? The mechanical work will be higher in the process at constant pressure, because pressure decreases when volume increases in isothermal processes. 58ª) Classify the following magnitudes between extensive and intensive: pressure, specific volume, internal energy, temperatura. Extensive: internal energy. Intensive: pressure, specific volume, temperature 59ª) Define adiabatic systems and adiabatic processes. An adiabatic system is a system that cannot exchange heat with the environment. An adiabatic process is a process heat exchange between the system and the environment is not allowed. 60ª) Write the Postulate of State. In a closed simple system in equilibrium, where the state variables are p,V and T, only two of them are independent and define completely the state of the system. 61ª) Which is the difference between mass and molar mass of a substance? How can you relate them? The molar mass is the mass of a mole of the considered substance. For a given amount of substance, the mass is the number of moles times the molar mass. 62ª) Draw schematically a crossflow heat exchanger and write the equation of energy conservation in this system. dm_1/dt·Δh_1 = - dm_2/dt·Δh_2 where dm_1/dt·Δh_1 and dm_2/dt·Δh_2 are the enthalpy changes of both fluids respectively...