General
Pressure, temperature, density, and specific volume are all examples of what?
Intensive Properties
What is a line on a map connecting points having the same temperature at a given pressure?
An isotherm
What are the different mechanisms for transferring energy to or from a control volume?
Energy can be transferred to or from a control volume as heat, various forms of work, and by mass.
Is ice water a pure substance? Why?
Yes. Because it has the same chemical composition throughout.
A can of soda at room temp is put into the fridge so that it will cool. Would you model the can of soda as a closed system or as an open system? Explain.
Should be analyzed as a closed system bc no mass is crossing the boundaries of the system.
What are specific properties?
Extensive property per unit mass
A gas in a piston-cylinder device is compressed, and as a result its temperature rises. Is this a heat or work interaction?
Work
What is the difference between exact and inexact differentials?
Exact (dx) - depend on states; "state function"
Inexact (δx) - depend on path; "path function"
If Ti > Tsat, what phase is the water in?
Superheated vapor
On a Pv diagram, what does the area under the process curve represent?
Work for a quasi-equilibrium process
Define mass and volume flow rates. How are they related to each other?
Mass flow rate is the amount of mass flowing through a cross-section per unit time, whereas the volume flow rate is the amount of volume flowing through a cross-section per unit time. dm/dt = rho (dV/dt).
If the pressure of a substance is increased during a boiling process, will the temperature also increase or will it remain constant? Why?
The temperature will also increase since the boiling or saturation temperature of a substance depend on pressure.
What is the general equation for Work for an isochoric process?
W = ∫PdV = 0!
What is the specific internal energy of water at 50 kPa and 200 oC?
u = 2660.0 kJ/kg (Table A-6)
When can kinetic and potential energy be neglected in energy balances?
If the system is stationary
Describe the difference between Wout and Win
Out = by = (-), In = on = (+)
An adiabatic closed system is accelerated from 0 m/s to 30 m/s. Negleting internal energy change, determine the specific energy change of this system in kJ/kg.
∆ke =(v22 - v12)/2 = (302 - 02)/2 = 450 m2/s2
∆ke = 450 m2/s2 x (1 kJ/kg) / (1000 m2/s2)
∆ke = 0.45 kJ/kg
What is the overall efficiency of a lightbulb powered by solar power if ηpanel = 20%, ηtransmission = 90%, ηbulb = 0.5%?
ηtotal = (ηpanel)(ηtransmission)(ηbulb)
ηtotal = 0.09%
What is the quality (x) of a substance in the superheated vapor region?
Undefined
In an open system, the energy associated with mass crossing across the boundary is θ = u + ke + pe + Pv. What does the Pv account for?
Flow energy
List two ways to determine change in enthalpy (Δh) for a given ideal gas.
1. Δh = h2 - h1 = (u + Pv)2 - (u + Pv)1
2. Δh = cpΔT
3. Determine h2 and h1 from tables (water)
A rigid container equipped with a stirring device contains 2.5 kg of motor oil. Determine the rate of specific energy increase when heat is transferred to the oil at a rate of 1 W and 1.5W of power is applied to the stirring device.
d∆E/dt = Qnet/dt + Wnet/dt
d∆E/dt = 1 + 1.5 = 2.5 W
∆e/dt = (d∆E/dt) / m
∆e/dt = 2.5 W / 2.5 kg = 1.0 J/kg s
Determine the pressure of nitrogen gas at T = 175 K and v = 0.00375 m3/kg.
Rnitrogen = 0.2968 (kPa m3)/(kg K)
P = RT/v = (0.2968 x 175) / 0.00375
P = 13,851 kPa
An 80-L vessel contains 4 kg of refrigerant-134a at a pressure of 160 kPa. Determine the quality (x).
v = V/m = 0.08 m3 / 4 kg = 0.02 m3/kg
vf < v < vg, two-phase
x = (v - vf)/vfg = (0.02-0.0007435)/(0.1228065)
x = 0.157
A rigid tank contains 50 kg of saturated liquid water at 90 oC. Determine the pressure in the tank and the volume of the tank.
P = Psat at 90C = 70.183 kPa
V = mvf = (50 kg)(0.001036 m3/kg)
V = 0.0518 m3