6 Weeks Exam (HW1-HW3)

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The quality of a two-phase liquid-vapor mixture of H2O at 40°C with a specific volume of 10 m3/kg is

0.511

A system consists of a two-phase liquid-vapor mixture of 5 kg of Refrigerant 134a. One kg is saturated liquid. What is the quality?

0.8

The quality of saturated vapor is

1.

Refrigeration cycle

A cycle where energy is transferred by heat into the system undergoing the cycle from the cold body and energy is transferred by heat from the system to the hot body.

Adiabatic

A process involving no energy transfer by heat.

Thermodynamic cycle

A sequence of processes that begins and ends at the same state.

If a process is modeled as occurring without heat transfer, the process is said to be an _________ process.

Adiabatic.

For a refrigeration cycle operating between two thermal reservoirs, the coefficient of performance is defined as the ratio of the energy input to the cycle be heat transfer from the _______ body divided by the net work into the system to accomplish this effect.

Cold.

A subcooled liquid is alternatively referred to as a _______ liquid.

Compressed.

The mode of energy transfer that can take place within solids, liquids, or gases as a result of the transfer of energy from more energetic particles to adjacent particles that are less energetic is commonly referred to as ________ heat transfer.

Conduction.

The mode of energy transfer between a solid at a temperature Tb and an adjacent gas or liquid at another temperature Tf is commonly referred to as _________ heat transfer.

Convection.

Energy transfer by work

Energy transfer for which the sole effect on everything external to the system could have been the raising of a weight.

Energy transfer by heat

Energy transfer induced only as a result of a temperature difference between a system and its surroundings.

Sign convention for work.

Energy transfer out of the system is considered positive.

When work is done, ______ is transferred or stored.

Energy.

The differential ⅆV, representing volume change, is said to be an _________ differential.

Exact.

Are the kinetic and gravitational potential energies of a closed system intensive or extensive properties?

Extensive.

Air can always be regarded as a pure substance.

False.

An electric battery is charged for 10 hours at a rate of 100 kW from a solar cell array on your roof. It is possible to later retrieve 1000 kW∙h of electricity for use in your home.

False.

Cooling of computer components achieved by a fan-induced air flow falls within the realm of radiation heat transfer.

False.

If a system's temperature increases, it must have experienced heat transfer.

False.

The change in gravitational potential energy of a 2-lb mass whose elevation decreases by 40 ft where g= 32.2 ft/s2 is -2576 ft·lbf.

False.

When a gas or a liquid expands, the work done on the gas or the liquid is positive.

False.

The symbol Δ is always used to denote:

Final value minus initial value.

The specific heat ratio, k, of an ideal gas must be

Greater than one.

What direction is the net energy transfer by work for a heat pump cycle: in or out?

In.

The differential δ⁢W is said to be ______ because the integral of δ⁢W for a process from state 1 to state 2 cannot be evaluated without specifying the details of the process.

Inexact.

The change in total energy of a closed system other than changes in kinetic and potential energies are accounted for by the change in ___________ energy.

Internal.

Each line of the following table gives data, in Btu, for a process of a closed system. Determine the missing table entries, in Btu.

LOOK AT FLACK'S ANSWER IN FOLDER.

The following table gives data, in kJ, for a system undergoing a power cycle consisting of four processes in series. Determine: the (a) missing table entries, each in kJ, and (b) the thermal efficiency.

LOOK AT FLACK'S ANSWER.

The line labeled a on the figure is a line of constant

Look at graph. Pressure.

According to the sign convention for heat transfer, energy transfer by heat from the system to its surroundings is considered

Negative.

A system contains a two-phase liquid-vapor mixture at equilibrium. Are the pressure and temperature of the mixture independently variable?

No.

Is the expression for work of a polytropic process W=(p2V2-p1V1)/1-n restricted to processes of an ideal gas?

No.

What direction is the net energy transfer by work for a power cycle: in or out?

Out.

According to the sign convention for work, energy transfer by work from the system to its surroundings is considered

Positive.

In order to evaluate work using ⅆW=∫V1V2 pⅆV, we must specify how p varies with V during the process. It follows that work is not a

Property.

A closed, rigid tank is filled with a gas modeled as an ideal gas, initially at 20°C and a gage pressure of 300 kPa. The gas is heated, and the gage pressure at the final state is 450 kPa. The local atmospheric pressure is 1 atm. Determine the final temperature, in °C.

T2 = 129.52 degrees Celsius

The specific enthalpy of an ideal gas is a function of ______ only.

Temperature.

In evaluating the change in gravitational potential energy, we usually assume that the elevation change is small enough that the acceleration of gravity, g, can be assumed constant.

True

A dielectric material in a uniform electric field can experience energy transfer by work if its polarization changes.

True.

A polytropic process for an ideal gas with n=k is adiabatic.

True.

A rotating flywheel stores energy in the form of kinetic energy.

True.

A two-phase liquid-vapor mixture has 0.2 kg of saturated water vapor and 0.6 kg of saturated liquid. The quality is 0.25 (25%).

True.

According to the Stefan-Boltzmann law, all objects emit thermal radiation at temperatures higher than 0 K (0°R).

True.

As pressure increases toward the critical pressure, the values of vf and vg approach each other.

True.

For a system at steady state, no property values change with time.

True.

For any cycle, the net amounts of energy transfer by heat and work are equal.

True.

For heat pumps, the coefficient of performance γ is always greater than or equal to one.

True.

For liquid water, the approximation v(T,p)≈vf(T) is reasonable for many engineering applications.

True.

For pure, simple, compressible systems, temperature and specific volume can be regarded as independent intensive properties.

True.

If the only force acting on a body is the force of gravity, the sum of the kinetic and gravitational potential energies remains constant, but energy can be converted from one form to another.

True.

In principle, expansion or compression work can be evaluated using ⅆ∫pⅆV for both actual and quasiequilibrium expansion processes.

True.

Power is related mathematically to the amount of energy transfer by work by integrating over time.

True.

The following assumptions apply for a substance modeled as an incompressible substance: The specific volume (and density) is constant and the specific internal energy is a function of temperature only.

True.

The problem solving format used in the text is intended to guide your thinking as you apply a systematic approach to solving problems in engineering thermodynamics.

True.

Thermal radiation can occur in vacuum.

True.

When a system undergoes a thermodynamic cycle, there is no net change in its energy.

True.

When an ideal gas undergoes a polytropic process with n= 1, the gas temperature remains constant.

True.

Work is not a property.

True.

For a simple compressible system, how many independent intensive properties must be specified to fix the state?

Two.

Determine the volume, in ft3, of 2 lb of a two-phase liquid-vapor mixture of Refrigerant 134A at 40°F with a quality of 50%. What is the pressure, in lbf/in.2?

V = 0.96 ft^3 p = 49.7 lbf/in^2

The quality of saturated liquid is

Zero.

For H2O, determine the specified property at the indicated state. (a) T = 140°C, v = 0.5 m3/kg. Find p, in bar. (b) p = 30 MPa, T = 80°C. Find v, in m3/kg. (c) p = 10 MPa, T = 550°C. Find v, in m3/kg. (d) T = 80°C, x = 0.4. Find p, in bar, and v, in m3/kg.

a.) p = 3.61 b.) v = 0.001029 m^3/kg. c.) v = 0.03619 m^3/kg. d.) p = 0.4739 bar v = 1.36 m^3/kg

Change in total energy.

delta KE + PE + delta U


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