Chapter 1 Sections 1-4

Calculate the net heat transfer (Qin - Qout) that occurs in a system where 80 J of heat energy is generated (Egen), and the change in the total energy of the system (ΔEsystem) is 400 J.

320 J

The differential changes in the enthalpy (h) of an ideal gas in terms of specific heat at constant pressure cp can be given by _____.

dh = Cp dT

The net change in the total energy of the system is the _____ of total energy entering the system and total energy leaving the system.

difference

Identify the equation that represents the thermal diffusivity (α) of a material, where ρ is the density of the material, cp is the specific heat of the material at constant pressure, and k is the thermal conductivity.

α = k/ρ×cp

Calculate the magnitude of the rate of net heat transfer of a steady-flow system when the mass flow rate into the control volume is equal to the mass flow rate out of the control volume. Given m˙m˙= 15 kg/s, cpcp = 50 J/kg··K, and ∆T∆T = 300 K, where m˙m˙ is the rate of mass transfer into or from the control volume, cpcp is the specific heat at constant pressure, cvcv is the specific heat at constant volume, and ∆T∆T is the change in temperature.

225 kJ/s

Calculate the convection heat transfer (Q˙conv) from a hot iron press (Ts) 120°°C left to cool in an environment (T∞) of 25°°C. The surface area (As) of the press is 75 cm2cm2 and the heat transfer coefficient (h) is 35 W/m2·Km2·K.

25W

Calculate the change in the temperature of a steady-flow system when the mass flow rate of the fluid flowing into the control volume is equal to the mass flow rate of the fluid flowing out of the control volume. Given: m˙ = 30 kg/s, cp = 50 J/kg··K, and Q˙ = 450 kJ/s, where Q˙ is the rate of net heat transfer from or into the control volume, m˙m˙ is the rate of mass transfer into or from the control volume, cp is the specific heat at constant pressure, cv is the specific heat at constant volume, and ∆T is the change in temperature.

300K

Identify the causes of heat conduction in liquids and gases.

Collisions of the molecules during random motion. Vibration of the molecules in the lattice.

The form of energy that can be transferred from one system to another as a result of temperature difference is _____ energy.

thermal or heat

The volume of a fluid flowing through a pipe or duct per unit _____ is called volume flow rate.

time

Identify the condition that represents a system in the absence of significant electric, magnetic, motion, gravity, boundary work, and surface tension effects, where ΔUsystem is the change in the internal energy of the system and ΔEsystem is the change in the total energy of the system.

ΔEsystem = ΔUsystem

If the surface area of an emitter (As) is 0.5 m2, the surface temperature of the emitter (Ts) is 350 K, and the maximum rate of emitted radiation (Q˙emit,max) is 250 W, calculate the emissivity (ε) of the body. Stefan-Boltzmann constant (σ) is 5.67 × 10−8W/m2K4.

0.59

Calculate the specific heat at constant pressure (cp) of dry air if the specific heat at constant volume (cv) of dry air is 0.718 kJ/kg⋅K and the gas constant R is 0.287 kJ/kg⋅K.

1.005 kJ/kg⋅K

If the density (ρ) of the material is 8000 kg/m3, the specific heat (cp) of the material at constant pressure is 490 J/kg.K, and the thermal conductivity (k) is 43 W/m.K, calculate the thermal diffusivity (α) of the material.

1.096 × 10-5 m2/s

Calculate the heat flux (q˙) through an area (A) of 25m2m2 through which a constant heat transfer occurs at a rate (Q˙) of 30 W.

1.2W/m2

Calculate the magnitude of the rate of heat conduction (Q˙condQ˙cond) through a wall of thickness 5cm, if its thermal conductivity (k) is 0.17 W/m··K. The cross-sectional area of the wall (A) is 50 m2m2 and the thermal gradient is found to be 20 K/m.

170 W

Calculate the change in enthalpy (Δh) in a system of unit mass whose temperature rises from (T1) 25°C to (T2) 85°C. The system has a specific heat at constant pressure (cp) of 45 kJ/kg⋅K.

2700 kJ/kg

If the total energy entering the system (Ein) is 60 J and the total energy leaving the system (Eout) is 30 J, calculate the change in the total energy of the system (Esystem).

30 J

If the total energy entering a system (Ein) is 75 J and the total energy leaving the system (Eout) is 15 J, then the change in the total energy of the system (ΔEsystem) is _____.

60 J

Calculate the heat transferred (Q) in a closed stationary system involving only heat transfer and no work. The mass (m) of the ideal gas in the system is 5 kg. The specific heat of the system (cv) is 300 J/kg⋅K and the initial and final temperatures (T1 and T2) are 25°C and 80°C, respectively.

82.5 kJ

Identify the reasons the specific heat at constant pressure is greater than the specific heat at constant volume for a given system.

Because the energy for work must be supplied to the system. Because the energy for work need not be supplied to the system.

One _____ is defined as the energy needed to raise the temperature of one pound-mass of water at 60°F by 1°F.

British thermal unit

Given the thermal conductivity (k) of the following materials, identify the one that can be best used to make a boiler.

Copper, k = 386 W/m⋅K

_____ represents the relationship between the two specific heats of an ideal gas, where cp is the specific heat at constant pressure, cv is the specific heat at constant volume, and R is the gas constant.

Cp = Cv+R

Identify the factors that cause high diffusion of heat in materials.

High thermal conductivity Low specific heat Low density

Identify the statements that describe the relationship between the rate of heat transfer and temperature gradient.

Higher the temperature gradient, higher is the rate of heat transfer. Lower the temperature gradient, lower the rate of heat transfer.

A cup of hot coffee which is at 70°C is placed in a 20°C environment. The temperature of the coffee is checked at 2 minutes intervals. It is observed that it took about 8 minutes for the coffee to reach thermal equilibrium with the environment. Rearrange the time intervals such that the interval with the highest rate of heat transfer is placed first and the interval with the least rate of heat transfer is placed last.

In order 0-2, 2-4, 4-6, 6-8

_____ is the international (SI) unit of thermal energy.

Joule

If Q˙ is the heat transfer rate and Δt is the time interval for the process, the amount of the heat transferred Q during a process can be calculated using the formula _______.

Q = Q˙Δt

The radiation emitted by real surfaces can be expressed as _____. Here, Q˙ is the maximum rate of emitted radiation, ε is the emissivity of a body. As is the surface area of the emitter, Ts is the surface temperature of the emitter, and σ is the Stefan-Boltzmann constant.

Q˙ = εσAsTs4 (W)

Identify Newton's law of cooling, where Q˙conv is the heat transfer through convection, h is the convection heat transfer coefficient, As is the surface area through which convection heat transfer takes place, Ts is the surface temperature , and T∞T∞ is the fluid sufficiently far from the surface.

Q˙conv = h × As × (Ts − - T∞)

Which of the following can be classified under internal thermal energies in a body that is under thermodynamic study?

Sensible Energy Latent Energy

Identify the correct statements about convection heat transfer.

The faster the fluid motion, the greater the convection heat transfer. In the presence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by convection.

Identify the causes of heat conduction in solids.

Vibrations of the molecules in the lattice Diffusion of the molecules during random motion

The thermal conductivity of iron (kiron) is 80.2 W/m⋅K and water (kwater) is 0.607 W/m⋅K. The specific heat cp of iron and water are 0.45 kJ/kg⋅°C and 4.18 kJ/kg⋅°C, respectively. Identify the statements that are true using the given information

Water is a poor heat conductor as the value of kwater is low. Water is a good medium for heat storage as the specific heat value is high. Iron is a good heat conductor as the value of kiron is high.

One _____ is defined as the energy needed to raise the temperature of 1 gram of water at 14.5°C by 1°C.

calorie

The mode of energy transfer that involves the combined effects of conduction and fluid motion is called _____.

convection

A measure of how closely a surface approximates a blackbody radiating energy is called _____.

emissivity

True or false: All good conductors of heat are good conductors of electricity.

false

The statement "Energy can neither be created nor destroyed during a process; it can only change forms" represents the _____.

first law of thermodynamics

The terms sensible and latent forms of energy in general, without the consideration of thermodynamics, are frequently referred to as _____.

heat

The rate of heat transfer per unit area normal to the direction of heat transfer is called _____.

heat flux

The amount of heat transfer per unit time is called _____

heat transfer rate

The sum of the kinetic and potential energies of the molecules of a system is termed _____.

internal energy

The internal energy related to the phase of a system is called _____.

latent energy

The mechanism of conductivity that occurs in crystalline solids and depends on the way the molecules are arranged is called the _____.

lattice vibrational waves

Identify the mechanisms of thermal conductivity in pure metals in the solid form.

lattice vibrational waves diffusion of particles

A closed thermodynamic system has a fixed _____.

mass

The absorptivity, α, of a blackbody is _____.

one

The portion of the internal energy of a system associated with the kinetic energy of the molecules is called _____.

sensible energy

The temperature gradient is the _____ the temperature curve in a T-x diagram (rate of change of temperature with thickness of the conducting medium) at location x.

slope of

The energy required to raise the temperature of a unit mass of a substance by one degree is called _____ of the substance.

specific heat

The specific heats of an ideal gas depend on _____ only.

temperature

The driving force for any heat transfer is _____.

temperature difference

The rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference is called _____.

thermal conductivity