Thermodynamics I - Unit 7 Quiz
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Questions and Answers

What is thermal energy?

Thermal energy is a form of energy associated with the random motion of atoms and molecules and is related to temperature.

What is the relationship between thermal energy and temperature?

Thermal energy is directly proportional to temperature.

What is meant by specific heat at constant volume?

Specific heat at constant volume (Cv) is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius while keeping the volume constant.

What is meant by specific heat at constant pressure?

<p>Specific heat at constant pressure (Cp) is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius while keeping the pressure constant.</p> Signup and view all the answers

What is enthalpy?

<p>Enthalpy is a thermodynamic property that represents the total heat content of a system.</p> Signup and view all the answers

What is the relationship between enthalpy and internal energy?

<p>Enthalpy is the sum of internal energy and the product of pressure and volume.</p> Signup and view all the answers

Which of the following is the correct formula for calculating the change in internal energy?

<p>ΔU = Q - W</p> Signup and view all the answers

Which of the following is the correct formula for calculating the change in enthalpy?

<p>ΔH = Q + W</p> Signup and view all the answers

What is the heat capacity relation for EOS substances undergoing single-phase processes?

<p>The heat capacity relation for EOS substances undergoing single-phase processes is du = cvdT.</p> Signup and view all the answers

How can we determine other final properties of a system using tables or EOS?

<p>Tables and EOS (Equations of State) provide information on the properties of substances at different conditions. By referencing these, we can determine the final properties of the system after a process has occurred.</p> Signup and view all the answers

What is the main type of energy involved in thermal energy?

<p>Kinetic Energy</p> Signup and view all the answers

What is the specific heat at constant volume (Cv) often used in calculations involving?

<p>Processes with constant volume.</p> Signup and view all the answers

What is Specific heat at constant pressure (Cp) often used in calculations involving?

<p>Processes with constant pressure.</p> Signup and view all the answers

Enthalpy is the sum of internal energy and the product of pressure and volume.

<p>True</p> Signup and view all the answers

Which of the following energy types is associated with the internal interactions of atoms?

<p>Potential Energy</p> Signup and view all the answers

Thermal energy is a type of potential energy.

<p>False</p> Signup and view all the answers

What is the relationship between specific heat and heat capacity?

<p>Heat capacity is the amount of heat required to raise the temperature of a substance by one degree Celsius, while specific heat is the heat capacity per unit mass of the substance.</p> Signup and view all the answers

What is the difference between true ΔU and approximate ΔU?

<p>True ΔU is the actual change in internal energy of a substance over a specific temperature range, while approximate ΔU is a simplified value calculated using the average specific heat at constant volume.</p> Signup and view all the answers

When can we use the heat capacity relation du = cvdT?

<p>The heat capacity relation du = cvdT is applicable for single-phase processes, where the specific heat doesn't change drastically with temperature.</p> Signup and view all the answers

What is the relationship between Cp and Cv for ideal gases?

<p>For ideal gases, the specific heat at constant pressure (Cp) is equal to the specific heat at constant volume (Cv) plus the gas constant (R).</p> Signup and view all the answers

What is the specific heat ratio (gamma) for ideal gases?

<p>The specific heat ratio (gamma) for ideal gases is defined as the ratio of the specific heat at constant pressure (Cp) to the specific heat at constant volume (Cv). It is given by the formula: γ = Cp/Cv.</p> Signup and view all the answers

What is the specific heat ratio (gamma) also known as?

<p>Both A and B</p> Signup and view all the answers

What is the assumption about incompressible substances?

<p>Incompressible substances are idealized substances that are assumed to have a constant volume, regardless of changes in pressure or temperature.</p> Signup and view all the answers

For truly incompressible substances, Cp = Cv = C.

<p>True</p> Signup and view all the answers

How does the specific heat of an incompressible substance vary with temperature?

<p>The specific heat of an incompressible substance can vary with temperature, even though its volume is assumed to be constant.</p> Signup and view all the answers

In the context of incompressible substances, what does the equation du = dh = cdT signify?

<p>Change in internal energy is equal to enthalpy change.</p> Signup and view all the answers

Water is considered an incompressible substance.

<p>True</p> Signup and view all the answers

What is the specific heat of water?

<p>The specific heat of water is approximately 4.182 kJ/kg⋅K.</p> Signup and view all the answers

How can we use tables and average specific heat methods to determine the temperature and volume of a system?

<p>Tables contain data on the properties of specific substances at various conditions, while average specific heat methods provide an approximate value of the specific heat over a certain temperature range. By combining these methods, we can estimate the temperature and volume of a system.</p> Signup and view all the answers

What is the importance of a property table in solving process problems?

<p>A property table allows us to organize and track the important thermodynamic properties of a system during a process. This helps us to understand how the system changes as it undergoes a process.</p> Signup and view all the answers

What is the purpose of sketching a system and showing energy interactions?

<p>Sketching a system and showing energy interactions helps us to visualize the problem and identify the relevant energy flows.</p> Signup and view all the answers

What is the first law of thermodynamics used to determine?

<p>Work done, heat transfer, and internal energy.</p> Signup and view all the answers

What is the importance of considering the type of substance and process when determining work?

<p>Different substances and processes require different equations to calculate the work done. For example, the work done by an ideal gas during isothermal compression is different than the work done by a solid.</p> Signup and view all the answers

Incompressible substances can always be assumed to behave like ideal gases.

<p>False</p> Signup and view all the answers

What is the main source of heat energy in the examples involving ideal gases?

<p>The main source of heat energy in the examples involving ideal gases is typically an electrical heater or a shaft doing work on the system.</p> Signup and view all the answers

What is an isothermal process?

<p>An isothermal process is a thermodynamic process that occurs at a constant temperature.</p> Signup and view all the answers

What is meant by the expansion of a gas and how does it relate to work?

<p>Expansion of a gas refers to an increase in its volume, often caused by an increase in temperature or a decrease in pressure. During expansion, the gas does work on its surroundings, often against a piston or other means of confinement.</p> Signup and view all the answers

What is the relationship between the change in internal energy and the work done by an ideal gas?

<p>The change in internal energy of an ideal gas is equal to the heat transferred to or from the gas minus the work done by the gas.</p> Signup and view all the answers

What is Joule's experiment and how does it relate to the specific heat of water?

<p>Joule's experiment demonstrated the equivalence of mechanical work and heat energy by measuring the amount of work required to raise the temperature of a given amount of water. This experiment established that the specific heat of water is approximately 4.182 kJ/kg⋅K.</p> Signup and view all the answers

What is the main principle behind the example involving the melting of ice in a picnic?

<p>The main principle behind the example is the heat transfer from the warmer malta drinks to the ice, causing the ice to melt.</p> Signup and view all the answers

What is the importance of knowing the specific heat of the malta drinks?

<p>Knowing the specific heat of the malta drinks is essential for calculating the amount of heat energy transferred from the drinks to the ice, which ultimately determines the mass of ice that will melt.</p> Signup and view all the answers

In the context of the example involving melting ice, what happens to the heat that melts the ice?

<p>The heat transferred from the malta drinks to the ice is used to break the bonds between the water molecules in the ice, causing the ice to change its phase from solid to liquid.</p> Signup and view all the answers

Study Notes

Thermodynamics I - Unit 7: The 1st Law and Thermal Energy in Closed Systems

  • Objectives:
    • Define thermal energy and its relationship to temperature.
    • Differentiate between specific heat at constant volume and constant pressure.
    • Define enthalpy and its relationship to internal energy.
    • Calculate internal energy and enthalpy changes for various substances (ideal gases, solids, liquids).
    • Utilize specific heat information for 1st Law analyses of various substances.

Approach to Solving Process Problems

  • Sketch the system and energy interactions across boundaries.
  • Create a property table for important properties.
  • Identify initial state properties (Units 2-4) and enter known values.
  • Determine other initial properties using tables or equations of state (EOS) (Units 2-4) and enter values.
  • Perform energy analysis (Units 5-7), applying the 1st law to determine work, heat, and other final properties, where appropriate. This may involve determining work using appropriate equations for different substances and processes, and heat capacity relations for equation of state (EOS) substances during single-phase process.
  • Determine other final properties using tables or EOS.

Internal Energy in Single-Phase Processes

  • Internal energy consists of potential and kinetic energy.
    • Potential energy: energy related to molecular interactions.
    • Kinetic energy: energy due to molecular motion (translation, rotation, vibration).
  • Thermal Energy: Energy manifested in temperature changes.
  • EOS (Equation of State) Processes: Internal energy and temperature are related.

Thermal Energy in a Gas

  • Systems with identical molecular speeds, identical numbers of molecules, but differing volume, have the same micro-kinetic energy therefore having the same temperature. However, the system with less volume has a higher pressure.

Internal Energy in Single-Phase Processes - Conclusion

  • Internal energy is related to temperature, not pressure or volume (a good approximation).
  • Heat capacity relates how temperature affects internal energy.
  • At high pressures/small volumes, latent energy (energy associated with phase changes) becomes significant.

Measuring Specific Heat Capacity

  • Constant Volume (cv): Heat substance in a rigid container, ΔU = Q or Q = ΔU.
  • Constant Pressure (cp): Heat substance in a piston-cylinder device, ΔU = Q - W or Q = ΔU + PΔV = ΔH.
  • Specific formulas for calculating both are presented in the slides.

Specific Heat Capacity

  • Constant Volume: The energy required for a unit mass substance increase by one degree while the volume is held constant. This is a change in internal energy per unit change in temperature at constant volume.
  • Constant Pressure: The energy required for a unit mass substance increase by one degree while the pressure is held constant. This is a change in enthalpy per unit change in temperature at constant pressure.

Specific Heat Capacity - Single-Phase Processes

  • For single-phase processes, the change in internal energy(Δu) is significantly approximated by using the average heat capacity and change in temperature.

Finding Δu Using cv

  • When a substance is tabulated, its internal energy at different conditions (u1 and u2) can be directly read from the table.
  • For an equation of state (EOS) defined substance, Δu= ∫ cv dT (over a given temperature range).
  • If cv(T) is tabulated, it's approximated based on its average over the specified temperature range.

Ideal Gas: Energy Analysis

  • For ideal gases, cp - cv = R and cp/cv = k = γ.
  • Specific values for air at 300 K are presented. Note that for air, constant heat capacities are assumed when not stated otherwise.

Incompressible Substances

  • Liquids and solids behave like incompressible substances.
  • For truly incompressible substances, cp = cv.
  • Even for incompressible substances, cv can vary with temperature.
  • Specific values for iron at 25°C are provided.
  • For water, cv = 4.182 kJ/kg⋅K.

Example Problems (E.g. 1, 2, 3)

  • Illustrate the application of the above concepts to solve for unknown variables such as pressure, temperature, volume, work, or heat transfer in thermodynamic processes.

  • E.g. 1: Ideal Gas Energy Analysis.

  • E.g. 2: Ideal Gas Energy Analysis in a different context.

  • E.g. 3: Real Steam Energy Analysis.

  • E.g. 4: Ideal Gas Energy Analysis.

  • E.g. 5: Isothermal Expansion for Ideal Gas.

  • E.g. 6: Incompressible Substance (e.g., cooling a beverage with ice).

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ENG2002 Thermodynamics I Unit 7 PDF

Description

Test your understanding of the First Law of Thermodynamics and thermal energy in closed systems. This quiz covers key concepts such as specific heat, enthalpy, and internal energy changes. Prepare to apply your knowledge in solving process problems related to various substances.

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