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3 The First Law and Energy Balances Online Quiz Questions

Use this free practice quiz with 20 questions to review 3 The First Law and Energy Balances, test your knowledge, and prepare for your next test or exam.

20 questions
01
True or false
1 point

True or false: Under the first law of thermodynamics, energy may cross a system boundary or change form, but it cannot be created or destroyed.

  1. A

    True

  2. B

    False

02
Choose one
1 point

For a closed system using the sign convention in the first-law balance, which signs apply to heat transferred into the system and work done by the system?

  1. A

    Heat into the system is negative, and work done by the system is positive.

  2. B

    Heat into the system is positive, and work done by the system is positive.

  3. C

    Heat into the system is positive, and work done by the system is negative.

  4. D

    Heat into the system and work done by the system are both negative.

03
Fill in the blank
1 point

Complete the closed-system energy balance using the stated sign convention:

04
True or false
1 point

True or false: Heat and work are properties stored in a thermodynamic system in the same way that internal energy is.

  1. A

    True

  2. B

    False

05
Choose one
1 point

A closed system undergoes a process in which changes in kinetic and potential energy are negligible. Which balance relates its internal-energy change to heat and work, using heat-in and work-by-system as positive?

  1. A

    ΔU=Q+W\Delta U=Q+W

  2. B

    ΔU=0\Delta U=0

  3. C

    ΔU=Q−W\Delta U=Q-W

  4. D

    ΔU=W−Q\Delta U=W-Q

06
Written response
1 point

What thermodynamic property accounts for energy at the molecular level, including molecular motion and interactions?

07
Fill in the blank
1 point

Complete the definition of specific enthalpy: h=h=

08
Choose all
1 point

Which specific energy terms are carried across a control-volume boundary by flowing mass? Select all that apply.

  1. A

    Specific enthalpy, hh

  2. B

    Heat-transfer rate, Q˙\dot Q

  3. C

    Specific kinetic energy, V22\frac{V^2}{2}

  4. D

    Specific gravitational potential energy, gzgz

  5. E

    Work rate, W˙\dot W

09
Choose one
1 point

Which description identifies a control volume rather than a closed system?

  1. A

    A fixed quantity of matter whose boundary cannot be crossed by mass.

  2. B

    A region defined only by the surface of a solid object.

  3. C

    A system in which no heat or work can cross the boundary.

  4. D

    A chosen region in space through which mass may flow.

10
Written response
1 point

A closed gas system receives 500 kJ500\ \text{kJ} of heat and does 180 kJ180\ \text{kJ} of work. If changes in kinetic and potential energy are negligible, what is its internal-energy change? Enter the value in kJ.

11
Choose all
1 point

Which conditions support using W˙=m˙(hin−hout)\dot W=\dot m(h_{\mathrm{in}}-h_{\mathrm{out}}) for a turbine? Select all that apply.

  1. A

    The turbine operates adiabatically.

  2. B

    The turbine operates at steady state.

  3. C

    The turbine's speed and elevation changes are significant.

  4. D

    There is one inlet and one outlet, and changes in speed and elevation are negligible.

12
Choose one
1 point

An adiabatic turbine operates with negligible changes in speed and elevation. The mass-flow rate is 2 kg/s2\ \text{kg/s}, the inlet enthalpy is 3200 kJ/kg3200\ \text{kJ/kg}, and the outlet enthalpy is 2600 kJ/kg2600\ \text{kJ/kg}. What is the shaft power output?

  1. A

    1200 kW1200\ \text{kW}

  2. B

    600 kW600\ \text{kW}

  3. C

    1200 kJ/kg1200\ \text{kJ/kg}

  4. D

    5800 kW5800\ \text{kW}

13
Open ended
1 point

Explain how the energy balance for a closed system differs from the balance for a control volume. Include what may cross each boundary and how flowing mass affects the control-volume balance.

14
Choose one
1 point

Why is enthalpy especially useful in energy balances for flowing fluids?

  1. A

    It measures the rate at which heat crosses a boundary.

  2. B

    Its pressure–volume term accounts for pressure-related work associated with pushing fluid into or out of a control volume.

  3. C

    It is the sum of kinetic and gravitational potential energy only.

  4. D

    It is a transfer term rather than a state property.

15
True or false
1 point

For a closed system, heat and work are energy transfers across the boundary rather than properties stored in the system.

  1. A

    True

  2. B

    False

16
Choose one
1 point

A flowing fluid has specific internal energy uu, pressure pp, and specific volume vv. Which expression gives its specific enthalpy?

  1. A

    h=u−pvh=u-pv

  2. B

    h=u+pvh=u+pv

  3. C

    h=pv−uh=pv-u

  4. D

    h=u+pvh=u+\frac{p}{v}

17
Choose one
1 point

Using the convention that work done by a closed system is positive, how should you assign the sign of work done on the system?

  1. A

    It is positive because energy enters the system.

  2. B

    It is zero because no work crosses the boundary.

  3. C

    It is negative under the stated convention.

  4. D

    Its sign depends only on the system's initial temperature.

18
Choose one
1 point

A control volume operates at steady state. Which statement must be true?

  1. A

    Energy stored in the control volume is not changing with time.

  2. B

    No mass can enter or leave the control volume.

  3. C

    Heat and work transfer rates must both be zero.

  4. D

    The fluid's enthalpy must be identical at the inlet and outlet.

19
Written response
1 point

An adiabatic turbine has one inlet and one outlet, with negligible changes in speed and elevation. If m˙=2 kg/s\dot m=2\ \text{kg/s}, hin=3200 kJ/kgh_{\mathrm{in}}=3200\ \text{kJ/kg}, and hout=2600 kJ/kgh_{\mathrm{out}}=2600\ \text{kJ/kg}, what is the shaft power output in kW?

20
Written response
1 point

What is the standard one-word descriptor for a process with no heat transfer?