First Law of Thermodynamics Quiz

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Questions and Answers

What does the First Law of Thermodynamics state regarding a system's internal energy?

  • It only considers heat transfer into the system.
  • It relates the change in internal energy to heat supplied and work done on the system. (correct)
  • It is independent of work done on the system.
  • It states that energy cannot be created or destroyed.

In the equation dU = dQ + dW, which of the following statements is true?

  • dU is considered a function of state. (correct)
  • dQ must always be negative for work to be positive.
  • dQ and dW are dependent on the system's initial state.
  • dW is a function of heat transfer only.

Which limitation is associated with the First Law of Thermodynamics?

  • It assumes all types of energies are equal. (correct)
  • It sets a maximum limit on heat transfer.
  • It provides a specific direction for energy transfer.
  • It does not take into account the conversion efficiency of work.

If a system experiences a loss of 45 J of heat and 450 J of work is done on it, what will be the change in internal energy?

<p>495 J (D)</p> Signup and view all the answers

What does the First Law of Thermodynamics imply about energy exchange in a system?

<p>It depends only on the initial and final state of the system. (B)</p> Signup and view all the answers

Which statement correctly describes a closed system?

<p>A closed system cannot exchange mass but can exchange energy. (A)</p> Signup and view all the answers

What is an example of a quasi-equilibrium process?

<p>Very slow compression of air in a cylinder. (B)</p> Signup and view all the answers

Which of the following is not a path function?

<p>Potential energy (C)</p> Signup and view all the answers

Match the process with its correct description: What matches Heat to work?

<p>Heat engine (D)</p> Signup and view all the answers

Which of the following is associated with irreversibilities?

<p>Entropy (D)</p> Signup and view all the answers

What does the first law of thermodynamics primarily assert?

<p>Energy cannot be destroyed, only transformed. (D)</p> Signup and view all the answers

If a system’s volume is constant while heat is added, what happens to its internal energy?

<p>It increases. (A)</p> Signup and view all the answers

What is characteristic of an open system?

<p>Allows both mass and energy to cross its boundaries. (C)</p> Signup and view all the answers

Flashcards

First Law of Thermodynamics

The change in a system's internal energy equals the heat added plus the work done on the system.

Internal Energy (dU)

The total energy stored within a system. It's a function of state (depends only on current conditions).

Heat (Ä‘Q)

Energy transferred between a system and its surroundings due to temperature difference. Not a state function.

Work (Ä‘W)

Energy transferred to or from a system through mechanical means. Not a state function.

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Change in Internal Energy Equation

ΔU = Q + W (where ΔU is the change in internal energy, Q is the heat added, and W is the work done on the system).

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Closed System

A system where no mass crosses its boundaries.

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Open System

A system where mass can cross its boundaries.

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Quasi-Equilibrium Process

A process that happens slowly enough that the system remains close to equilibrium at each step.

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Path Function

A property that depends on the path taken, not just the start and end points.

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Control Mass

A fixed amount of matter or a system with constant mass.

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Control Volume

A region of space that we focus on for analysis.

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Internal Energy

Energy contained within a system due to the motion and interactions of its molecules.

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Study Notes

First Law of Thermodynamics

  • The first law of thermodynamics is a statement of the conservation of energy
  • During interactions, energy can change forms, but the total energy remains constant.
  • Energy cannot be created or destroyed.
  • The first law asserts energy is a thermodynamic property.

Learning Goals

  • Students will be able to recognize basic terminology in thermal engineering.
  • Students will be able to understand the first law of thermodynamics.

System Definitions

  • Closed system = control mass
  • Open system = control volume
  • Closed system = control mass, a system where the mass remains constant
  • Open system = control volume, a system where mass can cross the boundary

Quasi-Equilibrium Process

  • A quasi-equilibrium process is a very slow compression of air in a cylinder.

Path Function vs. State Function (Example)

  • Not a path function: Thermal conductivity
  • Examples of path functions: Heat, Work, ∫vdP

Matching Terms (List I to List II):

  • Heat to work: Heat engine
  • Heat to lift weight: Hot air balloon
  • Heat to strain energy: Bimetallic strips
  • Heat to electromagnetic energy: Thermal radiation

Another Matching Set

  • Irreversibilities: Thermal equilibrium
  • Mechanical work: Inexact differences
  • Zeroth law: High grade energy
  • Heat: Entropy

The First Law Equation

  • ΔU = Uf - Ui = Q - W
  • ΔU = Change in internal energy
  • Uf = Final internal energy
  • Ui = Initial internal energy
  • Q = Heat
  • W = Work

Sign Conventions (for Q and W)

  • Positive Q: System gains heat
  • Negative Q: System loses heat
  • Positive W: Work done by the system.
  • Negative W: Work done on the system

Closed System Executing Cycle

  • Cyclic integral of work = cyclic integral of heat.
  • ∫dW = ∫dQ.

Closed System Undergoing a Process

  • Total energy entering the system – total energy leaving the system = change in total energy of system
  • â–³KE + â–³PE + â–³U = Q − W

Energy Balance of a System

  • The first law of thermodynamics (FLT) is a powerful tool for analyzing energy exchange between a system and its surroundings.
  • dU = dQ + dW (change in internal energy = heat + work)

Limitations of the First Law

  • The first law assumes all types of energies are equal.
  • It does not specify the direction of heat transfer and work transfer.
  • The first law sets no limit on how much work can be produced from supplied heat.

Total Energy Exchange

  • The total energy a system exchanges with its surroundings in any process depends only on the initial and final states, not how the change occurred.

Example Problem (Constant Pressure)

  • A gas loses 45 J of heat to its surroundings and has 450 J of work done onto it. Find the change in internal energy.
  • ΔU = 495J

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Related Documents

Unit I First Law of Thermodynamics PDF

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