First Law of Thermodynamics Quiz
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Questions and Answers

According to the first law of thermodynamics, what is the fundamental principle governing energy?

  • Energy can only be destroyed, not created
  • Energy can only be created, not destroyed
  • Energy can be neither created nor destroyed; it can only change forms (correct)
  • Energy can be destroyed, but not created

    • What is the sign convention for heat transfer to a system?

  • Negative for heat transfer to a system
  • Positive for heat transfer to a system (correct)
  • Positive for heat transfer from a system
  • Negative for heat transfer from a system

    • What is the unit of heat transfer per unit mass?

  • kJ
  • kW
  • kJ/kg (correct)
  • kW/kg

    • What is the characteristic of heat and work in thermodynamics?

    <p>They are path functions</p> Signup and view all the answers

    What is the definition of heat based on kinetic theory?

    <p>Energy associated with the random motions of atoms and molecules</p> Signup and view all the answers

    What is the fundamental difference between heat and work?

    <p>Heat is a vector quantity, while work is a scalar quantity</p> Signup and view all the answers

    What is the net energy transfer to or from a system as heat and work according to the first law of thermodynamics?

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

    What is the expression for the change in total energy of a system during a process?

    <p>E = U + KE + PE</p> Signup and view all the answers

    What is the first law of thermodynamics for a cyclic process?

    <p>Q - W = 0</p> Signup and view all the answers

    What type of energy is associated with the motion of an object?

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

    What is the energy stored in chemical bonds, which can be released or absorbed during chemical reactions?

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

    What type of system involves kinetic and potential energies?

    <p>Mechanical Systems</p> Signup and view all the answers

    What is the fundamental principle implied by the law of conservation of energy?

    <p>Energy cannot be created or destroyed, only converted or transferred</p> Signup and view all the answers

    What was the key observation made by James Joule in his experiment?

    <p>Mechanical work done by the falling weights resulted in a measurable increase in the water's temperature</p> Signup and view all the answers

    What is the mathematical representation of the work done on a system?

    <p>W = F dx</p> Signup and view all the answers

    What is the implication of the equation dU = dQ - P dV?

    <p>The change in internal energy is a function of the state of the system, not the path by which the system reached that state</p> Signup and view all the answers

    What is the condition for a process to be quasi-static?

    <p>The process is slow and reversible</p> Signup and view all the answers

    What is the implication of the equation ?dU = 0?

    <p>Internal energy is a state function</p> Signup and view all the answers

    What is the relationship between the internal energy and temperature for an ideal gas?

    <p>u is a function of T only</p> Signup and view all the answers

    What is the specific heat ratio k for monatomic gases?

    <p>1.67</p> Signup and view all the answers

    What is the assumption made for incompressible substances?

    <p>The volume work is negligible</p> Signup and view all the answers

    What is the expression for the enthalpy change of an incompressible substance?

    <p>?h = u + v?P</p> Signup and view all the answers

    What is the condition for a reversible adiabatic process?

    <p>Q = 0 and W ≠ 0</p> Signup and view all the answers

    What is the equation of state for a polytropic process?

    <p>pV^n = C</p> Signup and view all the answers

    A gas is heated in a closed container from 20°C to 50°C. What is the sign of the change in internal energy?

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

    A piston compresses a gas in a cylinder. What is the expression for the work done on the gas?

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

    A solid is cooled in a closed container. What is the sign of the heat transfer?

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

    A gas is expanded in a cylinder by a piston. What is the expression for the work done by the gas?

    <p>W = PdV</p> Signup and view all the answers

    A weight is lifted against gravity. What is the expression for the work done?

    <p>W = mgh</p> Signup and view all the answers

    A system undergoes an isothermal process. What is the change in internal energy?

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

    A gas is compressed in a cylinder without any heat transfer. What is the change in internal energy?

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

    A liquid is heated in a closed container until it vaporizes. What is the sign of the work done?

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

    A gas is stirred in a closed container. What is the expression for the work done?

    <p>W = Fdx</p> Signup and view all the answers

    A system undergoes a non-flow process. What is the expression for the change in internal energy?

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

    What is the primary function of internal energy in thermodynamics?

    <p>To represent the total energy contained within 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

    What is the difference between specific heat at constant volume and specific heat at constant pressure?

    <p>Cp is always higher than Cv</p> Signup and view all the answers

    What is the condition for the equation ?H = Q to be true?

    <p>Constant pressure process</p> Signup and view all the answers

    What is the primary difference between internal energy and enthalpy?

    <p>Internal energy is a function of temperature only, while enthalpy is a function of both temperature and pressure</p> Signup and view all the answers

    What is the physical interpretation of the specific heat at constant volume Cv?

    <p>The energy required to raise the temperature of a unit mass of a substance by one degree at constant volume</p> Signup and view all the answers

    Study Notes

    First Law of Thermodynamics

    • The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another.
    • The law can be mathematically expressed as: Q - W = ΔE (energy transferred as heat minus energy transferred as work equals the change in internal energy)
    • The law is based on experimental observations and cannot be proved mathematically.

    Energy Transfer Mechanisms

    • Heat (Q): energy transferred due to a temperature difference between the system and its surroundings.
    • Work (W): energy transferred when a force is applied over a distance or when a system changes its volume under pressure.

    Energy Conservation in a Closed System

    • For a closed system, the total energy (kinetic, potential, and internal energies) remains constant, although it can change forms.
    • The energy balance needs to account for the energy carried by the mass entering and leaving the system.

    Law of Conservation of Energy

    • The law of conservation of energy states that the total energy in an isolated system remains constant over time, although energy can transform from one form to another.
    • This fundamental principle of physics implies that energy cannot be created or destroyed, only converted or transferred.

    Joule's Experiment

    • James Prescott Joule's experiment demonstrated the mechanical equivalent of heat.
    • The experiment showed that mechanical work could be converted into heat, providing strong evidence for the conservation of energy.

    Energy as a Property of a System

    • Energy is a property of a thermodynamic system, meaning it is a state function that describes the energy content of the system.
    • Energy is a scalar quantity that encompasses various forms, including kinetic, potential, internal, thermal, chemical, electrical, nuclear, and radiant energy.

    Internal Energy

    • Internal energy (U) is a fundamental concept in thermodynamics that represents the total energy contained within a system.
    • Internal energy is a state function that depends on the temperature of the system.
    • For an ideal gas, internal energy (U) is a function of temperature (T) only.

    Enthalpy

    • Enthalpy (H) is a thermodynamic property that represents the total energy of a system.
    • Enthalpy is defined as: H = U + PV, where U is the internal energy, P is the pressure, and V is the volume.
    • Enthalpy is a useful concept for analyzing thermodynamic systems, especially in chemical engineering and chemistry.

    Specific Heats

    • The specific heat is defined as the energy required to raise the temperature of a unit mass of a substance by one degree.
    • There are two kinds of specific heats: specific heat at constant volume (Cv) and specific heat at constant pressure (Cp).
    • Cp is always higher than Cv because at constant pressure the system is allowed to expand and energy for this expansion must also be supplied to the system.

    First Law of Thermodynamics for Non-Flow Processes

    • The First Law of Thermodynamics can be applied to non-flow processes, also known as closed systems or batch processes.
    • The First Law for a closed system is: ΔE = Q - W, where ΔE is the change in internal energy, Q is the heat added to the system, and W is the work done on the system.

    Examples of Non-Flow Processes

    • Heating a solid: heat is added, and the internal energy increases.
    • Compression of a gas: work is done on the system, and the internal energy increases.
    • Phase change: heat is added, and the internal energy increases.
    • Expansion of a gas: work is done by the system, and the internal energy decreases.
    • Cooling a solid: heat is removed, and the internal energy decreases.

    Isothermal and Adiabatic Processes

    • Isothermal process: a process in which the temperature remains constant, and no heat is added or removed.
    • Adiabatic process: a process in which there is no heat transfer between the system and the surroundings, and the internal energy changes due to work done on or by the system.### Work Done in Different Scenarios

    Work Done in Compressing a Gas

    • Work done on the system: W = -PdV (negative sign indicates work done on the system)
    • Force applied by piston: F = PA (where P is pressure and A is area)
    • Displacement of piston: dx = dV/A (where dV is change in volume)
    • Work done: W = Fdx = PAdx = PdV

    Work Done in Expanding a Gas

    • Work done by the system: W = PdV (positive sign indicates work done by the system)
    • Force applied by gas: F = PA
    • Displacement of piston: dx = dV/A
    • Work done: W = Fdx = PAdx = PdV

    Work Done in Lifting a Weight

    • Work done by force applied to lift weight against gravity: W = mgh (where m is mass, g is acceleration due to gravity, and h is height lifted)
    • Force lifting weight: F = mg
    • Displacement of weight: h (height lifted)
    • Work done: W = Fdh = mgh

    Work Done in Stirring or Agitation

    • Work done by stirrer or agitator: W = τθ (where τ is torque and θ is angular displacement)
    • Torque applied: τ
    • Angular displacement: θ
    • Work done: W = τdθ = τθ

    Work Done in Isothermal and Adiabatic Processes

    • Isothermal compression or expansion: W = 0 (no work is done since process occurs at constant temperature and pressure)
    • Adiabatic compression: W = -PdV (compression)
    • Adiabatic expansion: W = PdV (expansion)
    • Sign convention for work done:
      • Negative sign (-) indicates work done on the system
      • Positive sign (+) indicates work done by the system

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    Test your understanding of the first law of thermodynamics, including energy conversion and transfer mechanisms.

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