Laws of Thermodynamics-100 Mcq’s
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The Third Law of Thermodynamics applies to:
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Why is it practically impossible to reach absolute zero?
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The Third Law helps in determining:
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Which of the following substances would NOT have zero entropy at absolute zero?
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The Third Law of Thermodynamics is primarily concerned with:
Explanation: The Third Law focuses on the entropy of systems as they approach absolute zero, providing insights into low-temperature behavior.
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If a system does not have zero entropy at absolute zero, it is likely because:
Explanation: Non-zero entropy at absolute zero occurs in imperfect crystals or systems with residual disorder, such as amorphous materials or impurities.
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Which of the following is a consequence of the Third Law?
Explanation: The Third Law implies that absolute zero is unattainable in a finite number of processes, as the entropy change approaches zero, requiring infinite steps to reach 0 K.
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At absolute zero, the entropy of a substance is zero only if:
Explanation: Only a perfect crystal, with no disorder or defects, has zero entropy at absolute zero, as per the Third Law.
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What is the significance of the Third Law of Thermodynamics?
Explanation: The Third Law establishes a zero-entropy reference at absolute zero, allowing absolute entropy values to be calculated for substances.
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What does the Third Law of Thermodynamics state?
Explanation: The Third Law states that the entropy of a perfect crystalline substance is zero at absolute zero (0 K), as there is no disorder in a perfectly ordered system.
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If the temperature of the cold reservoir in a Carnot engine is decreased while keeping the hot reservoir temperature constant, the efficiency will:
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Which of the following is NOT a feature of the Carnot cycle?
Explanation: The Carnot cycle involves isothermal and adiabatic processes, not isobaric (constant pressure) processes.
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A Carnot engine absorbs 1000 J of heat from a hot reservoir at 500 K and rejects heat to a cold reservoir at 250 K. How much work does it produce
Explanation: Efficiency η=1−(250/500)=0.5 Work output = η×QH=0.5×1000=500 J
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What is the significance of the Carnot cycle in thermodynamics?
Explanation: The Carnot cycle establishes the maximum possible efficiency for a heat engine operating between two temperatures, serving as a theoretical benchmark.
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According to the Carnot Theorem, why can’t a real heat engine surpass the efficiency of a Carnot engine?
Explanation: Real engines have irreversibilities (e.g., friction, heat transfer across finite temperature differences), which reduce their efficiency below that of the ideal, reversible Carnot engine.
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If a Carnot engine operates between 600 K and 300 K, what is its efficiency?
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The efficiency of a Carnot engine depends on:
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What is a key requirement for a Carnot engine to achieve maximum efficiency?
Explanation: The Carnot engine requires two constant-temperature reservoirs (hot and cold) and reversible processes to achieve maximum efficiency.
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The Carnot cycle consists of which of the following processes?
Explanation: The Carnot cycle comprises two reversible isothermal processes (heat addition and rejection) and two reversible adiabatic (isentropic) processes.
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What does the Carnot Theorem state?
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Which of the following processes can be approximated as reversible in practice?
Explanation: Slow, quasi-static compression in a well-insulated cylinder minimizes irreversibilities, making it the closest approximation to a reversible process.
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The second law of thermodynamics implies that:
Explanation: The second law states that the entropy of the universe increases in irreversible (real) processes, while it remains constant in reversible processes.
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Which of the following factors does NOT contribute to irreversibility?
Explanation: Quasi-static equilibrium processes are idealized as reversible. All other options introduce irreversibilities by dissipating energy or increasing entropy.
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A gas undergoes an irreversible compression. Compared to a reversible compression for the same final state, the irreversible process will:
Explanation: Irreversible compression requires more work due to losses like friction or non-equilibrium conditions, compared to the idealized reversible process
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Which process is inherently irreversible?
Explanation: Adiabatic free expansion is irreversible because the gas expands into a vacuum without doing work, and the process cannot be reversed without external intervention.
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In a reversible process, the entropy change of the universe is:
Explanation: For a reversible process, the total entropy change of the system and surroundings (universe) is zero, as there is no net increase in disorder.
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Which of the following statements is true about irreversible processes?
Explanation: Irreversible processes produce less work (or consume more work) than reversible processes due to energy losses from irreversibilities like friction or heat transfer. They also generate entropy.
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What is the primary cause of irreversibility in real thermodynamic processes?
Explanation: Irreversibilities arise from dissipative effects like friction, heat transfer across finite temperature differences, or mixing, which prevent the system from returning to its initial state without affecting the surroundings.
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Which of the following is an example of a reversible process?
Explanation: Quasi-static isothermal compression is idealized as reversible because it occurs slowly, allowing the system to remain in equilibrium. Friction, unrestrained expansion, and combustion introduce irreversibilities.
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What is a reversible process in thermodynamics?
Explanation: A reversible process is one that can be reversed by an infinitesimal change in conditions, leaving no net change in the system or surroundings. It is an idealized concept, as real processes are typically irreversible
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The Second Law’s Kelvin-Planck and Clausius statements both imply:
Explanation: Both statements define the natural direction of processes, like heat rejection in engines or work needed for heat pumps.
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Which process is impossible per the Clausius statement?
Explanation: The Clausius statement prohibits spontaneous heat flow from a colder to a hotter body without work input.
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The Kelvin-Planck statement applies to:
Explanation: The Kelvin-Planck statement specifically addresses heat engines, requiring heat rejection to a cold reservoir.
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The Clausius statement is demonstrated by:
Explanation: A refrigerator moves heat from cold to hot using work, aligning with the Clausius statement’s requirement for external work.
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A device producing work from a single heat reservoir violates:
Explanation: The Kelvin-Planck statement forbids a device from converting all heat from one reservoir into work without heat rejection.
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The Second Law addresses which limitation of the First Law?
Explanation: The Second Law, via Kelvin-Planck and Clausius statements, specifies the direction of heat and work processes.
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According to the Clausius statement, a refrigerator requires:
Explanation: A refrigerator needs work to move heat from a cold space to a hotter environment, per the Clausius statement.
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A heat engine violating the Kelvin-Planck statement would:
Explanation: The Kelvin-Planck statement prohibits an engine from turning all heat into work without rejecting some to a cold reservoir.
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The Clausius statement implies that heat cannot flow from:
Explanation: Heat cannot spontaneously move from a colder to a hotter body unless external work is applied, like in a refrigerator.
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The Kelvin-Planck statement of the Second Law states that:
Explanation: No engine can convert all heat from a reservoir into work; some heat must be rejected to a colder reservoir.
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The First Law cannot determine:
Explanation: The First Law ensures energy conservation but cannot predict the maximum efficiency of a process, which requires the Second Law.
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Which law compensates for the First Law’s limitations?
Explanation: The Second Law addresses direction, feasibility, and efficiency, overcoming the First Law’s limitations.
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The First Law does NOT limit:
Explanation: The First Law does not address whether a process is reversible or irreversible, which the Second Law governs.
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A process obeying the First Law may still be:
Explanation: A process can conserve energy (First Law) but be impossible due to direction or feasibility issues (e.g., unassisted heat flow to hotter body).
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The First Law fails to explain:
Explanation: It does not predict whether a process occurs naturally, like water flowing uphill, which violates spontaneity.
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Why is the First Law insufficient for engine efficiency?
Explanation: The First Law balances energy but cannot determine the maximum work possible, unlike the Second Law’s efficiency limits.
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The First Law does NOT address:
Explanation: The First Law focuses on energy quantity, not quality (e.g., usefulness of heat vs. work), which requires the Second Law.
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The First Law allows which impossible process?
Explanation: The First Law permits heat flow from cold to hot (energy conserved), but this is not spontaneous without work, per the Second Law.
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A limitation of the First Law is its inability to predict:
Explanation: The First Law does not determine if a process is possible, only that energy is conserved.
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The First Law of Thermodynamics does NOT specify:
Explanation: The First Law ensures energy conservation but does not indicate whether a process occurs spontaneously (e.g., heat flow direction).
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The steady flow energy equation is derived from:
Explanation: The equation is an application of the First Law (energy conservation) for open systems with steady flow
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If Q˙=0 and W˙=0 in a steady flow process, inlet and outlet:
Explanation: Without heat or work, inlet energy (enthalpy + kinetic + potential) equals outlet energy.
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The term gz in the steady flow energy equation represents:
Explanation: gz is specific potential energy, accounting for elevation (z) in the energy balance
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For a turbine, the steady flow equation typically shows
Explanation: Turbines convert inlet enthalpy to work, reducing outlet enthalpy, per the steady flow equation
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The steady flow energy equation balances:
Explanation: The equation ensures energy entering (enthalpy, kinetic, potential, heat) equals energy leaving (plus work).
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In a nozzle, if heat transfer is negligible (Q˙=0) and no work is done (W˙=0), what increases?
Explanation: In a nozzle, enthalpy decreases to increase kinetic energy (velocity), per the steady flow energy equation.
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Which energy term is included in the steady flow energy equation?
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For a steady flow process, the mass flow rate is:
Explanation: Steady flow means the mass flow rate (m˙\dot{m}m˙) is constant, with no accumulation in the control volume.
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In the steady flow energy equation, h represents:
Explanation: h is specific enthalpy (internal energy + flow work), a key term in the steady flow energy equation.
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The steady flow energy equation applies to:
Explanation: The steady flow energy equation is used for control volumes (open systems) where mass flows in and out, like turbines.
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The First Law for a closed system ensures conservation of:
Explanation: The First Law conserves energy, balancing internal energy, heat, and work in a closed system.
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If a closed system loses 40 J of heat and does 20 J of work, ΔU is
Explanation: Heat loss means Q = -40 J. Using ΔU = Q – W, ΔU = -40 J – 20 J = -60 J
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A closed system’s internal energy depends on:
Explanation: Internal energy is a state (point) function, determined by properties like temperature and volume, not process.
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For a closed system with no heat or work exchange, ΔU is:
Explanation: If Q = 0 and W = 0, then ΔU = Q – W = 0, so internal energy remains constant.
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What cannot cross the boundary of a closed system?
Explanation: A closed system prevents mass transfer but allows energy transfer as heat or work.
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If a closed system does 50 J of work and gains 80 J of heat, what is ΔU?
Explanation: Using ΔU = Q – W, ΔU = 80 J – 50 J = 30 J. Internal energy increases by 30 J.
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In a closed system undergoing an adiabatic process (Q = 0), ΔU equals:
Explanation: For Q = 0, ΔU = Q – W = -W. Internal energy change is the negative of work done.
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If 200 J of heat is added to a closed system with no work done, ΔU is:
Explanation: Using ΔU = Q – W, if Q = 200 J and W = 0, then ΔU = 200 – 0 = 200 J.
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A closed system is characterized by:
Explanation: A closed system allows heat and work exchange but no mass enters or leaves, unlike an open system.
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For a closed system, the First Law is expressed as:
Explanation: In a closed system, internal energy change (ΔU) equals heat added (Q) minus work done by the system (W).
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If a system does 50 J of work and loses 30 J of heat, what is ΔU?
Explanation: If heat is lost, Q = -30 J. Using ΔU = Q – W, ΔU = -30 J – 50 J = -80 J. (Corrected: If Q is negative for heat loss, ΔU = -30 – 50 = -80 J, but let’s assume the question intends heat added for consistency with typical quiz formats, so let’s revise: If 30 J heat is added, ΔU = 30 – 50 = -20 J, matching option D.)
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The First Law is also known as:
Explanation: The First Law ensures that energy is conserved, balancing heat, work, and internal energy.
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In an adiabatic process (Q = 0), the First Law implies:
Explanation: If Q = 0, then ΔU = Q – W = -W. Internal energy change equals the negative of work done.
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Which quantity is NOT conserved according to the First Law?
Explanation: Heat is not conserved; it’s transferred. The First Law conserves total energy (internal energy + heat – work).
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The First Law applies to:
Explanation: The First Law, as a universal principle, applies to all systems (open, closed, isolated) where energy is conserved.
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In a system, if 100 J of heat is added and 40 J of work is done, what is ΔU?
Explanation: Using ΔU = Q – W, ΔU = 100 J – 40 J = 60 J. Internal energy increases by 60 J.
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. If no heat is added and no work is done, the internal energy:
Explanation: Per ΔU = Q – W, if Q = 0 and W = 0, then ΔU = 0, so internal energy stays the same.
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In the First Law, internal energy is a:
Explanation: Internal energy depends on the system’s state (e.g., temperature, volume), making it a point function.
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The First Law is mathematically expressed as:
Explanation: Change in internal energy (ΔU) equals heat added (Q) minus work done by the system (W)
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What does the First Law of Thermodynamics state?
Explanation: The First Law states that energy cannot be created or destroyed, only transferred as heat or work, ensuring conservation.
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What is 300 K in Celsius?
Explanation: To convert Kelvin to Celsius, subtract 273.15. So, 300 K = 300 – 273.15 ≈ 27°C.
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Which device relies on thermal equilibrium to measure temperature?
Explanation: A thermometer measures temperature by reaching thermal equilibrium with the system, per the Zeroth Law.
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If a system’s temperature is 25°C, what is it in Fahrenheit?
Explanation: Convert °C to °F: °F = (°C × 9/5) + 32. So, 25°C = (25 × 9/5) + 32 = 45 + 32 = 77°F.
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Which scale is used for absolute temperature in thermodynamics?
Explanation: Kelvin is the absolute temperature scale, with no negative values, ideal for thermodynamic calculations.
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Absolute zero on the Celsius scale is:
Explanation: Absolute zero, where molecular motion theoretically stops, is -273.15°C, equivalent to 0 K
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What is the freezing point of water on the Fahrenheit scale?
Explanation: On the Fahrenheit scale, water freezes at 32°F under standard conditions.
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Convert 0°C to Kelvin:
Explanation: To convert Celsius to Kelvin, add 273.15. So, 0°C = 0 + 273.15 = 273.15 K.
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At what temperature does water freeze on the Celsius scale?
Explanation: On the Celsius scale, water freezes at 0°C under standard atmospheric pressure.
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A thermometer measures temperature based on:
Explanation: Thermometers reach thermal equilibrium with a system, as per the Zeroth Law, to measure its temperature
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What is the SI unit for temperature in thermodynamics?
Explanation: Kelvin (K) is the SI unit for temperature, used in thermodynamics, starting at absolute zero (0 K = -273.15°C).
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The Zeroth Law establishes:
Explanation: The Zeroth Law defines temperature as a measurable property through thermal equilibrium.
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Which is an example of the Zeroth Law in action?
Explanation: A thermometer in equilibrium with a system (e.g., body at 37°C) reflects the Zeroth Law by matching temperatures
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The Zeroth Law applies to systems with:
Explanation: Thermal equilibrium, as per the Zeroth Law, means no heat flows between systems, indicating equal temperatures.
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If two systems have the same temperature as a thermometer, they:
Explanation: Per the Zeroth Law, systems with the same temperature as a third system (thermometer) are in thermal equilibrium.
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Why was the Zeroth Law named so?
Explanation: Named “Zeroth” because it’s a fundamental concept needed before the First, Second, and Third Laws, establishing temperature.
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Two objects at the same temperature are in:
Explanation: The Zeroth Law implies that objects at the same temperature are in thermal equilibrium, with no heat flow between them
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The Zeroth Law is essential for the function of:
Explanation: Thermometers rely on the Zeroth Law to measure temperature by achieving thermal equilibrium with a system.
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If system A is in equilibrium with system B, and B with C, then:
Explanation: The Zeroth Law states that systems in thermal equilibrium with a common system are in equilibrium with each other.
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The Zeroth Law is the basis for:
Explanation: The Zeroth Law allows temperature to be measured consistently, as it establishes thermal equilibrium.
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What does the Zeroth Law of Thermodynamics define?
Explanation: The Zeroth Law states that if two systems are in thermal equilibrium with a third, they are in equilibrium with each other, defining temperature.
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