Thermodynamics and Heat

71 questions

All types71 Multiple choice36 True / False4 Numeric2 Matching15 Ordering14

Two metal brackets of equal mass are exposed to 500 Joules of thermal energy. Bracket A has a specific heat capacity that is exactly twice as large as that of Bracket B. If both brackets start at 20°C, which of the following statements correctly describes the outcome of this energy absorption?

Bracket B will experience a temperature increase twice as large as Bracket A.

Bracket A will experience a temperature increase twice as large as Bracket B.

Both brackets will experience the exact same temperature increase since they absorbed equal energy.

Bracket B will shrink in size because materials with lower heat capacity contract when absorbing energy.

Show answer & explanation

Answer: Bracket B will experience a temperature increase twice as large as Bracket A.

Answer

Bracket B will experience a temperature increase twice as large as Bracket A.

Specific heat capacity measures how much energy is needed to raise the temperature of a substance. Because Bracket A has twice the specific heat capacity of Bracket B, it requires twice as much energy to heat up by the same amount. Since both brackets receive the exact same amount of thermal energy (500 Joules) and have equal masses, Bracket B—with its lower resistance to temperature change—will experience a temperature increase that is twice as large as that of Bracket A.

Step-by-Step Solution

Identify the relationship between specific heat capacity and temperature change.

Temperature change is inversely proportional to specific heat capacity when mass and added heat are constant.

The formula for heat energy is

Q = mc\Delta T

. Rearranging for temperature change gives

\Delta T = \frac{Q}{mc}

Compare the specific heat capacities of the two brackets.

Bracket A has twice the specific heat capacity (

c

) of Bracket B.

This is stated directly in the problem.

Determine the resulting temperature changes.

Since Bracket B has half the specific heat capacity of A, its temperature change (

\Delta T

) will be twice as large.

A smaller denominator in the formula

\Delta T = \frac{Q}{mc}

results in a larger overall temperature change.

Key Concept

The specific heat capacity of a material determines how much thermal energy is required to change its temperature; materials with lower specific heat capacities will experience greater temperature changes for a given amount of heat input.

Estimated Time:1m 0s

Question 62Question →

Pair the specific thermal role or property with the correct engine bay material or component.

Click a left item, then click its matching right item

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Answer

The liquid coolant mixture matches with high specific heat capacity; the aluminum radiator core matches with excellent thermal conductivity; silicone wiring jackets match with flexible thermal insulation; and the ceramic exhaust shield matches with rigid radiant thermal barrier.

Each material is chosen for a specific thermodynamic property: water-based coolant has high heat capacity to absorb energy safely, aluminum has high conductivity to release energy rapidly, silicone acts as a flexible conductive insulator, and ceramic acts as a rigid high-temperature radiant barrier.

Step-by-Step Solution

Analyze the thermal requirements for liquid engine coolant.

Coolant must absorb massive amounts of engine heat without boiling, which requires a very high specific heat capacity.

To identify the matching property for the liquid coolant mixture.

Evaluate the function of an aluminum radiator core.

The core needs to transfer heat quickly from the coolant to the passing air, relying on the high thermal conductivity of aluminum.

To identify the matching property for the radiator core.

Assess the role of silicone wiring jackets in an engine bay.

Silicone provides a flexible, low-conductivity barrier that prevents conductive heat transfer from damaging electrical lines.

To distinguish the type of insulation needed for wiring.

Determine the thermal purpose of a ceramic exhaust shield.

Ceramic handles extreme exhaust temperatures and serves as a rigid barrier to block radiant heat transfer to surrounding parts.

To match the final component with its specific extreme-heat insulation role.

Key Concept

Thermal Properties of Materials (Heat Capacity, Conductivity, and Insulation)

Question 63Question →

In an automotive air conditioning system, high-pressure liquid refrigerant passes through an expansion valve just before entering the evaporator coils. This component causes a sudden, significant drop in the fluid's pressure. What is the primary thermodynamic purpose of this pressure drop?

To lower the boiling point of the refrigerant, allowing it to vaporize and absorb heat from the passenger cabin.

To compress the liquid refrigerant into a smaller volume, forcing it to absorb surrounding thermal energy more effectively.

To cause the refrigerant to contract in volume as it warms up, reducing the physical space it occupies in the evaporator.

To steadily increase the sensible temperature of the liquid while preventing any phase change from occurring.

Show answer & explanation

Answer: To lower the boiling point of the refrigerant, allowing it to vaporize and absorb heat from the passenger cabin.

Answer

To lower the boiling point of the refrigerant, allowing it to vaporize and absorb heat from the passenger cabin.

The core principle of a vapor-compression refrigeration cycle is using a phase change to move heat. By dropping the pressure at the expansion valve, the boiling point of the refrigerant is drastically lowered. This allows it to easily boil into a gas when exposed to the relatively warm air of the passenger cabin in the evaporator. Boiling is an endothermic process that absorbs massive amounts of latent heat, thereby effectively cooling the cabin air.

Step-by-Step Solution

Analyze the state of the refrigerant at the expansion valve.

The refrigerant enters the valve as a high-pressure liquid.

Understanding the starting state is necessary to determine what physical changes will occur.

Determine the physical effect of a sudden pressure drop on a fluid.

A drop in pressure significantly lowers the boiling point of a fluid.

This is a fundamental thermodynamic principle governing phase changes.

Evaluate the result in the context of the next component (the evaporator).

Because the boiling point is now lower than the cabin temperature, the liquid boils (vaporizes) in the evaporator, absorbing massive amounts of latent heat.

This phase change is the primary mechanism by which the air conditioning system removes heat from the air.

Key Concept

Phase changes in a refrigeration cycle

Question 64Question →

Arrange the following events in the correct chronological order to describe a single complete cycle of a standard four-stroke internal combustion engine, beginning with the intake of the fuel mixture.

Drag items to arrange them in the correct order

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Answer

The correct sequence is: 1) Intake valve opens / piston down, 2) Valves closed / piston up (Compression), 3) Spark ignites / piston down (Power), 4) Exhaust valve opens / piston up.

A standard internal combustion engine operates on a four-stroke cycle: Intake, Compression, Power, and Exhaust. First, the piston moves down to draw in air and fuel. Second, it moves up with valves closed to compress the mixture. Third, the spark plug ignites the mixture, causing a rapid expansion that forces the piston down to perform work. Finally, the piston moves up with the exhaust valve open to expel the burned gases, completing the cycle.

Step-by-Step Solution

Identify the starting phase (Intake).

The cycle begins by drawing in the air-fuel mixture. The piston moves downward with the intake valve open to create a vacuum.

An engine must first take in fuel before it can compress or burn it.

Determine the preparation phase (Compression).

The valves close and the piston moves upward to compress the mixture.

Combustion is highly inefficient unless the fuel mixture is tightly packed and pressurized first.

Identify the working phase (Power).

The spark plug fires, igniting the compressed gas. The resulting explosion forces the piston rapidly downward.

This is the phase that actually generates the mechanical energy to turn the crankshaft.

Determine the reset phase (Exhaust).

The exhaust valve opens and the piston moves upward, pushing out the leftover burned gases.

The cylinder must be emptied of spent gases so a fresh mixture can be drawn in during the next intake stroke.

Key Concept

Four-Stroke Engine Operating Cycle

Estimated Time:1m 0s

Question 65Question →

In a standard vapor-compression refrigeration cycle, the compressor significantly increases the pressure of the refrigerant vapor before it enters the condenser. What is the primary thermodynamic purpose of this pressure increase?

It raises the condensation temperature of the vapor, allowing the refrigerant to release heat to the surrounding environment.

It acts as a hydraulic pump to force the incompressible liquid refrigerant through the restrictive condenser coils.

It squeezes the refrigerant into a smaller volume, which directly causes its internal temperature to drop rapidly.

It provides the energy required to continuously raise the refrigerant's temperature while it changes from a gas to a liquid.

Show answer & explanation

Answer: It raises the condensation temperature of the vapor, allowing the refrigerant to release heat to the surrounding environment.

Answer

It raises the condensation temperature of the vapor, allowing the refrigerant to release heat to the surrounding environment.

The primary purpose of the compressor is to elevate the pressure and temperature of the refrigerant vapor. Because heat naturally transfers only from a warmer area to a cooler area, the refrigerant must be made hotter than the outside ambient air. By compressing the gas, its saturation temperature is raised, which allows the refrigerant to successfully reject absorbed heat into the surrounding environment when it passes through the condenser.

Step-by-Step Solution

Analyze the physical state of the refrigerant as it enters and leaves the compressor.

The refrigerant enters the compressor as a low-pressure gas and leaves as a high-pressure, high-temperature gas.

Understanding the state changes across the component is the first step in identifying its functional purpose in the cycle.

Evaluate the conditions required for heat transfer to occur in the condenser.

Heat must flow from the refrigerant to the ambient environment. This requires the refrigerant to be at a higher temperature than its surroundings.

The second law of thermodynamics dictates that heat flows naturally from hotter regions to colder regions.

Connect the compressor's function to the condenser's thermodynamic requirement.

By increasing the pressure, the compressor raises the saturation (condensation) temperature of the gas, ensuring it is hot enough to reject heat to the outside air.

The pressure and temperature of a gas are directly proportional during the compression phase.

Key Concept

Thermodynamics of Vapor-Compression Refrigeration

Question 66Question →

Match each thermodynamic process or thermal property to its correct physical description regarding phase changes and energy transfer.

Click a left item, then click its matching right item

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Answer

Sublimation matches the direct solid-to-gas transition. Latent Heat of Vaporization matches the energy absorbed during a liquid-to-gas phase change at constant temperature. Condensation matches the transition from gas to liquid. Sensible Heating matches a temperature increase within a single phase.

Each thermodynamic term has a specific mechanical definition. Sublimation skips the liquid phase. Latent heat drives phase changes at a constant temperature plateau, whereas sensible heat drives temperature changes without altering the phase. Condensation is the exothermic return from gas to liquid.

Step-by-Step Solution

Analyze the definitions for phase transitions that bypass the liquid state entirely.

Identify that Sublimation matches the transition directly from solid to gas.

Sublimation is a unique phase change property observed in substances like dry ice, skipping the melting phase.

Distinguish between latent heat and sensible heat.

Map Latent Heat of Vaporization to the process where state changes but temperature is constant, and map Sensible Heating to the process where temperature increases but state remains unchanged.

Latent heat plateau occurs during phase changes, whereas sensible heat causes a measurable temperature change on a thermometer.

Identify the remaining gas-to-liquid phase change.

Condensation matches the process of a gas releasing heat to become a liquid.

This is a standard thermodynamic reversal of boiling/vaporization.

Key Concept

States of Matter, Phase Transitions, and Thermal Energy Types

Estimated Time:1m 0s

Question 67Question →

A red-hot steel component is dropped into a vat of liquid coolant that is exactly at its boiling point. As the steel transfers extreme heat into the coolant, the fluid begins to boil vigorously. Assuming the vat is open to the atmosphere, what happens to the temperature of the remaining liquid coolant while this boiling phase change is actively occurring?

It remains constant because the absorbed thermal energy is used entirely to change the liquid into a gas.

It continues to rise steadily since extreme heat is being continuously transferred into the fluid.

It decreases momentarily because the rapid bubbling action creates strong convective cooling currents.

It increases at a significantly slower rate because the liquid expands, distributing the heat over a larger volume.

Show answer & explanation

Answer: It remains constant because the absorbed thermal energy is used entirely to change the liquid into a gas.

Answer

It remains constant because the absorbed thermal energy is used entirely to change the liquid into a gas.

During a phase change like boiling or melting, the temperature of a substance remains completely flat (forms a plateau) even though heat is continually being added. The thermal energy from the red-hot steel is absorbed by the coolant as 'latent heat of vaporization'. This energy strictly breaks the physical bonds between the liquid molecules to turn them into a gas, so the average kinetic energy (which dictates temperature) of the remaining liquid stays exactly the same.

Step-by-Step Solution

Identify the initial state of the coolant.

The coolant is a liquid already at its boiling point.

This establishes that the fluid is perfectly primed to undergo a phase change from liquid to gas without needing to warm up further.

Analyze the energy transfer occurring in the system.

Massive amounts of thermal energy are being transferred from the red-hot steel into the liquid coolant.

This continuous heat transfer drives the vigorous boiling observed in the vat.

Apply thermodynamic principles regarding phase changes.

The temperature of the liquid will not change; instead, the added heat is absorbed as latent heat of vaporization.

During a phase change, added energy breaks intermolecular bonds rather than increasing the kinetic energy (temperature) of the liquid molecules.

Key Concept

Latent Heat and Phase Change Plateaus

Estimated Time:1m 0s

Question 68Question →

During an air conditioning service, an automotive technician uses a recovery machine to extract refrigerant. The machine pulls hot, high-pressure refrigerant vapor from the vehicle and continuously removes heat from it until it becomes a subcooled liquid in a storage cylinder. Arrange the following thermodynamic events in the chronological order they occur during this continuous cooling process.

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Answer

The sequence follows a continuous cooling curve: initial sensible cooling of the vapor, reaching the saturation point, the constant-temperature condensation plateau, the end of the phase change, and finally the sensible cooling of the resulting liquid.

As heat is continuously removed from a superheated vapor, its temperature first drops due to sensible heat loss until it reaches the saturation temperature (condensation point). During the actual phase change from gas to liquid, the temperature remains entirely constant as latent heat is released. Once condensation is 100% complete, the temperature of the newly formed liquid begins to drop again.

Step-by-Step Solution

Identify the initial state and process described in the scenario.

The substance starts as a hot, superheated vapor and is continuously losing thermal energy.

Establishing the starting point is necessary to track the thermal sequence accurately.

Determine the first phase of cooling prior to condensation.

The vapor cools down steadily, lowering its molecular kinetic energy until it hits the saturation temperature.

A gas must lose sensible heat and cool to its exact condensation point before it can change state.

Analyze the phase change behavior.

The temperature stalls and remains completely constant while the vapor condenses into liquid.

During a phase change, heat removal changes the physical state of the matter (releasing latent heat) without changing the temperature.

Determine the final phase of cooling after condensation is complete.

Once the last bit of vapor transforms into liquid, the liquid's temperature begins to drop as sensible heat is removed.

After the phase change plateau ends, further heat removal resumes lowering the temperature of the substance.

Key Concept

States of Matter and Phase Changes

Question 69Question →

A stationary engineer is monitoring a heavy-duty industrial boiler operating at a constant standard atmospheric pressure. The pure water inside the boiler has just reached its boiling point, and steam is actively forming. The system's burner continues to supply a massive, steady flow of thermal energy to the tank. Which of the following best describes what happens to the internal temperature of the water while it is in the process of converting from a liquid to a gas?

The temperature remains perfectly constant until all of the liquid water has vaporized.

The temperature steadily increases because the burner continues to add thermal energy.

The temperature continues to rise, but at a slower rate because convection currents distribute the heat.

The temperature rapidly spikes because the specific heat capacity of the liquid has been overcome.

Show answer & explanation

Answer: The temperature remains perfectly constant until all of the liquid water has vaporized.

Answer

The temperature remains perfectly constant until all of the liquid water has vaporized.

The correct answer accurately describes the phenomenon of latent heat. When a substance reaches its boiling point at a constant pressure, any additional thermal energy supplied to the system is used to break the intermolecular bonds holding the liquid together. Because this energy is not increasing the average kinetic energy of the molecules, the temperature remains completely stable until the entire mass of liquid has transformed into a gas.

Step-by-Step Solution

Identify the current thermodynamic state of the substance inside the boiler.

The water is at its boiling point and is actively converting from a liquid phase to a gas phase (vaporization).

Understanding the specific phase transition is required to apply the correct thermodynamic principles.

Determine how the continuous thermal energy from the burner affects the substance during this transition.

The continuous thermal energy acts as latent heat of vaporization, breaking the physical bonds between water molecules rather than increasing their average molecular kinetic energy.

Latent heat alters the physical state of a substance without altering its sensible heat (temperature).

Conclude the effect on the measurable temperature of the liquid water.

Because the kinetic energy of the molecules is not increasing, the temperature remains constant throughout the entire phase change plateau.

Temperature is a direct measure of average kinetic energy, which stabilizes when energy is diverted strictly to changing the phase.

Key Concept

Latent Heat and Phase Change Plateaus

Estimated Time:1m 0s

Question 70Question →

An industrial oven used for curing aerospace composites is set to a precise temperature of $680^\circ\text{F}$ . To program a secondary backup sensor that operates exclusively on the Celsius scale, what numerical value must the technician enter?

Show answer & explanation

Answer: 360

Answer

360

The correct equivalent temperature is found by first subtracting 32 from the Fahrenheit value to adjust for the different zero points, resulting in

648

. Multiplying this difference by

\frac{5}{9}

accounts for the different degree increments between the scales, producing an exact value of

360^\circ\text{C}

Step-by-Step Solution

Set up the conversion equation.

C = \frac{5}{9}(680 - 32)

This is the standard formula to convert degrees Fahrenheit into degrees Celsius.

Perform the subtraction inside the parentheses.

680 - 32 = 648

According to the order of operations, the offset between the two temperature scales must be removed first.

Multiply the difference by the fraction

\frac{5}{9}

648 \div 9 = 72

, then

72 \times 5 = 360

Multiplying by

\frac{5}{9}

scales the Fahrenheit degree magnitude to match the Celsius degree magnitude.

Key Concept

Temperature Scale Conversion

Question 71Question →

During a performance test of an aviation hydraulic system, the operating fluid experiences a temperature increase of $45^{\circ}\text{C}$ . What is the equivalent temperature increase measured in degrees Fahrenheit?

25^{\circ}\text{F}

57^{\circ}\text{F}

81^{\circ}\text{F}

113^{\circ}\text{F}

Show answer & explanation

Answer: $81^{\circ}\text{F}$

Answer

The correct equivalent temperature increase is

81^{\circ}\text{F}

A change of

1^{\circ}\text{C}

corresponds exactly to a change of

1.8^{\circ}\text{F}

(which is the fraction

\frac{9}{5}

). Because we are evaluating a change in temperature (an interval) rather than converting an absolute point on the thermometer, we only multiply by the ratio and do not add the

32^{\circ}

offset. Multiplying the

45^{\circ}\text{C}

increase by

\frac{9}{5}

results in an

81^{\circ}\text{F}

increase.

Step-by-Step Solution

Recognize that the question asks for a temperature interval (an increase), not a specific absolute temperature.

The

+32

offset used in standard conversions is excluded.

The offset only aligns the starting points of the scales (

0^{\circ}\text{C}

32^{\circ}\text{F}

). When measuring a change (

\Delta T

), only the ratio of the degree sizes matters.

Apply the ratio of Fahrenheit degrees to Celsius degrees (

\frac{9}{5}

) to the given temperature increase.

\Delta F = 45 \times \frac{9}{5}

One degree Celsius is equal in magnitude to

1.8

(or

\frac{9}{5}

) degrees Fahrenheit.

Calculate the final value.

45 \div 5 = 9

;

9 \times 9 = 81^{\circ}\text{F}

Completing the arithmetic gives the equivalent magnitude of the temperature change.

Key Concept

Temperature Scale Intervals (

\Delta T

)

Alternative Method

You can pick any starting temperature to prove the interval. If the fluid starts at

0^{\circ}\text{C}

(

32^{\circ}\text{F}

) and goes to

45^{\circ}\text{C}

(

113^{\circ}\text{F}

), the difference is

113^{\circ}\text{F} - 32^{\circ}\text{F} = 81^{\circ}\text{F}

Estimated Time:45s

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