NAPT - Navy Advanced Programs Thermodynamics and Heat Transfer Questions and Answers — Questions and Answers
Question 1: Aboard a naval vessel, a sealed, rigid container filled with an ideal gas is moved from a cool storage room at 10°C to the deck where the temperature is 30°C. If the initial pressure was 150 kPa, what is the final pressure inside the container, assuming the volume remains constant?
- 140 kPa
- 150 kPa
- 160 kPa (Correct answer)
- 450 kPa
Correct answer: 160 kPa
This scenario is an application of the Ideal Gas Law (PV=nRT), specifically Gay-Lussac's Law, which states that for a constant volume and amount of gas, the pressure is directly proportional to the absolute temperature (P₁/T₁ = P₂/T₂). First, convert temperatures to Kelvin: T₁ = 10°C + 273.15 = 283.15 K and T₂ = 30°C + 273.15 = 303.15 K. Then, solve for the final pressure (P₂): P₂ = P₁ * (T₂ / T₁) = 150 kPa * (303.15 K / 283.15 K) ≈ 160.3 kPa. The closest answer is 160 kPa.
Question 2: Which of the following scenarios best exemplifies heat transfer primarily through convection?
- The handle of a metal wrench left in the sun becomes hot.
- A sailor feels the warmth of a nearby steam pipe without touching it.
- A ventilation system circulates cooled air throughout a ship's compartment. (Correct answer)
- The hull of a ship gets warm where it is in direct contact with a hot pier.
Correct answer: A ventilation system circulates cooled air throughout a ship's compartment.
Convection is the transfer of heat through the movement of fluids (liquids or gases). A ventilation system actively moves cooled air (a fluid) to distribute it, which is a clear example of forced convection. The sun heating a wrench is radiation. Feeling warmth from a steam pipe without contact is also primarily radiation. The hull warming through contact with the pier is conduction.
Question 3: The First Law of Thermodynamics is a statement of the conservation of energy. For a closed system, it is expressed as ΔU = Q - W. In this equation, what does ΔU represent?
- The work done by the system.
- The heat added to the system.
- The change in the internal energy of the system. (Correct answer)
- The total entropy of the system.
Correct answer: The change in the internal energy of the system.
The First Law of Thermodynamics states that the change in the internal energy of a system (ΔU) is equal to the heat added to the system (Q) minus the work done by the system (W). It accounts for the energy balance within the system. W is work done by the system, and Q is the heat transferred into the system.
Question 4: A 2 kg block of aluminum (specific heat capacity ≈ 900 J/kg°C) is used as a heat sink for an electronic component. If the component transfers 18,000 Joules of heat to the block, what is the temperature increase of the aluminum block?
- 5°C
- 10°C (Correct answer)
- 20°C
- 9,000°C
Correct answer: 10°C
The relationship between heat energy (Q), mass (m), specific heat capacity (c), and change in temperature (ΔT) is given by the formula Q = mcΔT. To find the temperature increase, we rearrange the formula to ΔT = Q / (mc). Plugging in the given values: ΔT = 18,000 J / (2 kg * 900 J/kg°C) = 18,000 / 1800 = 10°C.
Question 5: In the design of a ship's hull insulation, materials with low thermal conductivity are chosen. What is the primary mode of heat transfer that these materials are designed to minimize?
- Convection
- Radiation
- Advection
- Conduction (Correct answer)
Correct answer: Conduction
Thermal conductivity is the measure of a material's ability to conduct heat. Insulation materials are specifically chosen to have low thermal conductivity, meaning they are poor conductors of heat. Conduction is the transfer of heat through direct contact. Therefore, insulation's primary purpose is to minimize heat transfer by conduction through the structure.
Question 6: A heat engine, such as a ship's propulsion plant, operates by taking in heat (Q_H) from a high-temperature source, converting some of it into useful work (W), and rejecting the remaining heat (Q_C) to a low-temperature sink. Which of the following statements correctly describes the relationship between these quantities according to the First Law of Thermodynamics?
- W = Q_H + Q_C
- Q_H = W - Q_C
- W = Q_H - Q_C (Correct answer)
- Q_C = W + Q_H
Correct answer: W = Q_H - Q_C
The First Law of Thermodynamics, applied to a heat engine cycle, is a statement of energy conservation. The total heat energy put into the system (Q_H) must equal the energy that comes out, which is the sum of the useful work performed (W) and the waste heat rejected (Q_C). Therefore, Q_H = W + Q_C. Rearranging this equation to solve for work gives W = Q_H - Q_C.
NAPT - Navy Advanced Programs Thermodynamics and Heat Transfer Questions and Answers
Aboard a naval vessel, a sealed, rigid container filled with an ideal gas is moved from a cool storage room at 10°C to the deck where the temperature is 30°C.
If the initial pressure was 150 kPa, what is the final pressure inside the container, assuming the volume remains constant?