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Ch 19: Work, Heat, and the First Law of Thermodynamics
Knight Calc - Physics for Scientists and Engineers 5th Edition
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 19, Problem 45

The beaker in FIGURE P19.45, with a thin metal bottom, is filled with 20 g of water at 20°C. It is brought into good thermal contact with a 4000 cm3 container holding 0.40 mol of a monatomic gas at 10 atm pressure. Both containers are well insulated from their surroundings. What is the gas pressure after a long time has elapsed? You can assume that the containers themselves are nearly massless and do not affect the outcome.

Verified step by step guidance
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Identify the key principles involved: This problem involves the concepts of thermal equilibrium, the ideal gas law, and the specific heat capacity of water. The system will reach thermal equilibrium, meaning the final temperature of the water and the gas will be the same.
Calculate the initial temperature of the gas: Use the ideal gas law, \( PV = nRT \), to find the initial temperature of the gas. Rearrange the equation to solve for \( T \): \( T = \frac{PV}{nR} \), where \( P \) is the pressure (10 atm, converted to Pascals), \( V \) is the volume (4000 cm³ converted to m³), \( n \) is the number of moles (0.40 mol), and \( R \) is the ideal gas constant (8.314 J/(mol·K)).
Determine the heat transfer: The water will release or absorb heat based on its specific heat capacity, \( c_w = 4186 \ \text{J/(kg·K)} \). The heat transfer for the water is given by \( Q_w = m_w c_w \Delta T \), where \( m_w \) is the mass of the water (20 g converted to kg) and \( \Delta T \) is the change in temperature. Similarly, the heat transfer for the gas is \( Q_g = n C_v \Delta T \), where \( C_v \) is the molar heat capacity at constant volume for a monatomic gas (\( C_v = \frac{3}{2}R \)).
Apply the principle of conservation of energy: Since the system is thermally insulated, the heat lost by one part of the system is gained by the other. Set \( Q_w + Q_g = 0 \), and solve for the final equilibrium temperature \( T_f \). Substitute the expressions for \( Q_w \) and \( Q_g \) into this equation.
Calculate the final pressure of the gas: Once the final temperature \( T_f \) is determined, use the ideal gas law again to find the final pressure of the gas. Rearrange the equation \( P = \frac{nRT}{V} \), substituting \( T_f \) for the temperature and keeping \( n \) and \( V \) constant.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Thermal Equilibrium

Thermal equilibrium occurs when two systems in thermal contact reach the same temperature, resulting in no net heat transfer between them. In this scenario, the water and the gas will exchange heat until they stabilize at a common temperature, which is crucial for determining the final pressure of the gas.
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Ideal Gas Law

The Ideal Gas Law, expressed as PV = nRT, relates the pressure (P), volume (V), number of moles (n), the ideal gas constant (R), and temperature (T) of a gas. This law is essential for calculating the final pressure of the gas after thermal equilibrium is achieved, as it allows us to understand how changes in temperature affect pressure.
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Monatomic Gas Properties

Monatomic gases consist of single atoms and exhibit specific heat capacities that differ from diatomic or polyatomic gases. For a monatomic gas, the heat capacity at constant volume (Cv) is typically 3/2 R, which influences how the gas will respond to changes in temperature and pressure during the thermal exchange with the water.
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Related Practice
Textbook Question

A 6.0-cm-diameter cylinder of nitrogen gas has a 4.0-cm-thick movable copper piston. The cylinder is oriented vertically, as shown in FIGURE P19.49, and the air above the piston is evacuated. When the gas temperature is 20°C, the piston floats 20 cm above the bottom of the cylinder. What is the gas pressure?

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Textbook Question

512 g of an unknown metal at a temperature of 15°C is dropped into a 100 g aluminum container holding 325 g of water at 98°C. A short time later, the container of water and metal stabilizes at a new temperature of 78°C. Identify the metal.

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Textbook Question

The burner on an electric stove has a power output of 2.0 kW. A 750 g stainless steel teakettle is filled with 20°C water and placed on the already hot burner. If it takes 3.0 min for the water to reach a boil, what volume of water, in cm3, was in the kettle? Stainless steel is mostly iron, so you can assume its specific heat is that of iron.

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Textbook Question

When air is inhaled, it quickly becomes saturated with water vapor as it passes through the moist airways. Consequently, an adult human exhales about 25 mg of evaporated water with each breath. Evaporation—a phase change—requires heat, and the heat energy is removed from your body. Evaporation is much like boiling, only water's heat of vaporization at 35°C is a somewhat larger 24×105 J/kg because at lower temperatures more energy is required to break the molecular bonds. At 12 breaths/min, on a dry day when the inhaled air has almost no water content, what is the body's rate of energy loss (in J/s) due to exhaled water? (For comparison, the energy loss from radiation, usually the largest loss on a cool day, is about 100 J/s.)

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Textbook Question

A 6.0-cm-diameter cylinder of nitrogen gas has a 4.0-cm-thick movable copper piston. The cylinder is oriented vertically, as shown in FIGURE P19.49, and the air above the piston is evacuated. When the gas temperature is 20°C, the piston floats 20 cm above the bottom of the cylinder. What is the new equilibrium temperature of the gas?

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Textbook Question

A typical nuclear reactor generates 1000 MW (1000 MJ/s) of electric energy. In doing so, it produces 2000 MW of 'waste heat' that must be removed from the reactor to keep it from melting down. Many reactors are sited next to large bodies of water so that they can use the water for cooling. Consider a reactor where the intake water is at 18°C. State regulations limit the temperature of the output water to 30°C so as not to harm aquatic organisms. How many liters of cooling water have to be pumped through the reactor each minute?

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