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Ch. 19 - Heat and the First Law of Thermodynamics
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook
Chapter 19, Problem 50

If a heater supplies 1.8 x 10⁶ J/h to a room 3.5 m x 4.6 m x 3.0 m containing air at 20°C and 1.0 atm, by how much will the temperature rise in one hour, assuming no losses of heat or air mass to the outside? Assume air is an ideal diatomic gas with molecular mass 29.

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Determine the volume of the room using the formula for the volume of a rectangular prism: \( V = \text{length} \times \text{width} \times \text{height} \). Substitute the given dimensions \( 3.5 \ \text{m} \), \( 4.6 \ \text{m} \), and \( 3.0 \ \text{m} \) to calculate \( V \).
Calculate the mass of the air in the room. Use the ideal gas law \( PV = nRT \) to find the number of moles \( n \), where \( P \) is the pressure (1.0 atm), \( V \) is the volume (from step 1), \( R \) is the ideal gas constant (8.314 J/mol·K), and \( T \) is the temperature in kelvins (convert 20°C to K by adding 273.15). Then, multiply \( n \) by the molecular mass of air (29 g/mol or 0.029 kg/mol) to find the mass.
Determine the specific heat capacity of air. Since air is an ideal diatomic gas, its molar specific heat at constant volume \( C_v \) is approximately \( 5R/2 \), where \( R \) is the ideal gas constant. Convert this to specific heat capacity per unit mass by dividing \( C_v \) by the molar mass of air.
Relate the heat supplied to the temperature change using the formula \( Q = mc\Delta T \), where \( Q \) is the heat supplied (1.8 \(\times\) 10^6 \ \(\text{J}\)), \( m \) is the mass of air (from step 2), \( c \) is the specific heat capacity (from step 3), and \( \Delta T \) is the temperature change. Rearrange the formula to solve for \( \Delta T \).
Substitute the known values for \( Q \), \( m \), and \( c \) into the equation from step 4 to calculate \( \Delta T \), the temperature rise in one hour.

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Key Concepts

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

Heat Transfer

Heat transfer refers to the process of thermal energy moving from one object or substance to another due to a temperature difference. In this scenario, the heater provides energy to the air in the room, which will increase the air's temperature. Understanding how heat is transferred and measured is crucial for calculating the resulting temperature change.
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Ideal Gas Law

The Ideal Gas Law is a fundamental equation in thermodynamics that relates the pressure, volume, temperature, and number of moles of an ideal gas. It is expressed as PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature. This law helps in determining how the temperature of the air will change when heat is added.
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Specific Heat Capacity

Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius. For air, this value is essential to calculate how much the temperature will increase when a certain amount of heat is supplied. Knowing the specific heat capacity allows us to relate the energy supplied by the heater to the temperature change in the air.
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Related Practice
Textbook Question

Show, using Eqs. 19–7 and 19–16, that the work done by a gas that slowly expands adiabatically from pressure P₁ and volume V₁ , to P₂ and V₂, is given by W = (P₁V₁ - P₂V₂) / (γ - 1).

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Textbook Question

At a crime scene, the forensic investigator notes that the 6.2-g lead bullet that was stopped in a doorframe apparently melted completely on impact. Assuming the bullet was shot at room temperature (20°C), what does the investigator calculate as the minimum muzzle velocity of the gun?

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Textbook Question

A ceramic teapot (e = 0.70) and a shiny metal one (e = 0.10) each hold 0.55 L of tea at 85°C. (a) Estimate the rate of heat loss from each, and (b) estimate the temperature drop after 30 min for each. Consider only radiation, and assume the surroundings are at 20°C.

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Textbook Question

A 1.0-L volume of air initially at 3.5 atm of (gauge)pressure is allowed to expand isothermally until the (gauge) pressure is 1.0 atm. It is then compressed at constant pressure to its initial volume, and lastly is brought back to its original pressure by heating at constant volume. How much work does the 1.0 L of air do in this process?

Textbook Question

A copper rod and an aluminum rod of the same length and cross-sectional area are attached end to end (Fig. 19–35). The copper end is placed in a furnace maintained at a constant temperature of 205°C. The aluminum end is placed in an ice bath held at a constant temperature of 0.0°C. Calculate the temperature at the point where the two rods are joined.

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