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Rooms \(A\) and \(B\) are the same size, and are connected by an open door. Room \(A\), however, is warmer (perhaps because its windows face the sun). Which room contains the greater mass of air? Explain carefully.

Short Answer

Expert verified
Room B contains the greater mass of air.

Step by step solution

01

Understanding the Problem

Rooms A and B are of the same size and are connected. Room A is warmer due to factors like its windows facing the sun. The task is to determine which room contains the greater mass of air.
02

Applying Ideal Gas Law

The ideal gas law formula, \( PV = nRT \), relates pressure \(P\), volume \(V\), temperature \(T\), and number of moles \(n\) of a gas, where \(R\) is the universal gas constant. At a constant pressure and volume, the number of moles \(n\) is inversely proportional to the temperature \(T\). Since both rooms have the same volume and are connected (same pressure), the room with the lower temperature will have more moles of air, hence greater mass.
03

Comparing Temperatures and Masses

Since Room A is warmer (higher temperature) than Room B, the air in Room A will expand, resulting in fewer moles of air compared to Room B. Consequently, Room B, being cooler, will contain more moles and thus a greater mass of air. This is due to air's density being inversely related to temperature at constant pressure.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Pressure and Volume
Understanding how pressure and volume relate is crucial when discussing gases. The Ideal Gas Law, expressed as \( PV = nRT \), helps us here. This equation shows the relationship between pressure \( P \), volume \( V \), and the number of moles \( n \) of a gas, with \( R \) being the universal gas constant and \( T \) the temperature. At constant temperature, increasing the volume of a container will decrease the pressure if the number of gas moles remains constant. Conversely, decreasing the volume will increase the pressure. This inverse relation is known as Boyle's Law, a component of the Ideal Gas Law. When rooms are connected, as in the exercise, they tend to equalize in pressure if volume and temperature differences aren’t significantly altered. Since rooms \( A \) and \( B \) share a passage, they have an equal pressure if no other outside forces affect them. This condition is why the comparison focuses on temperature differences in this scenario.
Temperature and Density
Temperature influences the density of gases in an interesting way. Density is defined as mass per unit volume. In the context of gases and the Ideal Gas Law, if temperature increases, the gas molecules move more rapidly, causing them to occupy more space, and effectively decreasing density. This is why Room \( A \), being warmer, has less air density than Room \( B \). Since density decreases with increasing temperature at a constant pressure (the condition in our connected rooms), less dense air means fewer molecules are present in Room \( A \). Therefore, Room \( B \), being cooler, has air that is denser and more massed. This concept is central to understanding why the mass of air differs between two connected rooms at different temperatures.
Moles of Air
The concept of moles is crucial to understanding how much gas is present in a given space. In the Ideal Gas Law, \( n \), the number of moles, indicates the quantity of gas present. At a constant volume and pressure, temperature largely determines successful application of the Ideal Gas Law when comparing two spaces or containers. For Room \( A \) and Room \( B \), the room with a higher temperature, Room \( A \), contains fewer moles of air. Since the number of moles is inversely proportional to temperature, Room \( B \), being cooler, contains more moles of air. More moles translate to more mass, hence why Room \( B \) effectively has a greater air mass. These relations help explain various real-world phenomena, such as balloon deflation in cooler temperatures or how air conditioning can affect air density.

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Most popular questions from this chapter

By applying Newton's laws to the oscillations of a continuous medium, one can show that the speed of a sound wave is given by $$c_{s}=\sqrt{\frac{B}{\rho}}$$ where \(\rho\) is the density of the medium (mass per unit volume) and \(B\) is the bulk modulus, a measure of the medium's stiffness. More precisely, if we imagine applying an increase in pressure \(\Delta P\) to a chunk of the material, and this increase results in a (negative) change in volume \(\Delta V\), then \(B\) is defined as the change in pressure divided by the magnitude of the fractional change in volume: $$B \equiv \frac{\Delta P}{-\Delta V / V}$$ This definition is still ambiguous, however, because I haven't said whether the compression is to take place isothermally or adiabatically (or in some other way). (a) Compute the bulk modulus of an ideal gas, in terms of its pressure \(P,\) for both isothermal and adiabatic compressions. (b) Argue that for purposes of computing the speed of a sound wave, the adiabatic \(B\) is the one we should use. (c) Derive an expression for the speed of sound in an ideal gas, in terms of its temperature and average molecular mass. Compare your result to the formula for the rms speed of the molecules in the gas. Evaluate the speed of sound numerically for air at room temperature. (d) When Scotland's Battlefield Band played in Utah, one musician remarked that the high altitude threw their bagpipes out of tune. Would you expect altitude to affect the speed of sound (and hence the frequencies of the standing waves in the pipes)? If so, in which direction? If not, why not?

A 60-kg hiker wishes to climb to the summit of Mt. Ogden, an ascent of 5000 vertical feet \((1500 \mathrm{m})\) (a) Assuming that she is \(25 \%\) efficient at converting chemical energy from food into mechanical work, and that essentially all the mechanical work is used to climb vertically, roughly how many bowls of corn flakes (standard serving size 1 ounce, 100 kilocalories) should the hiker eat before setting out? (b) As the hiker climbs the mountain, three-quarters of the energy from the corn flakes is converted to thermal energy. If there were no way to dissipate this energy, by how many degrees would her body temperature increase? (c) In fact, the extra energy does not warm the hiker's body significantly; instead, it goes (mostly) into evaporating water from her skin. How many liters of water should she drink during the hike to replace the lost fluids? (At \(25^{\circ} \mathrm{C},\) a reasonable temperature to assume, the latent heat of vaporization of water is \(580 \mathrm{cal} / \mathrm{g}, 8 \%\) more than at \(100^{\circ} \mathrm{C} .\) )

Imagine some helium in a cylinder with an initial volume of 1 liter and an initial pressure of 1 atm. Somehow the helium is made to expand to a final volume of 3 liters, in such a way that its pressure rises in direct proportion to its volume.

In the course of pumping up a bicycle tire, a liter of air at atmospheric pressure is compressed adiabatically to a pressure of 7 atm. (Air is mostly diatomic nitrogen and oxygen.) (a) What is the final volume of this air after compression? (b) How much work is done in compressing the air? (c) If the temperature of the air is initially \(300 \mathrm{K},\) what is the temperature after compression?

The Rankine temperature scale (abbreviated \(^{\circ} \mathrm{R}\) ) uses the same size degrees as Fahrenheit, but measured up from absolute zero like kelvin (so Rankine is to Fahrenheit as kelvin is to Celsius). Find the conversion formula between Rankine and Fahrenheit, and also between Rankine and kelvin. What is room temperature on the Rankine scale?

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