Skip to main content
Ch. 20 - Second Law of Thermodynamics
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179당신이 사용하는 게 아니라요?교과서 변경
모든 교과서Giancoli Douglas 5th editionCh. 20 - Second Law of Thermodynamics문제 88
20장, 문제 88

A dehumidifier removes water vapor from air and has been referred to as a “refrigerator with an open door.” The humid air is pulled in by a fan and passes over a cold coil, whose temperature is less than the dew point, and some of the air’s water condenses. After this water is extracted, the air is warmed back to its original temperature and sent into the room. In a well-designed dehumidifier, the heat that is removed by the cooling coil mostly comes from the condensation of water vapor to liquid, and this heat is used to re-warm the air. Estimate how much water is removed in 1.0 h by an ideal dehumidifier, if the temperature of the room is 25°C, the water condenses at 8°C, and the dehumidifier does work at the rate of 550 W of electrical power.

검증된 단계별 안내
1
Step 1: Understand the problem. The dehumidifier removes water vapor by cooling the air below its dew point, causing condensation. The heat released during condensation is used to re-warm the air. The goal is to estimate the amount of water removed in 1 hour, given the power input of 550 W, the room temperature of 25°C, and the condensation temperature of 8°C.
Step 2: Calculate the total energy input to the dehumidifier in 1 hour. Use the formula for energy: \( E = P \cdot t \), where \( P \) is the power (550 W) and \( t \) is the time (1 hour = 3600 seconds). This gives the total energy available for the dehumidification process.
Step 3: Relate the energy to the mass of water condensed. The energy required to condense water is given by \( Q = m \cdot L_v \), where \( m \) is the mass of water condensed and \( L_v \) is the latent heat of vaporization of water. At 8°C, \( L_v \) is approximately 2.45 × 10^6 J/kg. Rearrange the formula to solve for \( m \): \( m = \frac{Q}{L_v} \).
Step 4: Assume that the dehumidifier operates ideally, meaning all the electrical energy input is used for condensation. Substitute the total energy \( E \) from Step 2 into \( Q \) in the formula \( m = \frac{Q}{L_v} \). This will give the mass of water condensed in kilograms.
Step 5: Convert the mass of water from kilograms to liters if needed, using the fact that 1 kg of water is approximately equal to 1 liter. This will provide the final estimate of the volume of water removed by the dehumidifier in 1 hour.

비슷한 문제에 대한 검증된 영상 답변:

이 영상 해법은 위 문제에 도움이 된다고 튜터들이 추천한 것입니다.
영상 길이:
8m

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Dew Point

The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid. In the context of a dehumidifier, the air is cooled below this temperature, causing water vapor to condense on the cold coils. Understanding the dew point is crucial for determining how much moisture can be removed from the air.
추천 영상:
가이드 코스
08:35
Angular Momentum of a Point Mass

Heat Transfer

Heat transfer refers to the movement of thermal energy from one object or substance to another. In a dehumidifier, heat is removed from the humid air as it passes over the cold coils, leading to condensation. This process involves both sensible heat (temperature change) and latent heat (energy absorbed or released during phase changes), which are essential for calculating the efficiency of moisture removal.
추천 영상:
가이드 코스
05:14
Overview of Heat Transfer

Power and Energy Calculations

Power is the rate at which energy is used or transferred, measured in watts (W). In this scenario, the dehumidifier operates at 550 W, indicating how much electrical energy it consumes per second. To estimate the amount of water removed, one must convert this power into energy over the time period (1 hour) and relate it to the latent heat of vaporization of water, which quantifies the energy required to condense water vapor.
추천 영상:
가이드 코스
06:00
Power
관련 실천
교과서 질문

A bowl contains many red, orange, and green jelly beans, in equal numbers. You are to make a line of 3 jelly beans by randomly taking 3 beans from the bowl. Construct a table showing the number of microstates that correspond to each macrostate.

897
views
교과서 질문

1.00 mole of an ideal monatomic gas at STP first undergoes an isothermal expansion so that the volume at b is 2.5 times the volume at a (Fig. 20–25). Next, heat is extracted at a constant volume so that the pressure drops. The gas is then compressed adiabatically back to the original state. Calculate the pressures at b and c.

1584
views
교과서 질문

Suppose a heat pump has a stationary bicycle attachment that allows you to provide the work instead of using an electrical wall outlet. If your heat pump has a coefficient of performance of 2.0 and you can cycle at a racing pace output of about 200 W for a half hour, how much heat can you provide?

1231
views
교과서 질문

Two 1100-kg cars are traveling 75 km/h in opposite directions when they collide and are brought to rest. Estimate the change in entropy of the universe as a result of this collision. Assume T = 20°C.

1410
views
교과서 질문

A bowl contains many red, orange, and green jelly beans, in equal numbers. You are to make a line of 3 jelly beans by randomly taking 3 beans from the bowl. Construct a table showing the number of microstates that correspond to each macrostate. Then, determine the probability of all 3 beans red.

700
views
교과서 질문

Rank the following five-card hands in order of increasing probability: (a) four aces and a king; (b) six of hearts, eight of diamonds, queen of clubs, three of hearts, jack of spades; (c) two jacks, two queens, and an ace; and (d) any hand having no two equal-value cards (no pairs, etc.). Discuss your ranking in terms of microstates and macrostates.

922
views