第一章 热力学基本概念.doc

第一章 热力学基本概念
英文习题
1. Expressing temperature rise in different units
During a heating process, the temperature of a system rises by 10℃. Express this rise in temperature in K, ℉ and R.
2. Absolute pressure of a vacuum chamber
A vacuum gage connected to a chamber reads 5.8 psi at location where the atmosphere pressure is 14.5 psi. Determine the absolute pressure in the chamber.
3. Measuring pressure with a manometer
A manometer is used to measure the pressure in a tank. The fluid used has a specific gravity of 0.85, and the manometer column height is 55 cm, as shown in Fig.1-1. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank.
4. Measuring pressure with a multi-fluid manometer
The water in a tank is pressurized by air, and the pressure is measured by a multi-fluid manometer as shown in Fig. 1-2. The tank is located on a mountain at an altitude of 1400 m where the atmospheric pressure is 85.6 kPa. Determine the air pressure in the tank if h1=0.1 m, h2=0.2 m, and h3=0.35 m. Take the densities of water, oil, and mercury to be 1000 kg/m3, 850 kg/m3, and 13 600 kg/m3 respectively.
5. Effect of piston weight on pressure in
a cylinder
The piston of a vertical piston-cylinder device containing a gas has a mass of 60 kg and a cross-sectional area of 0.04 m2, as shown in Fig.1-3. The local atmosphere pressure is 0.97 bar, and the gravitational acceleration is 9.81 m/s2. (a) Determine the pressure inside the cylinder. (b) If some heat is transferred to the gas and its volume is doubled, do you expect the pressure inside the cylinder to change?
6. Burning off lunch calories
A 90-kg man had two hamburgers, a regular serving of French fries, and a 200-ml Coke for lunch. Determine how long it will take for him to burn the lunch calories off (a) by watching TV and (b) by fast swimming. What would your answers be for a 45-kg man?
7. Burning of a candle in an insulated room
A candle is burning in a well-insulated room. Taking the room (the air plus the candle) as the system, determine (a) if there is any heat transfer during this burning process and (b) if there is any change in the internal energy of the system.
8. Boundary work during a constant-volume process
A rigid tank contains air at 500 kPa and 150℃. As a result of heat transfer to the surroundings, the temperature and pressure inside the tank drop to 65℃ and 400 kPa, respectively. Determine the boundary work done during the process.

9. Isothermal compression of an ideal gas
A piston-cylinder device initially contains 0.4 m3 of air at 100 kPa and 80℃. The air is now compressed to 0.1 m3 in such a way that the temperature inside the cylinder remains constant. Determine the work done during this process.
10. Heat transfer from a person
Consider a person standing in a breezy room at 20℃. Determine the total rate of heat transfer from this person if the exposed surface area and the average outer surface temperature of the person are 1.6 m2 and 29℃, respectively, and the convection heat transfer coefficient is 6 W/m2.℃ (Fig.1-7)

工程热力学与传热学
第一章 基本概念 习题
习 题

Expressing temperature rise in different units
During a heating process, the temperature of a system rises by 10℃. Express this rise in temperature in K, ℉ and R.
2. Absolute pressure of a vacuum chamber
A vacuum gage connected to a chamber reads 5.8 psi at location where the atmosphere pressure is 14.5 psi. Determine the absolute pressure in the chamber.
3. Measuring pressure with a manometer
A manometer is used to measure the pressure in a tank. The fluid used has a specific gravity of 0.85, and the manometer column height is 55 cm, as shown in Fig.1-1. If the local atmospheric pressure is 96 kPa, determine the absolute pressure within the tank.
4. Measuring pressure with a multi-fluid manometer
The water in a tank is pressurized by air, and the pressure is measured by a multi-fluid manometer as shown in Fig. 1-2. The tank is located on a mountain at an altitude of 1400 m where the atmospheric pressure is 85.6 kPa. Determine the air pressure in the tank if h1=0.1 m, h2=0.2 m, and h3=0.35 m. Take the densities of water, oil, and mercury to be 1000 kg/m3, 850 kg/m3, and 13 600 kg/m3 respectively.
5. Effect of piston weight on pressure in
a cylinder
The piston of a vertical piston-cylinder device containing a gas has a mass of 60 kg and a cross-sectional area of 0.04 m2, as shown in Fig.1-3. The local atmosphere pressure is 0.97 bar, and the gravitational acceleration is 9.81 m/s2. (a) Determine the pressure inside the cylinder. (b) If some heat is transferred to the gas and its volume is doubled, do you expect the pressure inside the cylinder to change?
6. Burning off lunch calories
A 90-kg man had two hamburgers, a regular serving of French fries, and a 200-ml Coke for lunch. Determine how long it will take for him to burn the lunch calories off (a) by watching TV and (b) by fast swimming. What would your answers be for a 45-kg man?
7. Burning of a candle in an insulated room
A candle is burning in a well-insulated room. Taking the room (the air plus the candle) as the system, determine (a) if there is any heat transfer during this burning process and (b) if there is any change in the internal energy of the system.
8. Boundary work during a constant-volume process
A rigid tank contains air at 500 kPa and 150℃. As a result of heat transfer to the surroundings, the temperature and pressure inside the tank drop to 65℃ and 400 kPa, respectively. Determine the boundary work done during the process.

9. Isothermal compression of an ideal gas
A piston-cylinder device initially contains 0.4 m3 of air at 100 kPa and 80℃. The air is now compressed to 0.1 m3 in such a way that the temperature inside the cylinder remains constant. Determine the work done during this process.
10. Heat transfer from a person
Consider a person standing in a breezy room at 20℃. Determine the total rate of heat transfer from this person if the exposed surface area and the average outer surface temperature of the person are 1.6 m2 and 29℃, respectively, and the convection heat transfer coefficient is 6 W/m2.℃ (Fig.1-7)

中文习题

1. 平衡状态与稳定状态有何区别？热力学中为什么要引入平衡状态的概念？
2. 表压力或真空度能否作为状态参数进行热力计算？若工质的压力不变，问测量其压力的压力表或真空计的读数是否可能变化？
3. 真空表指示数值越大，被测对象的实际压力愈大还是愈小？
4. 准平衡过程与可逆过程有何区别？
5. 不可逆过程是无法回到初态的过程，这种说法是否正确？
6. 没有盛满开水的热水瓶，其瓶塞有时被自动顶开，有时被自动吸紧，这是什么原理？
7. 用U形管压力表测定工质的压力时，压力表液柱直径的大小对读数有无影响？
8. 某容器被一刚性壁分为两部分，在容器不同部位装有3个压力表，如图示，压力表B的读数为1.75bar，压力表A的读数为1.10bar。如果大气压力计读数为0.97bar，试确定压力表C的读数及两部分容器内气体的绝对压力。
9. 如图所示的圆筒形容器，其直径为450mm，压力表A的读数为360kPa，压力表B的读数为170kPa，大气压力为760mmHg，试求：（1）真空室及1，2两室的绝对压力；（2）压力表C的读数；（3）圆筒顶部所受的作用力。
10. 气体初态时p1=0.5MPa，V1=0.4m3，在压力不变的条件下膨胀到V2=0.8m3。求气体所作的膨胀功。
11. 一热力系发生状态变化，压力随体积的变化关系为pV1.3=常数。若热力系初态为p1=600kPa，V1=0.3m3。问当系统体积膨胀至V2=0.5m3时对外作功为多少？
12. 利用储气罐中的压缩空气在温度不变的条件下给气球充气。开始时气球内没有气体，故可认为其初始体积为零；充满气体后气球的体积为2m3。若大气压力为0.09MPa，求充气过程中气体所作的功。
13. 上题中，若储气罐的体积为2m3，试求储气罐中气体压力为以下三种情况时气体所作的功：（1）压力为0.3MPa；（2）压力为0.182MPa；（3）压力为0.15MPa。

习 题 解 答

1 答：平衡着眼于时间，平衡状态是在没有外界作用的情况下，工质（或系统）的宏观性质不随时间而变化的状态。稳定着眼于固定的空间点，稳定状态是指对于系统中固定的空间点而言，该点的状态不随时间的变化而变化。系统是由无数个固定的空间点组成的，每一个固定空间点的状态都不随时间的变化而变化，但不同点的状态不同。
引入平衡的意义在于只有平衡状态，才能用确定的状态参数描述系统的状态。非平衡状态的系统，无法找到确定的状态参数来描述系统的状态。
2 答：表压力或真空度不能作为状态参数进行热力计算。工质的压力不变，若大气压力发生变化，压力表或真空计的读数也会发生变化。
3 答：真空度指示数值越大，被测对象的实际压力越小。
4 答：准平衡过程是假设过程中系统所经历的每一个状态都无限接近平衡态，着眼于系统内的平衡，可逆过程是指如果系统完成了某一过程之后，再沿着原路逆行，而恢复到原来的状态，外界也随之回复到原来的状态。可逆过程着眼于系统和外界的总平衡。一个准平衡过程不一定是可逆过程，但一个可逆过程一定是一个准平衡过程。可逆过程是无耗散的准平衡过程。
5 答：不正确。不可逆过程可以使系统回复到原来的状态，但此时相应的外界一定不能回复到原态。
6 答：当热水瓶内压力大于外界压力时，瓶塞会被自动弹开；当热水瓶内压力小于外界压力时，瓶塞会被自动吸紧。
7 答：U形管压力表直径的大小对读数无影响。
8 答：压力表C的读数为-0.64bar，两部分容器气体的压力分别为2.076bar，0.33bar。
9 答：（1）真空室压力2.0KPa，1室压力362KPa，2室压力192KPa；（2）压力表C的读数：190KPa；（3）圆筒顶部所受作用力1.58×104N。
10 答：气体所受膨胀功20kJ。
11 答：对外界作功85.2kJ。
12 答：充气过程中气体所作的功1.8×105J。
13 答：（1），（2）两种情况下，气体所作功为1.8×105J；（3）情况气体所作功为1.2×105J。