Molecular Physics | Thermodynamics Answers

.14. A closed system of constant volume experiences a temperature rise of 25°C

when a certain process occurs. The heat transferred in the process is 30 kJ. The specific heat at

constant volume for the pure substance comprising the system is 1.2 kJ/kg°C, and the system

contains 2.5 kg of this substance. Determine :

(i) The change in internal energy ;

(ii) The work done.

The power developed by a turbine in a certain steam plant is 1200 kW. The

heat supplied to the steam in the boiler is 3360 kJ/kg, the heat rejected by the system to cooling

water in the condenser is 2520 kJ/kg and the feed pump work required to pump the condensate

back into the boiler is 6 kW.

Calculate the steam flow round the cycle in kg/s

1. In a system, executing a non-flow process, the work and heat per degree

change of temperature are given by

dW

dT

= 200 W-s/°C and dQ

dT = 160 J/°C

What will be the change of internal energy of the system when its temperature changes

from

T1 = 55°C to T2 = 95°C ?

7. A cylinder containing the air comprises the system. Cycle is completed as

follows :

(i) 82000 N-m of work is done by the piston on the air during compression stroke and

45 kJ of heat are rejected to the surroundings.

(ii) During expansion stroke 100000 N-m of work is done by the air on the piston.

Calculate the quantity of heat added to the system.

4. When a stationary mass of gas was compressed without friction at constant

pressure its initial state of 0.4 m3 and 0.105 MPa was found to change to final state of 0.20 m3 and

0.105 MPa. There was a transfer of 42.5 kJ of heat from the gas during the process.

How much did the internal energy of the gas change ?

7. An air compressor commences compression when the cylinder contains 12 g at a pressure is 1.01

bar and the temperature is 20o

C. The compression is completed when the pressure is 7 bar and

the temperature 90o

C.

The characteristic gas constant R is 287 J/kg K. Assuming the process is reversible and

polytropic, calculate the index of compression and the work done.

Gas fills a cylinder fitted with a frictionless piston. The initial pressure and volume are 40 MPa

and 0.05 dm3 respectively. The gas expands reversibly and polytropically to 0.5 MPa and 1 dm3

respectively. Calculate the index of expansion and the work done

. A furnace wall consists of 200 mm layer of refractory bricks, 6 mm layer of

steel plate and a 100 mm layer of insulation bricks. The maximum temperature of the wall is

1150°C on the furnace side and the minimum temperature is 40°C on the outermost side of the

wall. An accurate energy balance over the furnace shows that the heat loss from the wall is

400 W/m2. It is known that there is a thin layer of air between the layers of refractory bricks and

steel plate. Thermal conductivities for the three layers are 1.52, 45 and 0.138 W/m°C respec￾tively. Find :

(i) To how many millimetres of insulation brick is the air layer equivalent ?

(ii) What is the temperature of the outer surface of the steel plate ?