Isothermal Process - Physical Engineering - Lecture Slides, Slides of Engineering Physics

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‘A thermodynamic process in which the temperature of the system remains constant during the supply of heat is called an Isothermal Process’

An isothermal process is a change of a system, in which the temperature remains constant: Δ T = 0. This typically occurs when a system is in contact with an outside thermal reservoir heat bath, and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange

Details for an ideal gas

For the special case of a gas to which Boyle's law applies, the product pV is a constant if the gas is kept at isothermal conditions. The value of the constant is nRT , where n is the number of moles of gas present and R is the ideal gas constant

Details for an ideal gas

In other words, the ideal gas law pV = nRT applies. This means that

P = nRT = Constant V V

holds.

The temperature corresponding to each curve in the figure increases from the lower left to the upper right

ISOTHERMAL COMPRESSION

Consider a cylinder of non-conducting walls and good heat conducting base. The cylinder is fitted with a frictionless piston. An ideal gas is enclosed in the cylinder. In the first stage pressure on the piston is increased and the cylinder is placed on a cold body. Due to compression , the temperature of the system increases but at the same time DQ amount of heat is removed from the system and the temperature of the system is maintained

ISOTHERMAL EXPANSION

In another situation the cylinder is placed over a hot body and the pressure on the system is decreased. Due to expansion, the temperature of the system is decreased but at the same time DQ amount of heat is absorbed from the hot body and the temperature of the system is again maintained.

According to the first law of thermodynamics:

DQ = DU+ DW Since temperature is constant, therefore, there is no change in internal energy of the system. i.e. DU = 0 As the work is done by the system, therefore, DW is positive, DQ = 0+ (DW) DQ = +DW

Applications

In Isothermal non flow Process, the work done by compressing the perfect gas (Pure Substance) is a negative work, as work is done on the system, as result of compression, the volume will decrease, and temperature will try to increase.

Applications

To maintain the temperature at constant value (as the process is isothermal) heat energy has to leave the system and enter the environment. The amount of energy entering the environment is equal to the work done (by compressing the perfect gas) because internal energy does not change

Applications

The product of pressure and volume is in fact, ' Moving Boundary Work' ; the systems boundaries are compressed. For Expansion the same theory is applied

Applications

As per Joule's Law for the perfect gas, Internal energy is the function of absolute temperature. In an Isothermal process the temperature is constant. Hence, the internal energy is constant, and the net change in internal energy is ZERO

Applications

So if the real pure gas undergoes an Isothermal process, there is a net change in internal temperature consistent with this component of internal energy