Adiabatic process

An adiabatic process occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work. The adiabatic process provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics, and as such it is a key concept in thermodynamics. An adiabatic process occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work. The adiabatic process provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics, and as such it is a key concept in thermodynamics. Some chemical and physical processes occur so rapidly that they may be conveniently described by the term 'adiabatic approximation', meaning that there is not enough time for the transfer of energy as heat to take place to or from the system. By way of example, the adiabatic flame temperature is an idealization that uses the 'adiabatic approximation' so as to provide an upper limit calculation of temperatures produced by combustion of a fuel. The adiabatic flame temperature is the temperature that would be achieved by a flame if the process of combustion took place in the absence of heat loss to the surroundings. In meteorology and oceanography, the adiabatic cooling process produces condensation of moisture or salinity and the parcel becomes oversaturated. Therefore, it is necessary to take away the excess. There the process becomes a pseudo-adiabatic process in which the liquid water/salt that condenses is assumed to be removed as soon as it is formed, by idealized instantaneous precipitation. The pseudoadiabatic process is only defined for expansion, since a parcel that is compressed becomes warmer and remains undersaturated. A process that does not involve the transfer of heat or matter into or out of a system, so that Q = 0, is called an adiabatic process, and such a system is said to be adiabatically isolated. The assumption that a process is adiabatic is a frequently made simplifying assumption. For example, the compression of a gas within a cylinder of an engine is assumed to occur so rapidly that on the time scale of the compression process, little of the system's energy can be transferred out as heat to the surroundings. Even though the cylinders are not insulated and are quite conductive, that process is idealized to be adiabatic. The same can be said to be true for the expansion process of such a system. The assumption of adiabatic isolation of a system is a useful one, and is often combined with others so as to make the calculation of the system's behaviour possible. Such assumptions are idealizations. The behaviour of actual machines deviates from these idealizations, but the assumption of such 'perfect' behaviour provide a useful first approximation of how the real world works. According to Laplace, when sound travels in a gas, there is no time for heat conduction in the medium and so the propagation of sound is adiabatic. For such an adiabatic process, the modulus of elasticity (Young's modulus) can be expressed as E = γP, where γ is the ratio of specific heats at constant pressure and at constant volume (γ = Cp/Cv ) and P is the pressure of the gas . For a closed system, one may write the first law of thermodynamics as : ΔU = Q – W, where ΔU denotes the change of the system's internal energy, Q the quantity of energy added to it as heat, and W the work done by the system on its surroundings.