Thermodynamics Test - 13

In a perfectly reversible system, there is no loss of energy. Losses can be minimised, friction can be reduced, the resistance in L-C oscillating system can also be negligible. But one cannot completely eliminate energy losses. This makes a perfectly reversible system, an ideal.

In reversible process, there always occurs some loss of energy. This is because energy spent in working against the dissipative force is not recovered back. Some irreversible process occur in nature such as friction where extra work to cancel the effect of friction. Salt dissolves in water but a salt does not separate by itself into pure salt and pure water.

As isothermal processes are very slow and so the different isothermal curves have different slopes so they cannot intersect each other.

We can change the temperature of a body without giving (or taking) heat to (or from) it. For example in an adiabatic compression temperature rises and in an adiabatic expansion temperature false, although no heat is given or taken from the system in the respective changes.

In adiabatic process, no heat transfer takes place between gas and surrounding

i.e., ΔQ = 0

From definition of specific heat,

C = ΔQ/mΔT = 0

Again, for isothermal process

ΔT = 0

∴ C = ΔQ/mΔT = ∞

According to first law of thermodynamics, ΔQ = ΔU + ΔW = ΔU + PΔV. If heat is supplies in such a manner that volume does not change ΔV = 0 i.e., isochoric process, then whole of the heat energy supplied to the system will increase internal energy only. But, in any other process it is not possible. Also heat may absorbed or evolved when state of thermal equilibrium changes.

When the door of refrigerator is kept open, heat rejected by the refrigerator to the room will be more than the heat taken by the refrigerator from the room (by an amount equal to work done by the compressor). Therefore, temperature of room will increase and so it will be warmed gradually. As according to 2nd law of thermodynamics, heat cannot be transferred on its own, from a body at lower temperature to another at higher temperature.

If an electric fan is switched on in a closed room, the air will be heated because due to motion of the fan, the speed of air molecules will increase. In fact, we feel cold due to evaporation of our sweat.

The internal energy of system depends only on its temperature. In isothermal process temperature does not change, therefore, internal energy of the system remains the same.

Basically isentropic process is just a process in which entropy of system remains constant. And adiabatic is the process in which the heat transfer is zero in the process. When the adiabatic process is carried out reversibally then it is called isentropic process.