Instant Notes

Need for Second Law: The First law allows us to calculate the energy changes, but it does not place any restriction on the direction of a process. Satisfying the first law alone does not ensure that the process can actually occur. A process cannot occur unless it satisfies both the first and the second laws of thermodynamics.

Thermal Reservoirs: Bodies with relatively large thermal masses can be modeled as thermal energy reservoirs. A source supplies energy in the form of heat, and a sink absorbs it. The temperature of these reservoirs remain constant.

Heat Engine: In the operation of a heat engine, heat is converted to work. The efficiency of such an engine is, \[\eta = \frac{\text{Net work done}}{\text{Heat absorbed}} = \frac{W_{\text{net,out}}}{Q_{\text{in}}}\] A heat-engine cycle cannot be completed without rejecting some heat to a low-temperature sink.

Refrigerator and Heat Pump: In these devices, heat flows from low temperature to high temperature with the aid of a compressor, which is taking a work of \(W\) in the form of electrical energy.