Work and Heat in Thermodynamic Processes
Figure 9.6 Work is done on a gas contained in a cylinder at a pressure P as the piston is pushed downward so that the gas is compressed.
(9.13)
(9.14)
Work and Heat in Thermodynamic Processes
Figure 9.7 A gas is compressed quasi-statically (slowly) from state i to state f. The work done on the gas equals the negative of the area under the PV curve.
The work done on a gas in a quasi-static process that takes the gas from an initial state to a final state is the negative of the area under the curve on a PV diagram, evaluated between the initial and final states. 
Work and Heat in Thermodynamic Processes
Figure 9.8 The work done on a gas as it is taken from an initial state to a final state depends on the path between these states.
Work and Heat in Thermodynamic Processes
Figure 9.9 (a) A gas at temperature Ti expands slowly while absorbing energy from a reservoir in order to maintain a constant temperature. (b) A gas expands rapidly into an evacuated region after a membrane is broken.
Energy transfer by heat, like work done, depends on the initial, final, and intermediate states of the system.
The First Law of Thermodynamics
the change in the internal energy of the system, independent of the path. 
the internal energy
the change in internal energy
(9.15)
The first law of thermodynamics
The First Law of Thermodynamics
First, consider an isolated system—that is, one that does not interact with its surroundings.
The internal energy Eint of an isolated system remains constant.
The First Law of Thermodynamics
Next, consider the case of a system (one not isolated from its surroundings) that is taken through a cyclic process—that is, a process that starts and ends at the same state.
In a cyclic process, the net work done on the system per cycle equals the area enclosed by the path representing the process on a PV diagram.
Quick Quiz 5.1
Two objects, with different sizes, masses, and temperatures, are placed in thermal contact. Energy travels (a) from the larger object to the smaller object (b) from the object with more mass to the one with less (c) from the object at higher temperature to the object at lower temperature.
Quick Quiz 5.2 
Consider the following pairs of materials. Which pair represents two materials, one of which is twice as hot as the other? (a) boiling water at 100°C, a glass of water at 50°C (b) boiling water at 100°C, frozen methane at -50°C (c) an ice cube at -20°C, flames from a circus fire-eater at 233°C (d) No pair represents materials one of which is twice as hot as the other