The First Law of Thermodynamics

From MyMCAT

Introduction

In all nonnuclear reactions, energy is conserved, and thermodynamics is no exception. Thermodynamic reactions often involve transferring energy to and from systems in the form of heat and applying or exerting work to and from the system. Before the complete conservation of energy formula is discussed, it is important to understand these two fundamental processes (heat and work).

For simplicity, on the MCAT, these changing systems always involve gases because of their ability to allow pressure, volume, and temperature to be easily controlled. As a result of this, one should always keep in mind the formula PV = nRT.

Heat Transfer

Heat can be added or removed from a gas system. If heat is added to the system while the pressure remains constant, then the temperature will rise, similarly if heat is removed then the temperature will fall. This should be a straight forward inference if one remembers that temperature is directly related to the total internal energy of the system.

In thermodynamic equations, heat is generally labeled Q. By convention, if heat is removed from the system, then Q < 0. Similarly, Q > 0 implies heat is absorbed/gained by the system from the outside. A process is considered adiabatic if no heat is exchanged during a process, in which case, Q = 0.

Work

Just as heat can be added to or removed from a gas system, so can work. Work can be applied on the system, or work can be done by the system on to the outside. Because we are considering gas systems, when work is done on the system, we are implying that we are compressing the "container", making the system smaller. If work is done by the system, then we are implying that the gas is expanding, "pushing" out the walls of the container in to the outside.

Consider a piston in an engine. When the gas inside explodes, a force is created which pushes the top of the piston up. This expansion of the container implies that the system (ie the gas inside the piston) doing work. Similarly, when the piston comes back down, it compresses the gas and decreases the total volume of the system, thus work is being done on the system.

P-V Diagrams and Work

This concept of working being done on or by the system is often represented in a Pressure vs. Volume graph. If the initial and final values of pressure and volume for a gas system are plotted, then the area on the curve represents the work done by the gas.

In an isobaric process, pressure does not change, thus the area under the PV curve is a simple rectangle, and the Work done by or on the system is P times the change in V.

In an isovolumetric process, the volume does not change, thus the area under the PV curve is zero, and the Work done by or on the system is zero. W = 0.