Thermodynamics - A-level Physics

Heat Drives Work and Internal Energy Increase In gaseous systems, supplied heat splits into work performed by the expanding gas and an increase in the kinetic energy of its particles. The process ensures that when heat is added, energy is divided, so if the gas does work during expansion, only the remaining energy augments its internal energy. In an isothermal change where temperature remains constant, every bit of added heat is completely expended as work, keeping internal energy unchanged as demonstrated by the constant product of pressure and volume.

Adiabatic Processes and the Direct Energy-Work Relationship Under adiabatic conditions, there is no heat exchange, so any work done by or on the gas comes entirely at the expense of its internal energy. The energy balance is governed by the relation where the change in internal energy equals the negative of the work performed. This behavior is further captured by the pressure-volume equation raised to a gas-specific exponent, highlighting how work directly translates into energy loss or gain without external thermal influence.

The p-V diagram illustrates how pressure and volume change during gas processes, capturing isothermal compressions and expansions with a curved line. In an isothermal scenario, a constant temperature makes pressure inversely proportional to volume, where compression increases pressure as volume decreases and expansion reverses that trend. Colder conditions shift these curves closer to the origin, while adiabatic changes drive the graph toward near-zero pressure. The work done by or on the gas is represented by the area under the curve, with vertical lines indicating no work and horizontal lines at constant pressure calculating work as P∆V, a concept that links together various thermodynamic cycles such as those in engines.

The explanation shows how a p-V diagram visually represents the work interactions during a gas cycle. It demonstrates that as the gas expands, the area under the curve indicates the work done by the gas, while during compression, the corresponding area signifies the work done on the gas. The narrative illustrates that both isothermal and adiabatic processes affect these work interactions, emphasizing the visual differentiation of energy transfer on the graph.

A four-stroke engine operates through a sequence where an air-fuel mixture is drawn into the cylinder at constant pressure during the intake phase. The piston then compresses the mixture, and a spark triggers a rapid combustion that sharply increases pressure. The resulting explosion forces the piston downward, and a final stroke expels exhaust gases at a slightly higher pressure. This process, illustrated on PV diagrams, transforms chemical energy into mechanical work.