EJC Physics H209 First Law of Thermodynamics 2024_1. Notes (FULL)

Aircon Compressor s. Air-conditioned environments provide a welcome respite from the tropical humidity . “Blower” units in interiors have refrigerant connected to the compressor which usually sits outdoors. The compressor pressurizes the refrigerant and passes the heated liquid through fins cooled down by a fan. The refrigerant is further cooled by adiabatic expansion, and re-circulated back into the blower units, ready to transport more thermal energy from the interiors. As a cyclic process, there is net work done on the refrigerant i.e. there is more energy usage to operate the aircon system than the thermal energy the aircon system is able to extract. Content • Specific heat capacity and specific latent heat • Internal energy • First law of thermodynamics Learning Outcomes Candidates should be able to: (a) define and use the concepts of specific heat capacity and specific latent heat (b) show an understanding that internal energy is determined by the state of the system and that it can be expressed as the sum of a random distribution of kinetic and potential energies associated with the molecules of a system (c) relate a rise in temperature of a body to an increase in its internal energy (d) recall and use the first law of thermodynamics expressed in terms of the increase in internal energy, the heat supplied to the system and the work done on the system. For the first part of this topic, we return to the macroscopic scale to discuss how heat and temperature relates to work and energy. Devices which convert thermal energy into mechanical motion are typically referred to as engines. We then bridge the above to the microscopic view and see how the interplay between energy and useful work is explained by the kinetic model of gases. 4-stroke internal -combustion petrol engine. The air -fuel mixture is compressed almost instantaneously – i.e. work is done on the gas. A spark ignites the air-fuel mix, resulting in great increases in pressure. The piston is pushed downwards and the gas does work by exerting the force (pressure applied on the piston surface) downwards and turning the crank shaft.

Similar to the idea of “GDP per capita” (normalizes economic output to per person basis), “population density” (normalizes number of people to per area basis) and “density” (normalizes mass to per unit volume basis), “specific” in thermal physics normalizes the quantity to a per unit mass basis. A specific quantity in thermal physics allows us to discuss an intrinsic property of a material. Since specific heat capacity discusses the property of a material, notice the definition involves substance rather than a body. Experiments for finding specific heat capacity need to a find the mass, b measure the temperature rise, c determine the quantity of thermal energy supplied (often by electrical means so E = Pt = Q) d(i) prevent unwanted heat loss from the setup to surrounding OR d(ii) permit the hea