DHS 12 First Law of Thermodynamics (Notes & Tutorial)

1 9749 Physics (2024) Topic 9: First Law of Thermodynamics Guiding Questions  How can temperature and heat be understood from both macroscopic and microscopic perspectives?  How does the energy transferred between a system and its environment relate to heat transfer and mechanical work done? 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. Introduction The first law of thermodynamics is central to understanding thermodynamic processes which involve heat and mechanical work . This law extends the principle of conservation of energy in Newtonian mechanics, by introducing the concept of internal energy. Thermodynamics and its applications are relevant to our daily lives. Many researchers still actively investigate the behaviour of gases using computer simulation and other techniques. Environmental problems connected with Earth’s atmosphere, such as the de pletion of ozone layer, are motivations for such work. A better understanding of the behaviour of gases might lead to an improved characterisation and possible solutions to these problems. The application of thermodynamics pervades modern society e.g. car engines, air conditioners and power stations. However, an inevitable consequence of this energy conversion is the discharge of heat. This unwanted release of thermal energy into the environment is known as thermal pollution. One approach to reduce this is the development of alternative sources of energy, e.g. solar power. Another approach is to make things more efficient. Reducing the demand for energy should also be a personal and policy priority. 9749 H2 Physics Topic 9 First Law of Thermodynamics Year 5 (2024) DUNMAN HIGH SCHOOL

2 9749 Physics (2024) Topic 9: First Law of Thermodynamics (a) define and use the concepts of specific heat capacity and specific latent heat Specific heat capacity The specific heat capacity of a substance is defined as the heat per unit mass required to cause unit change in temperature. If the heat Q supplied to a mass m of the substance can cause its temperature to rise by T, then the specific heat capacity of the substance c is expressed mathematically as Qc m T  The unit of c is J kg−1 K−1. For example, if c of a substance is