This lecture covers the first law of thermodynamics, describing it as inspired by the conservation of energy in mechanics. It explains the different quantities defined in the first principles, focusing on the sign convention. The lecture delves into thermodynamic transformations, the calculation of work done or received by a system, and the importance of energy conservation. It also discusses the generalization of the conservation of energy from mechanics to thermodynamics, emphasizing the role of heat. The first law, derived in the mid-1800s, highlights the crucial sign convention for work and heat. The lecture concludes by exploring the quantities regulated by the first law, including internal energy, work, and heat, with detailed examples and calculations.