First law of thermodynamics:
The first law of thermodynamics states that heat is a form of energy, and that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another location or converted to one from to other forms of energy.
Whenever a system goes through any change due to the interaction of heat, work and internal energy, it is followed by numerous energy transfers and conversions. However, during these transfers, there is no net change in the total energy. Thus, according to first law of thermodynamics the energy of the universe remains the same. This is known as the principle of conservation of energy. Any gain in energy by the system will correspond to a loss in energy by the surroundings, or any loss in energy by the system will correspond to a gain in energy by the surroundings.
ΔE = q + w
ΔE - Change in internal energy
ΔE = Efinal - Einitial
A positive value of ΔE results when Efinal > Einitial, indicating that the system has gained energy from its surroundings. A negative value of ΔE results when Efinal < Einitial, indicating that the system has lost energy to its surroundings.
Endothermic and Exothermic Processes:
When a process occurs in which the system absorbs heat, the process is called endothermic (endo- means “into”). During an endothermic process, such as the melting of ice, heat flows into the system from its surroundings. If we, as part of the surroundings, touch a container in which ice is melting, the container feels cold to us because heat has passed from our hand to the container. A process in which the system loses heat is called exothermic (exo- means “out of”). During an exothermic process, such as the combustion of gasoline, heat exits or flows out of the system into the surroundings.
To understand the meaning of the first law, we can take the common example of a heat engine. In a heat engine, the thermal energy is converted into mechanical energy. Heat engines are mostly categorized as open systems. The basic working principle of a heat engine is that it makes use of the different relationships between heat, pressure and volume of a gas. A heat engine is a system which converts heat into work by taking heat from the reservoir (hot body) to carry out some work. There is a discharge of some heat to the sink (cold body). In this system, there will also be some waste in the form of heat.
A very basic diagram of the heat engine is given below:
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