Explaination: First law of Thermodynamics

The 1st Law of Thermodynamics tells us that energy is neither created nor destroyed, thus the energy of the universe is a constant. However, energy can certainly be transferred from one part of the universe to another. To work out thermodynamic problems we will need to isolate a certain portion of the universe, the system, from the remainder of the universe, the surroundings. This is also known as the law of conservation of energy.

The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes. The first law makes use of the key concepts of internal energy, heat, and system work. It is used extensively in the discussion of heat engines. The standard unit for all these quantities would be the joule, although they are sometimes expressed in calories or BTU (British thermal unit).

The total energy in the universe is present either as kinetic energy or as potential energy.

A simple example can be of water stored in a dam. This water has a potential energy due to its height. now when this water falls down, this potential energy is converted into kinetic energy. This kinetic energy then rotates the turbines and electricity is produced. So the potential energy gets converted to kinetic energy which in turn helps us to produce electricity and thereby the total energy remains constant.

Energy Changes in relation to work and Heat changes:
Let us suppose that UA be the energy of a system in its state A and UB be the energy of a system in its state B. Frequently it is convenient to focus on changes in the assumed internal energy (U) and to regard them as due to a combination of heat (q) added to the system while undergoing change from state A to state B and work done by the system equal to w. The absorption of heat by the system tends to raise the energy of the system. The performance of work by the system, on the other hand, tends to lower the energy of the system because performance of work requires expenditure of energy. Therefore the change in internal energy ΔU, of a system is equal to the head added to the system minus the work done by the system:

ΔU = UB – UA = q – w

In general, if in a given process the quantity of heat transferred from the surrounding to the system is q and work done in the process is w, then the change in internal energy,

ΔU = q + w………………………………… (1)

Equation 1 is the mathematical statement of the first law of thermodynamics.

During the compression of a gas work is done by the surroundings on the system, so w is taken as positive so that ΔU = q + w.

During the expansion of a gas however work is done by the system on the surroundings, so w is taken as negative so that ΔU = q – w.