Equation of first law of Thermodynamics with example

Thermodynamics is a branch of Physics that deals with the macroscopic variables like Temperature, Pressure, Volume, etc. of a system. These type of systems are known as Thermodynamic systems. There are four laws for these thermodynamic systems – Zeroth Law, First Law, Second Law and Third Law. We already have discussed the Zeroth law, second law and third law in other articles. In this post, I am going to explain the First law of thermodynamics with example and its equation and uses.

Contents in this article:

  1. Definition of First Law of Thermodynamics
  2. Equation of First Law of Thermodynamics
  3. Examples for First law of Thermodynamics
  4. First law of Thermodynamics for open system
  5. First law of Thermodynamics for closed system
  6. Which conservation law does first law of Thermodynamics follow?
  7. Uses of 1st Law of T.D.

Simple definition of First Law of Thermodynamics

The First Law of Thermodynamics states that if some amount of heat is given to a thermodynamic system, the system uses some parts of this heat to increase its internal energy and work is done with rest of the heat. Here, heat is equivalent to energy.

Equation of First Law of Thermodynamics

Let, dQ amount of heat is given to a Thermodynamic system. The increase in internal energy of the system is dU and the work done by the system is dW. Then, according to the first law of Thermodynamics, dQ = dU + dW……….(1)

Equation-(1) gives the mathematical form of first law of Thermodynamics.

Examples for First law of Thermodynamics

If we heat a liquid in a pot, after a few time we can see the expand of the volume of the liquid. This expand of the volume occurs due to the work done by the system. Again, before the expanding of liquid, the molecules of the liquid become more energetic. That means the kinetic energy of the molecules increases. This rises the internal energy of the system.

So, due to the supply of heat to a system, the internal energy of the system increases first and then the system works with the rest of energy. This is a simple example of first law of Thermodynamics.

Equation of 1st Law of Thermodynamics for open system

An open system is a system that can exchange the information, matter and energy with its surroundings. So, the heat energy can be supplied to an open system.

Thus, the equation of the first law of Thermodynamics for an open system is dQ = dU + dW……….(1)

Where, dQ is supplied heat energy, dU is change in internal energy of the system and dW is the work done by the system.

Equation of First Law of Thermodynamics for closed system

A closed system or isolated system cannot exchange matter, energy and information with its surroundings. So, one cannot supply heat to an isolated system. Then, the first law of Thermodynamics is not applicable for a closed system? Not really. This law is applicable for any thermodynamic system. In this case, only the supplied heat is zero. The system still can work by using its internal energy.

So, heat absorbed by a closed system is zero and therefore, the system loses its internal energy to do some work. Then the formula of first law of Thermodynamics for a closed system is

0 = dU + dW

or, dW = – dU ………….(2)

Which conservation law does the first law of Thermodynamics follow?

According to the first law of Thermodynamics, if a system gets some amount of heat energy, it uses some parts of energy to increase its internal energy and works with rests of the energy. Since, work is also equivalent to energy, the first law of Thermodynamics gives a clear picture on how a thermodynamic system converts one form of energy into another form. So, the first law of Thermodynamics follows the law of conservation of energy.

Importance of First law of Thermodynamics

The first law of Thermodynamics is not only a form of the law of conservation of energy. It reveals the following things also –

  • The formulation of first law of Thermodynamics with the conservation of energy leads to the existence of internal energy function U which is a function of state.
  • First law of Thermodynamics defines heat as energy during transit by virtue of temperature difference.

Applications of 1st law of Thermodynamics

The first Law of Thermodynamics gives an idea that the heat can be converted into energy. Heat engine is a common system where the first law of thermodynamics is in use. Because, a heat engine converts heat into the work.

This is all from this article on the equation of first law of Thermodynamics with example. If you have any doubt on this topic you can ask me in the comment section.

Thank you!

Related posts:

  1. Zeroth Law of Thermodynamics
  2. Second Law of Thermodynamics
  3. Third Law of Thermodynamics
  4. All four Laws of Thermodynamics in simple way

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