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October 20, 2019

First Law of Thermodynamics – so easy to understand?

First Law of Thermodynamics – introduction

In one of our previous post we covered the Zeroth law of thermodynamics. Here in this post we will cover the first law of thermodynamics. It basically discusses on the effects of heat transfer in terms of internal energy and work done. As this law is based on the concept of internal energy, we will briefly cover this before stating the 1st law of thermodynamics.

Internal Energy – important component of 1st law of thermodynamics

Internal Energy (designated as U) of solid, liquid or gas is the result of 2 forms of energy- (1) Kinetic Energy of that object due to the translational, rotational and vibrational motion of its molecules and (2) Potential Energy of that object due to the configuration or separation of its molecules.

Heat and work are the two modes of energy transfer to a system that in turn changes the internal energy of that system.

Work can be done on a system or the system can be made to work. Either of these can be responsible for the change of internal energy of a system.

Similarly, Heat can be supplied to a system or heat can be extracted out of a system. Any such situation can modify the internal energy of a system. This understanding will help us to start with the first law of thermodynamics now.

First Law of Thermodynamics – statement

If energy is supplied to a system which is capable of doing work, then the quantity of heat energy accepted by the system will be equal to the sum of the rise in the internal energy of the system and the external work done by it. We can express it using a formula:

∆Q = ∆U + ∆W  ………………… (1)

here ∆Q is the energy supplied to the system. ∆U is the change in U and ∆W us the work done by the system, caused by the energy supplied.

From equation 1, we get the expression of ∆U.
∆U = ∆Q – ∆W  ……………………. (2)

Note: Now instead of “work done by the system’ if work is done on the system, then sign of ∆W becomes negative.
In that case, the above equations would differ, catering the -ive sign of ∆W.

The next section discusses this sign conventions for different possible scenarios.

Some conventions – First Law of Thermodynamics

∆Q is positive if heat is supplied to the system.

∆Q is negative if heat is taken out of the system.

∆W is positive if some work is done by the system.

∆W is negative if some work is done on the system.

∆U is positive if temperature of the system increases.

∆U is negative if temperature of the system falls.

Law of conservation of energy – first law of thermodynamics

The first law of thermodynamics can be seen as a version of the law of conservation of energy, specifically adapted for thermodynamic systems. The law of conservation of energy states that the energy can be transformed from one form to another, but can not be created or destroyed. It also states that the total energy of an isolated system is constant.

Zeroth Law of Thermodynamics

We will post numerical problems here on this topic. Just keep on visiting and reading.


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