High School Physics

first law of thermodynamics-heat transfer

First Law of Thermodynamics – so easy to understand?

Last updated on December 30th, 2021 at 03:25 pm

Here in this post, we will cover the first law of thermodynamics. It basically talks about 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 – an important component of the First 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 the 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 that is capable of doing work, then the quantity of heat energy supplied to 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.

First Law of Thermodynamics – formulas

We can express the First Law of Thermodynamics using this formula:
∆Q = ∆U + ∆W  ………………… (1)
where ∆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 the sign of ∆W becomes negative. In that case, the above equations would differ, catering to the -ive sign of ∆W.
The next section discusses these sign conventions for different possible scenarios.

Some conventions of this 1st 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 the temperature of the system increases.

∆U is negative if the temperature of the system falls.

Law of conservation of energy – 1st 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.

Conclusion | Related Study

While studying the first law of thermodynamics, you can have a look at a related post: Zeroth Law of Thermodynamics

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

See also  What are Heat Capacity and Specific Heat? | comparison | Specific Heat Data table | numerical problems solved
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