Jan 2021

A Simple Introduction To The First Law Of Thermodynamics

January 20, 2021

Many power plants and engines operate by converting heat energy to get work done. This is because a heated gas can do work (e.g. by pushing) on mechanical turbines or pistons, which causes them to move.

So what then is thermodynamics? From the name itself, ‘thermo’ refers to heat while ‘dynamics’ refer to the properties of temperature. So, thermodynamics is the study of temperature and how it relates to energy and work.

How is thermodynamics at work in our day-to-day lives? One example you may see in your household could be that of a whistling kettle. It represents energy in motion as the water in the kettle turns into water vapour as heat is being transferred from the stove to the kettle. As this system gets hotter, more work is done, resulting in the water evaporating and the kettle whistling.

The first law of thermodynamics

The first law of thermodynamics involves the conservation of different types of energy. Energy cannot be created or destroyed; it can only be transformed from one form into another. There are two ways of transferring energy to a system: heat and work. This transfer causes a change in the internal energy of the system, which is the sum of kinetic energy and potential energy of interaction of the molecules.

The first law of thermodynamics states that the change in internal energy of a system equals the heat transferred into the system minus the work done by the system. Thus, the equation for the first law of thermodynamics is:

ΔU = Q – W


ΔU is the increase in internal energy of the system

Q is the heat supplied to the system

W is the work done by the system

This simply means that by heating or doing work on the system, you can add to the internal energy. It is important to note that U, Q, and W can all be either positive or negative.

When Q is positive, there is a heat energy added into the system.

If Q is negative, heat is removed from the system.

When W is positive, there is work done by the system.

If W is negative, work is done on the system.

In physics context, the common scenario is one of adding heat to a gas volume and using the expansion of that gas to do work.

Examples related to the first law of thermodynamics

Many appliances apply the first law of thermodynamics in your home, such as refrigerators, heat pumps, or air conditioners. Many of these systems are heat engines that convert mechanical energy to heat.

In such closed systems, gas is compressed and therefore, the temperature increases. This hot gas can then transfer heat to its surrounding environment. When the compressed gas is able to expand, the temperature becomes colder than it was before as some of its heat energy was removed during the hot cycle. This cold gas can then absorb heat energy from its environment, thus cooling the system down.


If you are interested in learning more about the relationships between heat and other forms of energy, there are other principles and laws of thermodynamics to read up on. Physics tuition can help you gain a better understanding as the tutors guide you step by step and answer all your questions. If you are looking for O level physics tuition, Best Physics Tuition can be of service! Contact us today for any enquires.

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