Jun 1, 2022 • 8M

Thermodynamics Part 2 – Joule, Watt and the First Law

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The first law of thermodynamics makes it clear that the total energy in a closed system remains constant, despite multiple transformations of energy within it. As more studies regarding characteristics of energy were published and many forms of energy were discovered, scientists tended to believe that the first law of thermodynamics was true, but the problem before them was how they could prove it.  This required that the total amount of energy within a system be precisely measured. How were they to measure the heat energy of water boiling in a pot? It was not practical to experiment with a closed system, as energy always tried to flow from one place to another.

It was the English physicist James Prescott Joule who first conducted a fine experiment to prove the first law of thermodynamics. The unit of energy was named Joule in honour of his contributions.

The unit of power was named Watt in honour of the Scottish inventor James Watt, who had contributed greatly to improving the efficiency of the steam engine.

Joule, born in Salford, Lancashire, on 24 December 1818, received all his education at home.  He never worked officially throughout his life.  As he was born into a wealthy family, he had financial support from his family for his scientific experiments. He spent a lot of time and energy trying to improve the efficiency of electric motors. During those experiments, he learned much about the transformation of energy, which led him to test the validity of the first law of thermodynamics.


The first law of thermodynamics does not state how fast energy is converted from one form to another.  However, the rate of transformation of energy is important. As an example, consider a candle and a firecracker – both have chemical energy stored in them, but when lighted, they react differently. The candle will burn uniformly over a longer period of time while the firecracker will burn out in a matter of seconds. The rate of release of energy – that is, their power – in both these are different. It does not require much thinking to understand that the firecracker is more powerful than the candle because the firecracker releases energy faster.

The rate at which energy is released determines the power of an object, equipment, or person. When we divide the value of energy by time we get the value of power (P=E/t), where P is power, E is energy and t is time. The unit of power is Watt, and hence Watt = Joules per second.

The unit of power was named Watt in honour of the Scottish inventor James Watt, who had contributed greatly to improving the efficiency of the steam engine. James Watt was born in Greenock, Scotland, in 1736. He was an expert in matters related to engines. He went to London at the age of 18 to study the manufacture of instruments.

On his return to Scotland, Watt was appointed Mathematical Instrument Maker at the University of Glasgow. He started thinking about a design for a steam engine after seeing the model of the very old Newcomen steam engine at the university. The model at the university was not at all efficient. Watt made many useful modifications to that. Till his death in 1819, Watt kept on studying and modifying his steam engine. 

Watt is one of the units we use very commonly now. We buy bulbs of 50, 75, or 100 watts. At times, we refer to power in kilowatts and megawatts, but the ‘watt’ of James Watt was horsepower. This was a marketing idea as well. To indicate the immense power of his small steam engine, James Watt used the term ‘horsepower’. With this, James Watt wanted to motivate people to use the powerful engines in place of their horses.

The first law of thermodynamics is more than a mere statement about energy. Many scientists look upon it as an explanation of the structure and beauty of nature and the universe.


The equation E = P x t tells us how to find the value of E. To calculate the energy used by a 100-watt bulb, you need to multiply its power by the time it is used. Thus, E = P x t. For instance, if a 100-watt bulb is used for 10 hours, the energy used is 100 x10. This equation makes it clear that to reduce electricity consumption, we need to use equipment with less power or use them for a shorter duration, or limit both the power and usage time.

Energy has many forms, and, likewise, many units are used in our day-to-day life. However, those units are interconvertible with precise yardsticks. They all indicate the ability to exert a force over a given distance. For example, electric energy is measured in units of kilowatt/hour, while coal is measured in tons, and kerosene in gallons. They all refer to different forms of energy. The energy we derive from ingested food is recorded in calories.

The first law of thermodynamics is more than a mere statement about energy. Many scientists look upon it as an explanation of the structure and beauty of nature and the universe. 

About his experience with the first law of thermodynamics, Joule said: “Nothing is destroyed, nothing is ever lost, but all processes work in peace and harmony with all their complexities.” Joule also said that though those processes appeared to be very complex on minute examination, they were all protected with precision, and the ultimate will of God was eventually carried out. This can be considered to be a philosophical review.  For Joule, it was an example of God’s greatness, while others scientifically call it conservation of energy.

Dear teachers, we discussed the various aspects of the first law of thermodynamics in this article. Conservation of energy is at the core of this law. Energy transformation is another aspect of its conservation. Energy can neither be created nor destroyed. Instead, it only transforms into other forms.

The rate of change of energy is called power. The quantum of energy in a closed system is always constant. The first law of thermodynamics helped humankind understand the deep structure and harmony of the universe. It also created the framework for further investigations on energy.

Students can understand the practical implications of the first law only if they can assimilate the concept of conservation of energy. They must be given the opportunities through small experiments to understand conservation and transformation of energy.

Now put on your thinking hats and think about the following questions for a couple of minutes.

How would you explain the first law of thermodynamics to your students?

How would you describe the contributions of James Prescott Joule and James Watt in the field of Physical Science?

Write down your thoughts and discuss them with your students, children and your colleagues. Listen to their views and compare them with your own. As you listen to others, note how similar or different your views are to others’.

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Happy Teaching!