Conflicting Thoughts about the First Law of Thermodynamics

FROM THE LECTURE SERIES: THE JOY OF SCIENCE

By Robert M. Hazen, Ph.D., George Mason University

The first law of thermodynamics is more than just a useful statement about energy. To many scientists, it carries a very deep and profound significance about the underlying symmetry and the beauty of the natural order. In the conservation of energy, scientists saw something analogous to the immortality of the soul, and they made many connections.

An image of a lit bulb on a green background.
Energy can be converted from one form to another. (Image: PopTika/Shutterstock)

The Concept of Power in Science

Power is defined as energy divided by time, that is, P is equal to E divided by t (P = E/t). In other words, the faster the release of energy, the more powerful an object is.

This equation can be moved to give a way to calculate energy: energy is equal to power times time (E = Pt). So, for example, the energy used by a 100-watt light bulb in 10 hours can be calculated. 

Plugging into the equation E = Pt, energy is 100 watts times 10 hours, or 1 kilowatt-hour. That is what is paid for in the electric bill, and there are two choices for cutting down on electric bills—either by turning off the lights, thus lowering the time that the light bulb is on, or using a light bulb of lower wattage, thus cutting down the amount of power that is consumed by that bulb.

This is a transcript from the video series The Joy of Science. Watch it now, on The Great Courses Plus.

Energy and Its Units in Science

Energy is a  pervasive theme in science,  and every sub-discipline has to incorporate the concept of energy. Consequently, there have been a bewildering variety of energy units introduced into science.

An image of solar panels and wind turbines in a non-residential area.
Different scientific fields have different units to describe energy. (Image: petrmalinak/Shutterstock)

For example, home energy bills commonly use kilowatt-hours for electricity, they use therms for natural gas, gallons for heating oil, tons for coal, and cords for wood. In other countries, they sell energy by the British thermal unit, the BTU. The English system employs the foot-pound or the horsepower-hour (approx two million times larger).

Physicists use joules, but they also use ergs, which is one-millionth of a joule, for everyday objects. They all use electron volts as an energy source, or an energy term when dealing with atomic-scale processes. These are all just different units for energy.

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Lord Kelvin’s Agreement with the First Law of Thermodynamics

The great physicist William Thompson, also known as Lord Kelvin, certainly agreed with the beauty of the first law of thermodynamics. Kelvin was a towering science figure in the Victorian era. He was born in Belfast, Ireland, in 1824.

Thompson, at the age of 10, entered university. He was at the top of his class in mathematics, logic, and classics. He published the first of some 660 scientific papers at the young age of 16. He went on to Cambridge for what was, in fact, graduate work in theoretical physics. 

While there, he was also on the rowing team, and he helped found the Cambridge University Music Society, which is still thriving today. After a brief time in Paris, he became a professor of natural philosophy at Glasgow, and that was the position he held for 53 years until his death in 1907. He was buried in Westminster Abbey, near Newton and Darwin.

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Kelvin’s Conflict with Darwin’s Theory

Image of Charles Darwin statue on white background.
The conflict between Darwin (above) and Kelvin lasted many years until Rutherford resolved it by discovering a new source of energy. (Image: rigsbyphoto/Shutterstock)

Kelvin was drawn to the perfection of the first law of thermodynamics, the conservation of energy. He appreciated the economy, the symmetry of this law. On the other hand, Kelvin wrote with great distaste about Charles Darwin’s theory of evolution, in which random variations and chance seemed to play a central role. 

How could the creator of such a magnificent order of the first law of thermodynamics then resort to chance and probabilities when it came to his highest creation, life? So Kelvin set out to use the first law of thermodynamics to destroy Darwin’s idea. 

He said that every closed system such as the Earth and the Sun has a fixed budget of energy. For life to exist on the Earth, the Sun has to expend prodigious amounts of energy. In other words, the Earth only has a fairly short span of time for humans to evolve and to live on it. 

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Kelvin’s Assumption about the Earth 

According to the few simple assumptions that Kelvin made about all the different known energy sources in his time, for example, the Sun, the various kinds of fuels, gravitational collapse of the Sun, and so forth, he decided that life on Earth must have been around for significantly less than 100 million years, perhaps only 10 million years. 

What he has stated was very much less than the hundreds of millions, or perhaps billions of years required by Darwin and his theory of evolution. Kelvin was very wise to add, when he said this, “unless sources of energy now unknown to us are prepared in the great storehouse of creation”. So, he kind of hedged his bets.

This conflict between physics and biology was resolved in 1904 when Ernest Rutherford announced the discovery of radioactivity, a powerful new energy source. Einstein’s famous equation, E=mc2 appeared the following year. So clearly, the Sun and the Earth had major energy sources that no one had recognized before, and that conflict between physics and biology was finally resolved.

Learn more about the chemical evolution of life.

To Sum Up

Energy can change from one form to another in different ways. Energy can change form many, many times, and that the rate of change of energy, or power, can vary widely. Nevertheless, the total amount of energy in a closed system is constant. 

Energy is neither created nor is it destroyed. The first great law of thermodynamics provides a glimpse of the order and the symmetry of the universe. The first law also provided a framework for investigating energy, but the first law is only half of the story.

Common Questions about the Conflicting Thoughts about the First Law of Thermodynamics

Q: What is the definition of power in science?

Power is defined as energy divided by time, that is, P is equal to E divided by t (P = E/t). In other words, the faster the release of energy, the more powerful an object is.

Q: What are some of the different units that describe energy?

Energy has so many units to describe it. For example, home energy bills commonly use kilowatt-hours for electricity, therms for natural gas, gallons for heating oil, tons for coal, and cords for wood. Also, physicists use joules and ergs, which is one-millionth of a joule.

Q: Who was William Thompson?

William Thompson, also known as Lord Kelvin, was one of those scientists who followed and appreciated the first law of thermodynamics. He wrote with great distaste about Darwin’s theory of evolution, in which random variations and chance seemed to play a central role. 

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