Niels Bohr

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Niels Bohr (1885 – 1962) was a Danish physicist who significantly improved our understanding of the structure of the atom.  His improved model of the atom solved the problems associated with classical physics and was based on the newer quantum mechanic physics.

Early Life and Education

Niels Bohr was born on October 7th, 1885 in Copenhagen, the capital of Denmark.  He was born into an intellectual and supportive family, and his upbringing consisted of a rich educational environment. His father, Christian Bohr, was a professor of physiology at the University of Copenhagen. His mother, Ellen Bohr, came from a prominent Jewish-Danish banking family with influence in parliamentary circles. The combination of his fathers scientific work and his mothers cultural pedigree together created a home environment that nurtured curiosity and scholarly pursuit. It is no surprise that young Niels Bohr grew up strong in mathematics and was naturally attracted to science at a young age. 

In 1903 he enrolled as an undergraduate at the University of Copenhagen, where he received his master’s and doctorate in physics by 1911.  That same year he traveled to England to work in the Cavendish Laboratory where he met J.J. Thomson. The two scientists didn’t work well together at first, but Bohr soon met another scientist at a different laboratory whom he worked better with. In March 1912 Bohr traveled to Manchester to work with Ernest Rutherford, who had recently won a Nobel Prize in Chemistry for his work on radioactivity.

Scientific Career

Niels Bohr
Niels Bohr

Bohr returned to Denmark in 1912 after having secured a teaching position at the University of Copenhagen.  He brought with him new ideas about the structure of the atom.  It was becoming clear that Rutherford’s model of the atom, based on classical physics, was unstable.  According to classical physics, the electrons moving around the nucleus of an atom in orbit would emit electromagnetic radiation, causing the electron to lose energy and eventually spiral into the nucleus.  The form of electromagnetic radiation being emitted came to be called photons, with each photon having its own precise wavelength and amount of energy.  Quantum physics states that objects emit photons in discrete packets rather than in continuous streams. Using quantum physics, Bohr proposed that electrons are confined to fixed orbits, each with their own distinct energy level.  They only suddenly jump to lower or higher orbits as a precise amount of energy is emitted or absorbed in the atom.  For example to move an electron to a lower energy level it emits a photon of the precise amount of energy that is the difference between the two orbits.  Using Plank’s constant, the frequency of photons, and some information about the electrons mass and charge, Bohr was able to obtain an accurate mathematical formula for the hydrogen atom.  In 1922, Niels Bohr was awarded Nobel Prize in Physics for this work.

Bohr continued to work on quantum physics for the remainder of his life.  He founded the Niels Bohr Institute at the University of Copenhagen.  Later in life he was a part of the Manhattan Project during the Second World War, working with many other great physicists.  He died in Copenhagen at the age of 77.

1781: Discovery of Uranus

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In 1781 Sir William Herschel announced the discovery of a new planet that became named Uranus in the tradition of naming planets after classical mythology. The discovery of Uranus, the seventh planet from our Sun, was a pivotal moment in the history of astronomy. In antiquity, the planets referred to the seven visible points of light that moved across the fixed background of the stars. These included the Sun and the Moon, as well as the classical planets of Mercury, Venus, Mars, Jupiter, and Saturn. The discovery of Uranus marked the first time a new planet was discovered since ancient times and ushered in a new era of exploration within our solar system.

Sir William Herschel Makes a Monumental Discovery

Uranus photo NASA
Photo of Uranus
(Credit: NASA)

Herschel was a German-born astronomer who resided in England.  Born in 1738, Herschel was a polymath with a keen interest in music, mathematics, and of course astronomy.  In 1757 he moved to England where he worked as an organist in Bath. He began his foray into the world of astronomy with a simple, homemade telescope and began observing the stars. He would eventually construct more than 400 telescopes during his lifetime, including a great 40-foot telescope, and most of which were superior to the ones available at the time.

On the night of March 13, 1781, Herschel was conducting a routine survey of the night sky using a 6.2-inch aperture telescope he had constructed himself.  During this survey, he stumbled upon an object that appeared to be a faint, nonstellar object.  Herschel initially reported it as a comet due to its slow movement across the sky and dimness but he continued to observe it over the following nights.  As the months passed by Herschel and the other astronomers he reached out to for input began to suspect differently, as its orbit suggested it was a planet and no tail was visible.  Eventually it became clear the object was a planet as the orbit was calculated by Pierre-Simon Laplace and Alexis Bouvard, two French mathematicians and astronomers.  Their calculations confirmed that Uranus followed a nearly circular orbit around the Sun, consistent with it being a planet.  For the first time in history, our solar system had expanded from the six previously known planets.  

The discovery of Uranus sparked a debate over its name.  Hershel, the discoverer, felt that he had the right to name the planet and he proposed the name “Georgium Sidus” or “George’s Star,” in honor of King George III.  However this name was not well received in the international community, as it broke with the tradition of naming the planets after ancient Roman Gods.  Several alternative names were suggested, but in the end the name Uranus was chosen and eventually accepted as the planet’s name.  

The Planet Uranus and the Impact of its Discovery

Uranus is the seventh planet from the Sun and approximately 2.6 billion kilometers from Earth.   It takes about 84 years for it to complete a full orbit around the Sun.  Its mass is about fifteen times Earths with a diameter of about four times Earth, making it the third largest planet in our solar system.  Accompanying the planet are thirteen rings and 27 named moons.  Composed mostly of rock and ice, it is one of the coldest planets in the solar system with an average temperature of -216 Celsius.  This is due to its low core temperature – it doesn’t generate much heat unlike Jupiter and Saturn.

The discovery of Uranus marked a new era in the exploration of our Solar System.  Most significantly, it showed that our Solar System was much larger than previously thought.  This provided much motivation for further exploration. Perturbations in Uranus’s orbit could not be explained by Newton’s laws of gravity if the only known planets were considered. This led to a hypothesis that there might be other unknown planets leading to the discrepancies. The discovery of Neptune in 1846 and later Pluto in 1930 confirmed this hypothesis, although Pluto was later reclassified as a dwarf planet in 2006. The discovery of Neptune was a triumph for Newton’s laws of gravity because its position was predicted based on the gravitational influence it had on Uranus.

The discovery of a planet more distant than Saturn provided motivation for advances in telescope technology as astronomers recognized the new for more powerful telescopes to study distant objects.  Lastly, the naming controversy that followed highlighted the significance of tradition in the scientific community.  

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Albert Einstein

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Albert Einstein portrait
Albert Einstein

The scientific career of German-born physicist Albert Einstein (1879 – 1955) was one of the most impactful ever in the history of science.  His work led to significant and meaningful changes in our understanding of several fundamental laws of nature including gravity, light, and time.

Albert Einstein was born on March 14, 1879 in Ulm, Germany.  He became interested in nature and mathematics at an early age.  There were two key moments in his childhood that inspired a wonder of curiosity in him.  The first moment was when he was sick at the age of five.  His father brought him a compass and this device stirred his curiosity and sparked his intellect.  At age twelve he stumbled upon a book of geometry that has much the same effect on him as the compass.

In 1896 Einstein enrolled for a science degree at the Swiss Federal Institute of Technology in Zurich.  He graduated in 1900 with his teachers being largely unimpressed with him.  Two years later he would obtain a post of an examiner in the Swiss Federal patent office where he would go on to make some of his greatest discoveries.

The year 1905 was called the year of miracles for Einstein.  He published four scientific papers each with immense importance for science.  His papers topics were on the photoelectric effect, Brownian motion, the Theory of Special Relativity, and the equivalence of mass and energy.  These four papers gained him international respectability and propelled his academic career.  He won the Nobel Prize for Physics in 1921 for his work on the photoelectric effect.

As Einstein began teaching physics at various institutions he began formulating his General Theory of Relativity, which he finally published in 1915. This theory shows how gravity works as a geometric feature of space and time and how its curvature is directly related to the energy and momentum of the present mass and radiation.

As Einstein aged he still worked in science but became more involved in politics.  He emigrated to the United States in 1933 due to the rise of Nazi power in Germany.  During World War 2 he would work on the Manhattan Project which developed the atomic bomb.  This was a difficult moral decision for him since he was a pacifist, but he was uneasy that Germany would develop the bomb first and ultimately decided to help the US develop it before Germany.  Einstein died in 1955 with the legacy of being one of the most impactful scientists in all of history.

3000 BCE: Number Systems

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The invention of number systems designated another high mark for the civilization. Its development led to formal mathematics just as the development of writing systems led to reading and literature. These two tools are responsible for preserving and transmitting the vast, accumulated knowledge humanity has attained in the past 5000 years. These disciplines form the foundation of our modern academic curriculum.

For tens of thousands of years there was not an organized number systems. People counted things using their fingers and toes, if they needed to count anything at all.  Around 25,000 ago there is evidence that people started placing marks on wood and bone, a practice known as tally systems, to keep track of things.  Later on people used markers, counters, or tokens in what is called a token system. These tokens corresponded directly to the goods and things that they represented. Tally systems followed by token systems were proto-numeral systems. This proved was useful for counting and keeping track of smaller amounts but it was not very practical when counting large numbers or attempting to complete more complex mathematical operations.

When humans were hunter-gathers there was not a pressing need to tally items. People lived in smaller groups and generally shared goods in the community. Human emotions such as resentment and distrust were sufficient to regulate fairness in the community. With the advent of agriculture the human condition changed. Soon their were sprawling city-states with tens of thousands of people and a division of labor. Accountants were needed to record debts and taxes owed. This necessity provided the kindling spark to devise a more capable system of keeping track items than a tally system.

Number Systems Take Their Position in Civilization

Babylonian Number System
Babylonian Sexagesimal Number System

Around 3000 BCE the Babylonians developed one of the first known positional number systems.  It was written in cuneiform and was a sexagesimal (base 60) number system.  The major achievement of the Babylonian number system over previous number systems was that it was positional.  This mean that the same symbol could be used to represent different orders of magnitude, depending on where the symbol was located within the number.

The Babylonian system was a significant advancement in the development of mathematics. It provided for the addition and subtraction of numbers and allowed for fractions. It did have some many shortcomings. One such shortcoming is the absence of the number zero. Today we use a base 10 positional number system however there are still some relics of the base 60 number system in our culture.  For example, the circle is 360 degrees and there are 60 seconds in a minute.

Many other civilizations further developed number systems. They Chinese, Egyptians, Aztecs, Mayans, and Inca’s all made use of them. The Greeks in particular showed an intense interest in math. When the conquests of Alexander the Great spread Greek culture throughout the ancient world it marked a turning point in science and math that still lingers, along with so much else from the Greek culture, with us today.

The Story of our Number System

The number system we used today is referred to as Arabic Numerals despite its oldest preserved samples being discovered in India from around 250 BCE. It is uncertain whether this system developed entirely within India or had some later Phoenician and Persian influence. What is certain is that the Arabic’s fully developed and institutionalized this system. A book written around 820 by the mathematician Al-Khwarizmi provides us with the oldest fully developed description of this system. Titled On the Calculation with Hindu Numerals, it is responsible for introducing this Hindu-Arabic numeral system to Europe.

Arabic Numerals
Various Styles of Arabic Numerals
(Credit: Wikimedia Commons)

The Arabic’s designed different sets of symbols which can be divided into two main groups – East Arabic numerals and West Arabic numerals. Although the Arabic language is written from right to left, Arabic numerals are arranged from left to right. The European numeral system was primarily modeled on the now extinct West Arabic numeral system.

The Importance of Number Systems

We would be lost in our world without numbers. They are used to represent goods and things.. They allow the measurement of objects. They are used in the tracking of time. But maybe most importantly, number systems are necessary for mathematics, the bedrock of science.

Pythagorean Theorem
Pythagorean Theorem
(Credit: www.mathworld.wolfram.com)

Much of the world can be expressed in mathematics. It has been echoed by many great scientists that nature speaks to us in the language of mathematics. This is why science depends so much on math. Math has wide-ranging applications ranging from engineering, accounting and finance, navigation, physics and cosmology, computers and coding. Geometry and calculus allows us to construct buildings to live in. Algebra allows us to calculate our loan payments when we purchase that new home. The examples the benefits of using math, just like are numbers, are infinite.

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Dmitri Mendeleev

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Dmitri Mendeleev picture
Dmitri Mendeleev

Dmitri Mendeleev (1834 – 1907) was a Russian chemist who is most famous for publishing his periodic table of elements.  The period table is one of the most recognizable symbols in science.

Mendeleev was born in the Russian province of Siberia in 1834, the youngest of seventeen siblings.  At the age of sixteen Mendeleev moved  with his family to the the Russian capitol of St. Petersburg and enrolled in his father’s old school, St Petersburg’s Main Pedagogical Institute.  At age 21 he got a teaching job in the Crimea but soon returned to St. Petersburg to study for a master’s degree in chemistry from the University of St. Petersburg which he obtained in 1856.

Mendeleev was becoming more passionate about science and more concerned that Russia was falling behind Germany in the field.  He believed improving Russian educational textbooks was one way to close the gap.  In 1861 he published the 500-page textbook Organic Chemistry which won him the Demidov Prize of the Petersburg Academy of Sciences.  He continued to be a a teacher of chemistry and write additional textbooks over the next few decades.

During his time teaching and writing textbooks Mendeleev began to notice patterns and relationships among the known elements.  He found many similar properties in groups of elements such as the halogens and the alkaline earth metals.  He noticed the atomic weight’s of the elements could be used to arrange elements within groups, and also to arrange groups themselves. In 1869 he published publish The Relation between the Properties and Atomic Weights of the Elements, revealing his periodic table to the world.

Mendeleev’s periodic table was impactful for its predictive power.  Due to its ordering, it predicted that some of the atomic weights of known elements may be wrong.  It also predicted the existence of unknown elements and it predicted what properties these elements would possess.  Both of these predictions ended up being true.

Mendeleev received substantial fame and recognition for his periodic table.  In 1905, the British Royal Society awarded him its highest honor, the Copley Medal.  The same year he was elected to the Royal Swedish Academy of Sciences.  He died just after the turn of the century in 1907 from the influenza virus.  Element number 101 is named Mendelevium in his honor.

1869: Mendeleev’s Periodic Table of Elements

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The periodic table of the elements is a cornerstone of modern chemistry and an iconic visual representation almost all school children are familiar with.  It was originally developed in 1869 by the Russian chemist Dmitri Mendeleev, when he published his periodic table of elements.  Aside from a few minor changes we still use Mendeleev’s system of organization for our modern periodic table.

The Development and Organization of Mendeleev’s Period Table of Elements

There had been a total of 63 elements discovered and isolated by 1869.  As the number of known elements was increasing, several scientists began to notice relationships among some of elements and patterns in how they combined with each other.  Scientists had been trying to develop a classification system of elements for decades for the known elements for decades but no agree upon system had been reached.  For one example, an English scientist named John Newlands proposed the Law of Octaves in 1865.  He noticed that every eighth element shared similar characteristics when arranged by atomic weight.  There were however limitations to his law, and it was not accepted by all scientists of the time. 

Mendeleev's Periodic Table of Elements
Mendeleev’s Periodic Table of Elements

Dmitri Mendeleev changed this when he published his periodic table of elements in 1869. He had been working on publishing a chemistry textbook beginning two years back titled Principles of Chemistry and it was his research for his textbook that led him develop this relationship between the chemical properties of the elements to their atomic weights.

He organized his table in order of increasing atomic weight.  He then placed elements with similar properties underneath each other.  In a few instances where it made sense, he swapped some elements out of order of increasing atomic weight to better line up the chemical properties. In doing this he inadvertently set up his table by increasing atomic number rather than atomic weight. By the time his table was finished he had discovered what is now called the Periodic Law, which means that the physical and chemical properties of the elements repeat in a periodic manner.  

Mendeleev’s true genius came in the fact that he left spaces on his table, correctly predicting the existence of elements that had yet been discovered.  He even predicted the properties of these missing elements based on their position in his table.  For instance, he correctly predicted the existence of elements that would later be known as gallium, scandium, and germanium.  Although his table was initially met with skepticism, when these elements were eventually discovered and their properties closely matched Mendeleev’s predictions it provided a strong validation of his table, quickly leading to its acceptance.

The Modern Periodic Table

Mendeleev’s table has continued to evolve as new elements were discovered and the understanding of the atomic structure increased. It is a product of the collaborative efforts of many scientists involving a few important changes and additions.

Most notably, thanks to the work of Henry Moseley, we now organize the table by atomic number, which is the total number of protons in the nucleus.  This change happened as a result of the discovery of isotopes and lead to the realization that atomic number is the fundamental basis for the organization of the elements.  In addition to that change the modern periodic table now includes over 100 elements, up from the 63 known to Mendeleev when his first table was published.

The Modern Periodic Table of Elements
The Modern Periodic Table of Elements: The table today contains 118 known chemical elements
(Credit: American Chemical Society)

The are a few other changes and additions to make note of.

  • Noble gases: the original table did not include the specific group of noble gases as these elements were not yet discovered.
  • Electron structure: the modern periodic table is often presented with electron configurations for each element.
  • Improved measurements: significant advances in technology have allowed for more precise measurements of atomic weight and structure.
  • Filling of d- and f- blocks: these blocks were not fully understood during Mendeleev’s time and have been refined to reflect their electronic structures and chemical properties.
  • More comprehensive periodic trends:  the modern periodic table provides more information about periodic trends such as atomic radius, ionization energy, and electron affinity.

The Importance of the Periodic Table

The periodic table is an indispensable tool for chemists and educators worldwide.  Its organization captures the complexity of the natural world in a simple framework that easily shows the relationships, properties, and reactivity between the chemical elements.

The periodic table also provides a wealth of information about the chemical elements.  It contains information about the atomic structure and weight, electron configuration, valence electrons, and chemical reactivity.  All of this information provides insights into studying the elements and for manipulating matter at the molecular level.  One area of study where this information is particularly useful is in the field of material science.  By understanding the periodic trends, scientists have been able to design and engineer new materials with specific characteristics for a wide range of applications such as in electronics, energy production, agriculture, and medicine.  

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Alessandro Volta

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Alessandro Volta portrait
Alessandro Volta

Alessandro Volta (1745 – 1827) was an Italian physicist and chemist who was a pioneer in electricity.  He is best known for inventing the battery, which was the first artificial source of continuous electrical current.

Volta was born in Como, Italy, was educated first at home and then at a Jesuit school.  He became interested in electricity at an early age and began publishing papers on the topic in his early twenties.  By 1774 he had  became a professor of physics at the Royal School in Como. One of his first major accomplishments was the discovery and isolation of methane gas. 

In 1791 his friend Luigi Galvani published his views of animal electricity, a form of electricity Galvani believed to be generated in the bodies of animals which would flow through the nerves, causing muscles to move.  Galvani experiments showed that when two different metals came into contact with a dead frog, the frogs muscles would twitch.  Volta disagreed with Galvani on his animal electricity hypothesis as he believed that it was the through the contact of two dissimilar metals that caused the electric current to flow.  In an attempt to prove this Volta built what he called the Voltaic pile, the first ever electric battery. 

The Voltaic pile provided a powerful too for other scientists to made additional observations and discoveries in the field of electricity.  It was easy enough for anyone to make.  Within weeks it was used to dissolve water into hydrogen and oxygen and within a few years newer and more powerful batteries were being created and used to isolate new chemical substances.

In 1791 Volta was already deemed an expert in electricity and he was elected to be a Fellow of the Royal Society.  Impressed by his battery and work on electricity, Napoleon Bonaparte made him a Count in 1809.  Volta died in 1827 but his legacy would live on.  In 1881 in recognition of his fantastic contributions to electrical science the term volt would be the official SI unit of electric potential.

Luigi Galvani

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Luigi Galvani portrait
Luigi Galvani

Luigi Galvani (1737 – 1798) was a pioneer of bioelectricity who made important contributions in physics, chemistry, and biology.  He was the first person to demonstrate the electrical basis for nerve impulses when he made a dead frogs muscle move when he jolted it with electricity.

Galvani was born in Bologna, Italy and obtained a degree in medicine at the University of Bologna in 1759.  After graduation he applied for a position at the university and became a lecturer of anatomist of the university.  In the 1770s Galvani had started to become interested in the relationship between electricity and life.

During experiments he conducted, Galvani realized that he could use electricity to make the dissected legs of a frog contract.  For example, when Galvani used a scalpel made of steel to cut through the leg of a frog hanging from a brass hook, the leg visibly twitched.  Based on his observations Galvani concluded that the body contained a type of electrical fluid which he dubbed animal electricity.  He knew that his conclusions would be controversial and he delayed publishing his work until 1791 when he published Commentary on the Effect of Electricity on Muscular Motion.

Some of his scientific colleagues accepted his views, but he received opposition from the Italian physicist Alessandro Volta.  Volta believed that it was through the contact of two dissimilar metals, such as the steel in the scalpel and the brass of the hook, that caused the electric current to flow.  In response to Galvani Volta invented the Voltaic Pile, the first battery.  He realized that the frog’s leg served as a conductor of electricity (electrolyte) and he replaced the leg with brine-soaked paper placed in between to pieces of metal.  Volta’s conclusion ultimately proved correct however Galvani was still correct in attributing the muscular contractions to the electrical stimuli.  Where Galvani was wrong was in the idea of an inherent animal electricity operating within the body.

Galvani continued to investigate animal electricity until the end of his life.  It is his work that inspired Mary Shelly to write her famous work Frankenstein in 1818.  Galvani died at his brothers home in December 1798.

James Lind

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James Lind portrait
James Lind

James Lind (1716 – 1794) was a Scottish doctor who is famous for his medical insight that eating citrus fruits will prevent and cure scurvy.  He instituted other recommendations which stressing the importance of ventilation of ships and hygiene to the Royal Navy.

Lind was born in Edinburgh and began his education as an apprentice at the College of Surgeons at Edinburgh University before entering the Navy as a surgeons mate.   In 1747 he had become surgeon of the 50 gun vessel HMS Salisbury. 

As surgeon he carried out experiments, or clinical trials, to discover the cause of scurvy.  Scurvy was a disease that was extremely deadly to sailors during the Age of Exploration.  It is estimated that over 2 millions sailors were killed by the disease.  Its symptoms weakness and fatigue, muscle soreness, loose teeth, bleeding gums and hemorrhages.  It is caused by a lack of Vitamin C in the diet which is why citrus fruits prevent and cure it.

To carry out his experiment, Lind selected twelve men from his ship who suffered from scurvy and divided them into two groups of six, feeding each group a different diet.  One of the groups contained citrus fruit, such as lemons and oranges, and this was the group that showed remarkable recovery results.  This can be considered one of the first reported, controlled clinical trials in history.  In 1753 he published A Treatise of the Scurvy to little fanfare.  Lind recognized the importance of citrus friends in his treatise but he believed that there were multiple causes and hence many cures for the disease.   He notes that the group that had the oranges and lemons recovered yet he is unclear on his recommendations to scurvy’s cure.

Lind continued to work on improving the diet and hygene of sailors.  In 1762 Lind published an Essay on the Most Effectual Means of preserving the Health of Seamen rehashing some of his recommendations of his first book while adding further remarks on improving sailors heath.  It wasn’t until 40 years later that the Navy mandated a supply of lemon juice on all ships, and with the scurvy virtually disappeared. Lind continued to do work on typhus, tropical disease, and other areas until he died in 1794.

1824: Carnot Cycle

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The Carnot cycle is the ideal cycle of operations that provides the maximum theoretical efficiency for any engine that utilizes heat.  It was first proposed by the French physicist Sadi Carnot in 1824 prior to its expansion by other scientists.

Reflections on the Motive Power of Fire

Nicolas Leonard Sadi Carnot is often referred to as the “father of thermodynamics”.  In 1824, at the age of 27, he published his only book titled Reflections on the Motive Power of Fire in which he introduced the concept of the Carnot cycle while laying down the foundations for a new discipline – thermodynamics.  By this time the steam engines industrial and economic importance had been established, although little scientific work had been done by studying them. Carnot’s book was one of the first scientific studies of steam engines.

Carnot explore some key ideas in his book. He recognized heat as a form of energy that can be converted from one state to another. This concept eventually led to the first law of thermodynamics, which states that energy is conserved in any thermodynamic process. He introduced the notion of an idealized heat engine, known as the Carnot engine, as a theoretical construct to study the maximum efficiency of heat conversion. This engine could be served as a reference point against which all other real work engines could be compared. Additionally, he noted that the efficiency of a heat engine depends of the temperature difference between the source and the sink, and that maximum efficiency is only achieved when it operates in a recoverable manner.

To understand the principles of an ideal heat engine, Carnot devised the Carnot cycle which is a series of four reversible processes. It is a theoretical cycle that provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work.  The Carnot cycle heat engine is not a practical engine that can be made because its processes are reversible, which violate the second law of thermodynamics.

The Four Stages of the Carnot Cycle

The Carnot cycle consists of several operations.  First, the engine absorbs energy in the form of heat from a reservoir.  Work is done by the heat which causes an expansion.  Next is compression where the heat is given out.  The net result is that heat is taken from a hot source to a cooler one, while some of the heat does work.  The Carnot cycle establishes the highest possible efficiency – measured by dividing the work that is done by the energy absorbed from the reservoir – for any heat engine.  In the real work efficiency of an engine is never as high as that predicted by the Carnot cycle due to factors such as friction.

Here are the four stages of the Carnot cycle.

The Four Stages of the Carnot Cycle
The Four Stages of the Carnot Cycle
  1. Isothermal expansion – gas expands on its surroundings as heat is transferred from hot reservoir at a constant temperature; work being done = heat supplied.
  2. Adiabatic expansion – gas continues to expand on its surroundings with no heat exchange, causing temperature to cool; work being done > heat supplied.
  3. Isothermal compression – surroundings compress gas as heat is transferred to a low temperature reservoir at a constant temperature.
  4. Adiabatic compression – surroundings compress gas with no heat exchange, resetting the system to its original state (before stage 1).

The Importance of the Carnot Cycle

Sadi Carnot’s work brought about a revolution in our understanding of the underlying principles of heat transfer and energy conversion. Most directly, it set the stage for subsequent advancements in the field of thermodynamics. Work on the Carnot cycle heat engine led to the fundamental thermodynamic principle of entropy, a fundamental concept related the the second law of thermodynamics.  Rudolf Clausius introduced the term entropy in the mid-19th century to explain the irreversible nature of energy transfer and transformations in physical systems. Clausius observed that heat naturally flows from a higher temperature region to a lower temperature region and that the process is irreversible. He recognized the importance of Carnot’s work with heat and sought to put in on a more rigorous, mathematical footing.
Another Rudolf was to later be inspired by the Carnot cycle when he was developing a more efficient engine. In 1982 Rudolf Diesel submitted design patents for an internal combustion engine.  He knew gasoline engines were very inefficient, wasting around 90% of its heat in the process. He sought to make an engine closer to the theoretical framework of the Carnot cycle engine.

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