Why do some chemical reactions give off heat and others don’t? The answer depends on the amount of energy absorbed when chemical bonds are broken compared with the amount released when new bonds are formed.

There are two types of chemical reactions: endothermic and exothermic. Endothermic reactions are those that require energy, and exothermic reactions are those that release energy. Both words are made by combining the Greek word “therme”, which means “hot,” with “endon”, meaning “within”, or “exo” meaning “outwards”.

An endothermic reaction is a chemical process that needs energy, so it absorbs heat from the surroundings. This is because a chemical bond is being broken. When two atoms are bonded, they are in a low-energy, stable state. To pull them apart, you have to overcome the electrical attraction that is holding them together. That requires adding energy, and depending on the element, it can take a lot of energy. Endothermic reactions reduce the thermal energy in the surrounding environment because it is being absorbed by the reaction.

An exothermic reaction is the opposite. It is a chemical process that releases energy, so it expels heat into its surroundings. This happens because chemical bonds are forming. When two atoms come together, two hydrogen atoms, for example, they each have one positively charged proton and one negatively charged electron. As the atoms get closer, each electron is attracted to the other atom’s proton, and once they get close enough, the atoms settle into a stable arrangement, sharing electrons. The new bonded atoms have less energy than the two atoms had on their own, and this excess energy is released as heat.

Endothermic and exothermic reactions can be seen in the cold packs and hot packs you can buy in shops. A cold pack is one where you press an activator on it, and it very rapidly becomes cold. You can use it if you are too hot, or if you need to ice a muscle injury. It works because of an endothermic process. Inside the cold pack is usually water and a plastic bag or tube containing a chemical reactant. Usually something like ammonium nitrate, urea, or calcium ammonium nitrate. When you activate the pack, the chemical reactant comes into contact with the water, and the water starts to dissolve it. Ammonium nitrate consists of positively charged ammonium ions and negatively charged nitrate ions held together in a crystal structure. The water molecules surround the ammonium nitrate molecules, and the positive end of the water molecule (the hydrogen) is attracted to the negatively charged nitrate ions, and the negative end of the water molecule (the oxygen) is attracted to the positively charged ammonium ions. These attractions pull the ions away from the crystal structure and keep them separated in the water. Separating the ions requires more energy than is released when the water molecules surround them. This difference in energy is absorbed as heat from the water and its surroundings, and the pack very quickly gets cold. It stays cold until the reaction finishes.   

A heat pack works in the opposite way, and it uses an exothermic reaction. There are different kinds of hand warmers, but the cheapest ones are the disposable pouch type. They come in a sealed plastic bag, and they start to get warm as soon as you open the bag. They can stay warm for several hours. They work because they contain iron powder, which reacts with the oxygen in the air. These pouches contain cellulose, iron powder, activated carbon, vermiculite, and salt. When you open them, air enters the pouch through holes in its covering. The oxygen and water in the air react with the iron to produce rust. When you open the packet, oxygen from the air enters through tiny holes in its covering and begins reacting with the iron powder. The salt and water help speed up the process. The iron reacts with oxygen to produce iron oxides, commonly called rust. The products have less chemical energy than the original iron and oxygen, and the difference is released as heat. You might wonder why oxygen reacting with iron to produce rust becomes so hot when your bicycle left in the front garden for six months never warms up. The reason is down to the speed of the reaction. The rust appearing on your bicycle is producing heat, but it is doing it in such small amounts and so slowly that you never notice. A hand warmer reaction is much faster. The iron in a hand warmer is a fine powder, giving it an enormous surface area exposed to oxygen. This allows much more iron to react at once, releasing enough heat for us to feel. And this is what I learned today.

Sources

https://www.bbc.co.uk/bitesize/articles/zb7wwnb#zjynn9q

https://en.wikipedia.org/wiki/Exothermic_process

https://en.wikipedia.org/wiki/Endothermic_process

Image Public Domain, https://commons.wikimedia.org/w/index.php?curid=642519

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What is quantum tunneling? Quantum tunneling is a phenomenon where an electron or an atom can pass through an energy barrier without having enough energy to get through it.

To get through barriers, energy is usually required. A brick wall is a barrier. If I want to get through that wall I need to have enough kinetic energy. Kinetic energy is calculated as half mass multiplied by velocity squared. However, the kinetic energy required to get through that brick wall is higher than the kinetic energy any human could muster. Usain Bolt, for example, weighs 94 kg and his top speed in his world record 100 m run was 44.72 km/h, which is 12.422 meters per second. That gives him a kinetic energy of 7252.38 joules. To break through a brick wall, you would need far more kinetic energy than a person could ever produce. A car could get through that wall because it has a much higher mass and a much higher velocity. However, when it does break through that wall, its energy goes into breaking and moving bricks, deforming the car, producing sound and generating heat. This is conventional physics. A particle can sometimes pass through a potential barrier without losing any of its kinetic energy. This is quantum tunneling.  

My brick wall was just an easy visual example. In theory, a particle could tunnel through a brick wall, but the probability would be so incredibly small that it would effectively never happen. So, what is quantum tunneling?

We need to remember that a particle, such as an electron, is both a particle and a wave. However, it is not the same as a wave in the water or a sound wave in the air. These types of waves are disturbances in a medium and they have a frequency and a wavelength. An electron’s wave is a probability wave. When scientists measure electrons, they don’t know where it will be until they measure it, but they do know that it must be somewhere and they can give a probability to the likelihood of where it will be when they do measure it. All of these probabilities will add up to 100% because it must be somewhere, but there could be a 72% chance it will be here and a 0.0003% chance it will be somewhere else. Some locations may have a high probability, others an incredibly small probability, and some may have no probability at all.

This is demonstrated with the famous double split experiment. Electrons are fired at a board with two slits cut in it. If the electrons were only particles, you would expect to detect them in two lines on the other side of each slit. However, because they have this probability wave function, they produce several lines, which is what a wave would do. Waves can interfere with each other. Sometimes they reinforce each other, and sometimes they cancel each other out. This produces a pattern of alternating bands, and electrons produce the same kind of pattern.

Because electrons have this probability wave, they can tunnel through solid barriers. When scientists look at the path an electron might take, the wavefunction can extend into the barrier they are firing the electrons at. The probability of finding the electron along this path will decrease with the thickness of the barrier, but if the barrier is not too thick, there will be a non-zero chance of finding an electron on the other side of the barrier. And, when the electron is on the other side of the barrier, it will not have lost any of its kinetic energy. It will have effectively teleported through a solid object.

Quantum tunneling is important in modern electronics. As transistors have become smaller, their insulating barriers have become thinner, allowing electrons to tunnel through them and create unwanted electrical leakage. This can waste power and generate heat. However, quantum tunneling can also be useful. Flash memory uses tunneling to move electrons through thin insulating barriers, allowing information to be stored or erased.

However, quantum tunneling is vital for something like the sun. The core of the sun is incredibly hot, but that is still not enough kinetic energy to force two positively charged protons together because of the electrical repulsion they have. If they can get close enough, the strong nuclear force can bind them, but getting them close enough for the strong force to take over is impossible because the amount of energy required increases as the particles get closer. At least, it would be impossible if it weren’t for quantum tunneling. Very rarely, quantum tunneling allows two protons to get close enough for a nuclear reaction to occur. In the first step of the Sun’s fusion process, one proton changes into a neutron, forming a heavier type of hydrogen called deuterium. Further reactions eventually produce helium and release energy. Each individual reaction is extremely unlikely, but there are so many protons in the Sun that fusion happens continuously. Without quantum tunneling, the Sun could not produce energy in the way it does, and we would not be here. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/Quantum_tunnelling

https://www.quera.com/glossary/quantum-tunneling

https://www.calculatorsoup.com/calculators/physics/kinetic.php

Photo by Nicola Narracci: https://www.pexels.com/photo/visual-representation-of-magnetic-field-lines-38032287/

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Why do physicists think dark energy exists? Physicists think dark energy exists because something must be overcoming the pull of gravity to be speeding up the expansion of the universe.

13.8 billion years ago, the Big Bang happened and the universe began to expand. This is often described as an explosion into space, but it wasn’t an explosion. It was space itself expanding. In roughly the first 10 to the power of -36 seconds, the universe underwent incredibly rapid expansion. This is called Cosmic inflation. Distances increased exponentially. Then, in the first few minutes, the universe cooled enough for protons and neutrons to combine, producing mostly hydrogen and helium nuclei. About 380,000 years later, it had cooled enough for electrons to join those nuclei and form neutral atoms. At this point, the universe became transparent. After hundreds of millions of years, the first stars and galaxies start to form. Matter has gradually been gathering under gravity and it starts to concentrate in clumps. The universe is still expanding, but the rate of expansion is decelerating. Just like if you throw a ball straight up in the sky, it starts to slow down as gravity pulls on it. At some point, it stops rising, and starts to fall back. Oddly, and this is where dark energy comes in, that doesn’t happen to the universe.

After several billion years in the life of the universe, galaxies and galaxy clusters have grown. Gravity is still slowing down the rate at which the universe expands. And this is where things start to get interesting. Up to now, the universe has been matter dominated, but from this point on that changes. The entire amount of the matter in the universe hasn’t changed since the big bang, but, as the universe expands, it spreads out and the density of the universe drops. About 5 to 6 billion years ago, when the universe is roughly 8 billion years old, the expansion stops decelerating and starts accelerating again. And this is where we find ourselves today. The universe is still accelerating, which means the rate of expansion is increasing. Something therefore has to account for this acceleration. Physicists call the unknown cause dark energy.

So, what is dark energy? Nobody knows. However, whatever dark energy is, its effect appears to be causing the expansion of the universe to accelerate. Whatever it is, it was most likely there from the very start of the universe, but it is an extremely low source of energy. It doesn’t have enough power to counteract gravity. But that changes as the density of the universe falls and when the universe reaches a certain level of density, which it hit about 5 billion years ago, the dark energy does have enough power to overcome gravity. And, if that is true, as matter becomes increasingly diluted, dark energy will dominate the evolution of the universe more and more.

What is dark energy Nobody has directly identified what dark energy is, but its apparent effects on the expansion of the universe can be measured, just like dark matter. Physicists know that dark matter is there because they can see far more gravity than the visible matter can possibly produce. There must be five times more dark matter than visible matter for gravity to make sense. The same is true for dark energy. The universe is expanding in a way that is different to the way matter and gravity alone could predict. Something must be pushing it.

How do we know that the expansion of the universe is accelerating? In 1998, two independent teams of astronomers were studying distant Type Ia supernovae. These exploding stars are extremely bright and can be used to estimate enormous cosmic distances. The astronomers expected their observations to show that the expansion of the universe had been gradually slowing because of gravity. Instead, the distant supernovae appeared dimmer than expected, meaning they were farther away than the astronomers’ models predicted. The best explanation was astonishing: the expansion of the universe had actually been accelerating. The two teams announced their results in 1998, providing the first strong evidence for what we now call dark energy. The discovery was so important that Saul Perlmutter, Brian Schmidt, and Adam Riess were awarded the 2011 Nobel Prize in Physics for it.

There are three possible theories for what dark energy might be. The first, and simplest, is that it is a cosmological constant, sometimes associated with the energy of empty space. The strange thing about this energy is that its density would remain constant as the universe expands. In the early universe, matter and radiation were incredibly dense, so the tiny amount of dark energy would have been insignificant. As the universe expanded, matter became more spread out, while the density of dark energy stayed the same. Eventually, dark energy became dominant and the expansion of the universe began to accelerate. The problem is that physicists have no idea why the energy of empty space should have the tiny value that observations suggest. Quantum physics actually predicts that empty space should have energy, but attempts to calculate how much can give values enormously greater than the amount needed to explain the expansion of the universe. This enormous disagreement between theory and observation is known as the cosmological constant problem.

Another theory is something called quintessence, which is a fluid or field that is slowly filling space. If that is true, then nobody knows where it could be coming from or why it could be forming.

And the third theory is that there is no dark energy and our theory of gravity is incomplete. Out of all of these theories, the cosmological constant seems the most likely, but it probably won’t be solved in my lifetime, And this is what I learned today.

Sources

https://science.nasa.gov/dark-energy

https://news.uchicago.edu/explainer/dark-energy-explained

https://en.wikipedia.org/wiki/Dark_energy

https://en.wikipedia.org/wiki/Expansion_of_the_universe

Photo by Francis Glenn Marciano: https://www.pexels.com/photo/the-milky-way-galaxy-in-the-sky-12940327/

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How did we discover DNA? DNA was first discovered in 1869, but it wasn’t identified as the substance carrying hereditary information until 1944, and its structure wasn’t discovered until 1953.

DNA stands for deoxyribonucleic acid. Broken down, “deoxyribo” refers to the sugar part of the DNA, “nucleic” means it is found within the nucleus of the cells, and “acid” means it is a weak acid. DNA is often called the instruction manual for life because it contains the genetic code that is necessary for everything that we do. DNA is hereditary, which means it is passed down from the parents, and we all carry part of our mother’s DNA and part of our father’s DNA, which makes our own unique DNA. If we have children, they will carry half of our DNA.

DNA is found in the nucleus of almost every cell in our body, and when the cells replicate, they read the information in the DNA. It is made up of two intertwined strands that are linked together by pairs of compounds made from nitrogen. These are called bases, and they are divided into adenine (A), cytosine (C), guanine (G), and thymine (T). Each strand has bases attached to it that point inwards towards the other strand. The bases on opposite strands pair together, but they can only make certain pairs. As always pair with Ts, and Cs always pair with Gs. The order of the bases contains genetic information, including instructions for making proteins.

The first person to discover DNA was a Swiss physician and biologist called Johannes Friedrich Miescher, but he didn’t know what he had found. Miescher was interested in white blood cells, and he wanted to isolate and describe the protein components of the white blood cells. To do this, he needed a supply of white blood cells, so he approached his local hospital. They supplied him with bandages that had pus on them. Pus contains enormous numbers of white blood cells, so it was perfect for Miescher. He planned to wash the bandages to get out the white blood cells, and then he could extract the proteins. What he ended up extracting completely surprised him. He found a substance that had a much higher phosphorus content than regular proteins, and it was very resistant to being digested. He didn’t know what it was, but he knew that it was a new substance. He called it nuclein because it came from the nucleus. This was in 1869.

Over the next fifty or so years, more work was done on the nuclein. Scientists established that nuclein was an acidic substance, leading eventually to the name nucleic acid. Phoebus Levene worked out that it was made of phosphate, sugar, and a nitrogenous base. But still nobody knew what it was for. Then a scientist called Frederick Griffith made a great discovery. He had two types of bacteria, one lethal to the mice and the other harmless. When he killed the dangerous bacteria and injected them into mice, nothing happened. However, when he injected the dead dangerous bacteria with live harmless bacteria into the mice, the mice died, and he was left with live dangerous bacteria. Something from the dead bacteria had entered and transformed the living bacteria.

A group of scientists called Avery, MacLeod, and McCarty worked out what was doing the transforming in 1944. They repeated Griffith’s experiment, but they kept removing different bits of the cells until the dangerous bacteria stopped transforming the harmless bacteria. They removed everything, and it still worked, until they removed the DNA (nuclein), and then it stopped working. They had discovered that DNA carries hereditary information.

In 1950, Erwin Chargaff made another important discovery. He measured the amounts of the four bases in DNA and found that the amount of adenine was always roughly equal to the amount of thymine, while the amount of cytosine was roughly equal to the amount of guanine. He didn’t know why, but this would become an important clue to the structure of DNA. Other scientists worked out that the DNA carried genetic instructions to create new cells.

At King’s College London, Rosalind Franklin and Raymond Gosling used X-ray diffraction to study DNA. In 1952, they produced the famous Photo 51. It wasn’t a photograph of the DNA molecule itself, but the pattern produced by X-rays passing through DNA fibres provided crucial information about its shape and dimensions and strongly indicated that it had a helical structure.

Finally, in 1953, James Watson and Francis Crick put together the information that had been discovered by all of these scientists and built a model of DNA. They realized that it consisted of two strands twisted into a double helix, with the bases pointing inwards. Adenine paired with thymine and cytosine paired with guanine, explaining Chargaff’s mysterious ratios. Even more importantly, the structure suggested how DNA could copy itself. If the two strands separated, each one could act as a template for building its partner. They had finally discovered the structure of DNA. And this is what I learned today.

Sources

https://www.jax.org/news-and-insights/minute-to-understanding/what-is-dna

https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397

https://www.yourgenome.org/theme/the-discovery-of-dna-the-first-building-blocks

https://www.bbc.co.uk/bitesize/articles/z4pd382

https://en.wikipedia.org/wiki/DNA

https://en.wikipedia.org/wiki/Friedrich_Miescher

Image By Raymond Gosling – King’s College London Archives: KDBP1/1/867. Taken from “The double helix: “Photo 51” revisited” by Thoru Pederson (https://doi.org/10.1096/fj.202000119), Public Domain, https://en.wikipedia.org/w/index.php?curid=38068629

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Who came up with the idea for the periodic table? The periodic table was first made by a Russian chemist called Dmitri Mendeleev in 1869.

You have definitely seen the periodic table of elements. Currently, it has 118 elements on it. The first 94 of those are naturally occurring elements and the other 24 have to be synthesized in a lab. Theoretically, there are more elements that could be synthesized, but as elements become heavier, they become much more unstable. This is because as nuclei get larger, proton–proton repulsion increases, and it becomes harder for the strong nuclear force to keep the nucleus stable, so very heavy elements tend to be radioactive and decay.

The periodic table organizes elements by their atomic numbers. Atomic numbers are the number of protons that are in an element’s nucleus. An atom of any element has a nucleus that is surrounded by orbiting electrons. The nucleus is made up of protons and neutrons. The number of protons is the atomic number of an element and an atom always has the same number of electrons as protons. (Unless they have been knocked off or added to.) Protons have a positive charge, and electrons have a negative charge. Neutrons are neutral. Several different forces hold an atom together. The negative electrons are attracted to the positive protons through the electromagnetic force. The protons and the neutrons are attracted to each other by the strong nuclear force.

Dmitri Mendeleev came up with the periodic table idea, but he organized it by atomic mass and not atomic number, because atomic numbers would not be discovered until the early 20^th century. The atomic number is the number of protons in the nucleus. The mass number is the number of protons plus neutrons in a particular atom. Atomic mass had been proposed by an English chemist called John Dalton in about 1800.

Mendeleev wasn’t the first person to attempt to organize the elements. Chemists had been trying to do it for about 50 years with no real success. They tried to group them together in groups, such as metals, and non metals. That is tricky because that doesn’t give a very useful table. A German chemist called Johann Wolfgang Dobereiner tried to order them in groups of three. He noticed that the middle element in each group of three had an atomic weight that was the average of the other two. This worked for a lot of groups, but it didn’t work for all of his groups. Meanwhile, people had been working out the atomic weights of as many elements as they could and in 1860 a fairly comprehensive list of elements and their atomic weights was published. Hydrogen was taken as 1 because it had the lowest atomic weight and all the other elements were ordered by their comparison with hydrogen.

A British chemist called John Newlands tried to make a table with the elements organized by atomic mass. It worked for the main elements, but didn’t work for all of the elements. The elements seemed to follow a pattern and then suddenly they didn’t. He couldn’t solve the problem.

Dmitri Mendeleev decided he was going to work it out and he realized what the problem was. It took him about a day. The 17^th of February 1869. He was looking at all of the elements and he suddenly realized that the reason they didn’t seem to fit in the system was because the atomic weights had either been calculated incorrectly or there were still elements that hadn’t been discovered. He didn’t know what these elements were, but he could clearly see how they would go in the spaces. He wrote up his periodic table and published it in the journal of the Russian Chemical Society. He was proved correct a few years later when a French chemist discovered gallium in 1875, an element that Mendeleev had predicted existed. Then, again, in 1879 when a Swedish chemist found scandium, which was another element Mendeleev had predicted. The last one he was missing, germanium, was discovered shortly after that.

Today, the periodic table is ordered by atomic number rather than atomic weight. Despite all of Mendeleev’s work, there were still a few elements that didn’t fit into the table, and it was only after the discovery of atomic numbers in the early 20^th century that the reason was known. Some elements, such as iodine, have a higher atomic number but a lower atomic mass. These discoveries fixed that and gave us the periodic table that we have today. And this is what I learned today. 

Sources

https://www.asbmb.org/asbmb-today/science/020721/a-brief-history-of-the-periodic-table

https://en.wikipedia.org/wiki/Periodic_table

https://ehss.energy.gov/ohre/roadmap/achre/intro_9_3.html

https://en.wikipedia.org/wiki/Atom

https://en.wikipedia.org/wiki/Henry_Moseley

https://en.wikipedia.org/wiki/Atomic_number

https://en.wikipedia.org/wiki/John_Dalton

Image By Lekritz – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=185001042

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Why is fire hot? Fire is hot because thermal energy is released from the materials when chemical bonds are rearranged during a combustion reaction. New, stronger bonds form and the energy difference comes out as heat and light

When you burn most fuels, you are releasing the chemical potential energy that they have stored inside them. Almost all fuels that we burn today were alive at one point. Wood was part of a tree. Charcoal was wood, which was part of a tree. Coal, oil, and natural gas were formed when algae, plankton, bacteria, and other organisms were crushed through millions of years of intense pressure. As autotrophs, when they were alive, they photosynthesized and turned energy from the sun into energy that they could use. The energy that they didn’t use, they stored inside them. When we burn them, we are releasing that stored energy and basically running photosynthesis backwards. To make their energy, plants need sunlight (energy), carbon dioxide, and water. In the same process, they store energy and produce oxygen. When we burn them, we need energy and oxygen, and we produce more energy, carbon dioxide, and water. It is reverse photosynthesis.

The process of turning oxygen and fuel into carbon dioxide, water, and released energy is called combustion. For combustion to start, the fuel needs an initial input of energy, and the combustion process needs to release enough energy to start a chain reaction and keep the process going. If we use wood as an example, it obviously doesn’t burst into flame when it is exposed to oxygen. The wood needs enough energy to start the chemical reaction and that comes from heating. If you heat the wood to a few hundred degrees Celsius, it reaches its ignition point. At that temperature, the wood starts to break down in a process called pyrolysis. Wood is made of several substances, especially cellulose and lignin, which contain carbon, hydrogen, and oxygen. Pyrolysis breaks these large molecules into smaller, flammable gases and vapors, and it leaves behind a carbon-rich solid called char. When the gas comes into contact with oxygen, it can ignite. In the flame, old chemical bonds (in the fuel and in O₂) are broken and new bonds form, mainly the strong C=O bonds in carbon dioxide and the O–H bonds in water. Breaking bonds costs energy, but forming these new bonds releases even more energy than was needed to break the old ones. The “extra” energy is given off as heat (fast-moving molecules) and as light, which is why the fire is hot and bright. A fire needs fuel, oxygen, and energy to burn. If you take away any one of these three things, the fire will go out, which is what fire extinguishers try to do. The fire will also extinguish itself when it runs out of fuel.

The thermal energy from the fire can travel through radiation, convection, and conduction. The chemical reaction in the fire produces thermal energy. Some of that energy is released as infrared radiation. This radiation travels in a straight line and imparts its energy into whatever it comes into contact with. That is why, if you are standing side on to a fire, one side of your body will be hot and the other cold. The heat also moves by convection, but this is only above the fire. The hot gases heat the air molecules above the fire. Those molecules start to move more as they are heated, become less dense, and rise. This carries the heat upwards from the fire and is why trees over a fire can start to burn as well. And the heat also travels through conduction. If anything is touching the fire, the heat will transfer through into that thing as well, unless it is a bad conductor.

Before the chemical reaction in combustion was understood, scientists thought materials that burned contained a substance called phlogiston. When these substances burn, they dephlogisticate and release their phlogiston into the air. They also reasoned that the reason a fire extinguished in a closed space was not because it ran out of oxygen but because the air became completely saturated with phlogiston. They also thought that when matter was burned, it lost mass because it was losing phlogiston. This theory was finally disproved by Antoine-Laurent Lavoisier in the 1770s.

The sun produces heat in a different way, but the thermal energy from the sun travels through space in the same way.  Anything that gets in the way of that infrared radiation will absorb its energy and start to heat up. The difference between the sun and a fire on Earth is that there are no molecules in space to absorb the thermal energy until it hits Earth. The space between stars and planets is almost at absolute zero because there is nothing to heat there. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/Fire

https://www.thoughtco.com/why-is-fire-hot-607320

https://www.plasticstoday.com/materials/sorry-folks-oil-does-not-come-from-dinosaurs

https://saunacloud.com/when-sitting-next-to-a-fire-are-you-feeling-infrared-heat-or-heated-air

https://en.wikipedia.org/wiki/Phlogiston_theory

Photo by Zion Smith: https://www.pexels.com/photo/vibrant-campfire-flames-in-north-carolina-wilderness-38741836/

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What is highly enriched uranium? It is uranium that contains 20% or more uranium-235, which is the type of uranium most useful for a sustained nuclear chain reaction.

Uranium is a fairly common element that can be found in rocks, soil, and water. It is not as rare as many people probably imagine. There is about as much uranium in the Earth’s crust as there is tin, and it is far more common than gold. Traces of uranium occur almost everywhere, and vast amounts of it are dissolved in the oceans, although in very low concentrations. It is possible to extract uranium from seawater, but it is difficult and expensive because so much seawater would need to be processed. Mining uranium from rock is much more practical.

Natural uranium on Earth is mostly uranium-238, with only a small amount of uranium-235. About 99.3% of natural uranium is uranium-238, and about 0.7% is uranium-235. The difference between the two is the number of neutrons they have. All uranium atoms have 92 protons. That is what makes them uranium. Uranium-238 has 146 neutrons, while uranium-235 has 143 neutrons. They are the same element, but they are different isotopes.

Uranium-235 is important because it can split more easily than uranium-238. This is partly because uranium-235 has an odd number of neutrons. Neutrons tend to be more stable when they are paired. When a uranium-235 nucleus is struck by a slow neutron, it can absorb that neutron. The new neutron pairs with the unpaired neutron, and this releases extra binding energy inside the nucleus. For a tiny fraction of a second, the uranium-235 has become excited uranium-236. That excited nucleus is unstable, so it can split apart.

When it splits, it splits into two smaller atoms, called fission products, and it also releases two or three neutrons and a large amount of energy. The two new atoms are usually radioactive and are much smaller than the original uranium atom. The total mass after the split is slightly less than the mass before the split, and that missing mass has been converted into energy. E=mc2. This is the basic idea behind nuclear power and nuclear weapons.

The released neutrons are the key. If those neutrons hit other uranium-235 atoms, they can make those atoms split as well. Those atoms then release more neutrons, which can split more uranium-235 atoms. This is called a chain reaction. In a nuclear reactor, the chain reaction is carefully controlled so that it releases heat steadily. In a nuclear weapon, the chain reaction is designed to happen extremely quickly.

Natural uranium dug out of the ground does not usually sustain a chain reaction by itself because there is not enough uranium-235 in it. A uranium-235 atom might absorb a neutron and split, but the neutrons it releases are unlikely to hit another uranium-235 atom before they escape or are absorbed by uranium-238. There needs to be enough uranium-235 close together for the reaction to continue. This is where enrichment comes in.

Enrichment does not change uranium into a different element. It changes the mixture of isotopes. Natural uranium contains about 0.7% uranium-235. Enriched uranium contains a higher percentage of uranium-235. The more uranium-235 there is, the easier it is for neutrons to find another uranium-235 atom and continue the chain reaction.

There are several ways to enrich uranium, but the most common method today uses gas centrifuges. Uranium itself is a heavy metal, so it is not easy to separate the isotopes as a solid. First, the uranium is chemically changed into uranium hexafluoride. This is a compound made of one uranium atom and six fluorine atoms. When uranium hexafluoride is heated, it becomes a gas. That gas can then be fed into centrifuges.

A centrifuge spins extremely quickly. The uranium hexafluoride molecules that contain uranium-238 are slightly heavier than the ones that contain uranium-235. Because of that tiny difference, the heavier molecules move a little more toward the outside, while the lighter ones stay a little closer to the center. The separation in one centrifuge is very small, so many centrifuges have to be connected together. Each stage makes the uranium slightly richer in uranium-235. Over many stages, the percentage can be raised.

There are different names for different levels of enrichment. Ordinary low-enriched uranium is below 20% uranium-235, but most of the fuel used in today’s commercial nuclear power reactors is only enriched to about 3% to 5%. High-assay low-enriched uranium, or HALEU, is usually between 5% and 20% uranium-235. It may be used in some advanced reactors because it can make fuel last longer and produce more energy from a smaller amount of material. Uranium enriched to 20% or more is called highly enriched uranium.

Highly enriched uranium is sensitive because it is much closer to material that could be used in nuclear weapons. That does not mean all highly enriched uranium is used for weapons. Some research reactors, naval reactors, and special uses have used highly enriched uranium. However, once uranium is enriched beyond 20%, it becomes much more worrying from a security point of view. Ordinary reactor fuel is usually far below that level. Weapons material is generally enriched much more highly.

That is why the phrase “highly enriched uranium” matters so much. It does not mean the uranium is more radioactive in a simple way, or that it has become a new element. It means the mixture contains a much higher percentage of uranium-235. Uranium-235 is the isotope that can most easily keep a chain reaction going and with enough of it in the right conditions, the reaction can be sustained. With much higher concentrations, the material becomes dangerous for a completely different reason. Countries with highly enriched uranium might not be making weapons, but they could. And this is what I learned today.

Sources

https://www.energy.gov/ne/articles/uranium-enrichment-explained

https://en.wikipedia.org/wiki/Enriched_uranium

https://www.nrc.gov/reading-rm/basic-ref/glossary/high-enriched-uranium

https://science.anu.edu.au/news-events/news/what-uranium-enrichment-and-how-it-used-nuclear-bombs-scientist-explains

https://www.iaea.org/newscenter/news/what-is-uranium

https://www.smithsonianmag.com/smart-news/this-weird-new-chemical-can-pull-uranium-out-of-the-ocean-37326105

https://en.wikipedia.org/wiki/Uranium_hexafluoride

https://ukinventory.nda.gov.uk/wp-content/uploads/2021/03/20201222-Official-Rep-PO023346_fact-sheet-05.pdf

Image By Unspecified – https://web.archive.org/web/20050829231403/http://web.em.doe.gov/takstock/phochp3a.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2287275

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Why does friction produce heat? Friction produces heat because mechanical energy has to go somewhere, and some of it is converted into the random movement of atoms and molecules, which is heat.

Sometimes we need friction to produce heat and sometimes we don’t want it to. If you have ever been out in the cold and rubbed your hands together to warm them up, that is an example of friction producing heat. If you have ever tried to start a fire by rubbing two sticks together, that is another example of wanting friction to produce heat. On the other hand, if you have tripped over and slid along a carpet, the resulting carpet burn is an example of when we don’t want heat from friction. Or, if you ride a bicycle and touch the gears after cycling, they may feel hot. That is another example, because some of the energy put into the bicycle has been wasted as heat.

Why does friction produce heat? It comes down to the First Law of Thermodynamics, which is also called the conservation of energy. In our universe, we cannot make energy from nothing and we cannot destroy energy. All we can do is convert it from one form into another. When dinner is cooked over a gas burner, the chemical energy stored in the gas is converted into thermal energy and light.

There are two basic kinds of energy: kinetic energy and potential energy. Kinetic energy is the energy something has because it is moving. Potential energy is stored energy. Gas has chemical potential energy because of the way its atoms are bonded together. When the gas burns, that stored energy is released and converted into heat, light, and the movement of hot gases. Heat is basically the random kinetic energy of atoms and molecules. Friction takes organized kinetic energy, such as a moving hand, wheel, or car, and turns some of it into this disorganized thermal energy.

Friction happens on a microscopic level. The surface of anything may look smooth to our eyes, but if it is magnified enough, nothing is perfectly smooth. Every surface has tiny bumps, pits, and rough areas. These are sometimes called asperities. When two surfaces rub together, these tiny high points press into each other, catch, bend, break, and slide. The atoms in the two surfaces are pulled and pushed around. They vibrate more strongly, and that extra vibration is heat.

This is the important point. When a hand moves across another hand, the movement is organized. The whole hand is moving in one direction. But when the surfaces rub together, some of that organized motion is broken up into tiny, random motions inside the skin. Atoms and molecules jiggle faster. The faster they jiggle, the hotter the material becomes. The energy has not disappeared. It has just become harder to use.

That is why friction usually feels like a loss. If a bicycle chain and gears get hot, that heat came from the energy used to pedal. Some of the energy moved the bicycle forward, but some of it was converted into heat by friction in the chain, gears, bearings, and tires. Engineers try to reduce this wasted energy by using smooth surfaces, good bearings, and lubricants such as oil. Oil does not remove friction completely, but it keeps surfaces from grinding directly against each other, so less energy is turned into heat.

Sometimes, however, converting kinetic energy into heat is exactly what is needed. Brakes are a good example. When a car is moving, it has a lot of kinetic energy. To stop the car, that kinetic energy has to go somewhere. In ordinary brakes, friction between the brake pads and the brake discs converts much of that energy into heat. The brakes can become extremely hot, which is why they are designed to handle high temperatures. This causes wear over time, but it is necessary because the kinetic energy of the car has to be removed somehow. Regenerative braking, used in many electric and hybrid vehicles, reduces this waste by converting some of the kinetic energy into electrical energy and storing it in a battery. Even then, friction brakes are still needed.

A spacecraft reentering the atmosphere is another example of motion turning into heat, although it is not quite the same as rubbing two solid objects together. When a spacecraft enters the atmosphere at enormous speed, it compresses the air in front of it and collides with air molecules. The air becomes extremely hot, and heat transfers to the outside of the spacecraft. This slows the spacecraft down to a speed where it can land or deploy parachutes safely, but the heat is so intense that spacecraft need heat shields or special protective tiles to protect the people and equipment inside.

Friction can be reduced, but it can never be removed completely in ordinary life. Air resistance can be reduced by making cars, trains, and planes more streamlined. It can be reduced even more in a vacuum because there are fewer air molecules to collide with. Contact friction between solid objects can be reduced with smoother surfaces, wheels, bearings, or lubricants. However, as long as objects touch or move through matter, some energy will usually be converted into heat. And this is what I learned today.

Sources

https://www.ucc.ie/en/physics/study/undergraduate/thelaboratories/firstyearphysicslabs/heatfromfriction

https://cs.stanford.edu/people/zjl/pdf/friction.pdf

https://en.wikipedia.org/wiki/Conservation_of_energy

https://www.eia.gov/energyexplained/what-is-energy/forms-of-energy.php

Photo by Mike Bird: https://www.pexels.com/photo/photo-of-a-ferrari-tire-11629476/

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What is tuberculosis? Tuberculosis (TB) is a contagious, airborne bacterial infection caused by Mycobacterium tuberculosis.

If you have read any books set in Victorian London, you will have seen character after character die of “consumption.” It killed two of the famous Bronte sisters, as well as two of their other sisters and their brother. In the early 19th century, it is thought to have been responsible for a quarter of all deaths in Europe. The Industrial Revolution was in full swing and people were moving to cities, living in overcrowded and often unclean houses. It was very easy for a contagious disease like tuberculosis to spread in that environment. It is not generally as contagious as the flu, but, if there are enough people in a close area, it will spread.

Tuberculosis is not often seen in developed countries these days, although it is still there. It is more common in poorer countries. Roughly 10 million people become ill with tuberculosis every year. It can be cured, but it still causes well over a million deaths per year. It is estimated that over a quarter of the world’s population have been infected with tuberculosis at some point. In many of those people, the infection stays hidden and inactive for years, which is one of the reasons it is so difficult to eliminate completely.

Tuberculosis is caused by a bacterium called Mycobacterium tuberculosis, and it is thought that it has infected humans since very early in our history. Skeletons have been found that show evidence of TB. The bacterium doubles every 20 hours, which is much slower than most other bacteria. Tuberculosis bacteria spread through the air when people cough or sneeze, and they can stay airborne and alive for several hours before finding a new host. There is also a form called Mycobacterium bovis, which causes tuberculosis in cows, goats, and sheep. It can be transmitted in milk, and a lot of people developed tuberculosis from drinking raw milk.

When a person inhales a tuberculosis bacterium, it enters the lungs and reaches the alveoli. The immune system goes into action and the bacterium is swallowed up by a macrophage, whose job is to digest invaders using enzymes. Rather than being digested, the tuberculosis bacterium takes over the inside of the macrophage and starts to multiply. The immune system sees what has happened and traps the infected macrophage, trying to stop it from spreading around the body. This grows into a small round nodule called a tubercle, which is where the name tuberculosis comes from. In some people, the immune system can keep the bacteria inside these tubercles and the disease never takes off. This is called a latent infection, and the person may not even know that they have it. However, if the immune system cannot contain it, the bacteria can get into the bloodstream and infect other areas of the body. They can severely damage the lungs, and tuberculosis can also spread to places such as the bones, kidneys, or brain. Some of the tissue in the lungs is killed and comes away, causing the person to cough. When they cough up these pieces, they cough up bacteria as well, which spreads the disease.

Tuberculosis can mostly be treated with antibiotics, which is why it kills nowhere near as many people now as it did in the 19th century. Without treatment, it kills roughly two-thirds of people within about three years. The problem in recent years is that several strains of the bacterium have become resistant to the antibiotics used to kill it. If that resistance spreads further, tuberculosis will become much harder to treat.

In 1819, a French doctor, Rene Laennec, realized that the tubercles in the lungs were causing consumption. In 1865, another French doctor, Jean Antoine Villemin, showed that tuberculosis could be spread. In 1882, a German doctor, Robert Koch, discovered the bacterium that caused tuberculosis. He later received the Nobel Prize for this discovery. All of these discoveries led to cures and a lot of methods of prevention. The bacterium could be killed by several antibiotics. It could also be controlled and prevented by pasteurizing milk. A vaccination was developed as well. The larger efforts went more into prevention than cure. Pasteurizing milk and food were huge game changers, as were increased sanitation in hospitals and improvements in the cramped places where people were living. There were public campaigns to stop people spitting and to encourage them to cover their mouths when they coughed. It took a long time, but, over the course of the 20th century, the number of cases of tuberculosis declined. Tuberculosis could, in theory, be eradicated. However, that would cost an enormous amount of money, and finding all of the hidden bacteria would probably be impossible. And this is what I learned today.

Sources

https://www.mayoclinic.org/diseases-conditions/tuberculosis/symptoms-causes/syc-20351250

https://www.who.int/news-room/fact-sheets/detail/tuberculosis

https://en.wikipedia.org/wiki/Tuberculosis

https://www.cdc.gov/world-tb-day/history/index.html

Image By Rensselaer County Tuberculosis Association. – U.S. National Library of Medicine Transferred from en.wikipedia, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4352690

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What does E = mc² mean? E = mc² means that mass and energy are two ways of describing the same physical thing. In everyday life, mass feels like “stuff” and energy feels like “activity,” but Einstein showed that they are connected. If a system loses energy, it also loses mass. If it gains energy, it gains mass. Usually the change is far too small to notice, but the relationship is always there.

In the equation, E stands for energy, m stands for mass, and c stands for the speed of light. The c comes from the Latin celeritas, meaning “speed.” The speed of light in a vacuum is about 300,000,000 meters per second. In Einstein’s relativity, that speed is special because everyone measures the same value for it, even if they are moving relative to each other. If you stand on a train moving at 100 km/h and throw a ball forwards at 30 km/h, the ball is moving away from you at 30 km/h. However, a person standing on a train platform watching the train move past will see the ball moving at 130 km/h. This doesn’t happen for light. Everybody, no matter where they are, will see the speed of light at exactly the speed of light. That strange fact forces physics to treat space and time differently than was once thought, and it is one reason c appears in so many relativity equations.

A common source of confusion is that E = mc² is really about rest energy. Rest energy is the energy an object has simply because it exists and has mass, even when it is not moving. Physicists sometimes write it as E₀ = mc² to make that clearer. There are lots of different types of energy. There is potential energy, kinetic energy, thermal energy, chemical energy, and others. The rest energy that Einstein was talking about is the energy that every object in the universe just has all the time.

Why is c squared? One simple answer is that it makes the units work. Mass is measured in kilograms, energy in joules, and multiplying by c² converts between them. But the deeper message is about scale. Because c is so large, c² is unimaginably large. That means even a tiny amount of mass corresponds to a huge amount of energy. A single gram of matter has an enormous rest energy. That does not mean that a gram of matter can easily be turned into usable energy in everyday life. It means that if a process can convert even a small fraction of mass into other forms of energy, the result can be dramatic.

This is where nuclear reactions enter the story. In the Sun, energy is produced mainly by fusion. Hydrogen nuclei combine through a chain of reactions that ends with helium, plus energy carried away as light and heat. The surprising part is that the final helium nucleus has slightly less mass than the starting hydrogen nuclei. That missing mass is called the mass defect. It is not “lost” in a mysterious way. It left the system as energy. When energy leaves, the mass of the system goes down by an amount equal to E/c².

The same basic idea applies to fission reactors. In fission, a heavy nucleus such as uranium splits into smaller nuclei, along with energy and particles. Again, the products end up with slightly less mass than the original nucleus. The difference shows up as released energy. E = mc² is the conversion rule that connects that tiny mass change to the large amount of energy that comes out.

An easier way to think about the mass defect is to connect it to binding energy. When particles bind together into a stable nucleus, energy is released. That released energy reduces the total energy of the bound system compared to the same particles separated far apart. Because mass and energy are linked, a bound nucleus can have less mass than the sum of its parts. The “missing” mass is really the energy that was given off when the nucleus formed. This is not only true for nuclear physics. In principle, any bound system has a total mass that includes all its energy—motion, heat, and binding energy—though in chemistry the changes are usually too small to measure easily.

E = mc² also helps explain the speed limit of the universe. As an object with mass moves faster and faster, its total energy rises. Near the speed of light, enormous amounts of energy are required for smaller and smaller increases in speed. In the limit, reaching the speed of light would require an infinite amount of energy. That is why objects with rest mass can approach c but cannot reach it. Light and other massless particles travel at c because they do not have rest mass in the first place.

Einstein first came up with the mass–energy connection in 1905 in a short German paper translated as “Does the Inertia of a Body Depend Upon Its Energy Content?” Using a thought experiment involving an object emitting light, he argued that if an object gives off energy, its inertia must decrease. Einstein imagined an object emitting light. The light carries away energy, and light also carries momentum. To keep the laws of physics consistent for observers in different states of motion, the emitting object must lose a tiny amount of inertia. Since inertia is what mass measures, the object must lose a tiny amount of mass when it loses energy. When energy leaves an object, the object becomes very slightly easier to accelerate, so its mass must have dropped — which means energy and mass are connected. That is the link behind E = mc².

Sources

https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence

https://www.theguardian.com/science/2014/apr/05/einstein-equation-emc2-special-relativity-alok-jha

https://en.wikipedia.org/wiki/Speed_of_light

https://www.energy.gov/science/doe-explainsrelativity

Photo by JESHOOTS.com: https://www.pexels.com/photo/woman-illustrating-albert-einstein-formula-714698/

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