- 3672 - THERMOCOUPLES AND HEAT PUMPS. Thermocouples are simple devices consisting of two different metals in contact at two different places. If one contact is at a hotter temperature than the other an electric current flows. Even more amazing is that it works backwards too. If electricity flows from one contact to the other than heat is transferred. It can act as a heat pump or a refrigerator, or a thermometer.
--------------------- 3672 - THERMOCOUPLES AND HEAT PUMPS
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- In the 1800’s electric currents were limited to short duration sparks. Static electricity was the only source used to move electric charges from one point to another. Ben Franklin was tying church keys to kites in thunderstorms to see if lightning was electric.
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- In order to get a continuous flow of electric charges we needed a way to resupply the charges as fast as they moved away. The thermocouple discovery was made all because the cooks were putting frog legs in the soup that day.
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- In 1741 Luigi Galvani ( 1737 - 1798) discovered that sparks from a Leyden jar would cause the thigh muscles of dissected frogs to jump. Leyden jars were the rage back then. They were big capacitors that could store an electric charge.
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- Luigi was studying medicine and the cooks had the frog legs laying around before they went into the soup de jour. Once Luigi saw the frog legs jump from the electric spark he gave them he wondered if he could get the same effect in a thunderstorm. After all, lightning was an electric spark.
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- Luigi hung the frog legs on brass hooks on an iron railing on the window. Sure enough the frog legs jumped. But, they jumped whether the thunderstorm was there or not. All he needed was for the 2 different metals to come in contact with the muscle and it would twitch from an electric shock. Galvani mistakenly thought that this was caused by “animal electricity”.
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- Luigi Galvani was born is 1737 in Bolonga, Italy. He first studied theology and turned to medicine. It just so happened that Leyden jars and frog legs were laying around and he was interested in muscles.
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- Galvani’s friend Alessandro Volta (1745 - 1827) did not believe this. He had the theory that the electricity came from the combination of dissimilar metals. Volta started his own experiments. He started with bowls of salt water and connected them with bridges of 2 different metals copper and zinc. He called this a “crown of cups” and when he completed the circuit electricity would flow.
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- He could tell this using a compass needle. But, he kept spilling the bowls so he decided to use cardboard disks soaked in salt water and put them between copper disks (coins worked fine) and zinc disks. Sure enough the stack of disks were a source of electric current. He had invented the battery. He called it a “voltaic pile”.
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- Allessandro Giuseppe Volta was born in 1745 in Como, Italy of a noble family of nine children. His family thought he was retarded and Allessandro did not talk until he was 4 years old. When he was 14 he got interested in electricity and later became a teacher of physics at Como High School. Volta and Galvani were friends but Volta did not buy the idea of animal electricity in frog legs and in 1794 started is own experiments and invented the battery.
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- Here is how Volta’s battery worked and how Galvani’s thermocouples worked: All matter is made of atoms with an internal structure of negatively charged electrons and positively charged protons. So, the battery is always there you just need a way to get the charges separated otherwise the matter is neutral.
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- The two metals we brought in contact were copper and zinc. Copper has 29 electrons and zinc has 30 electrons. The electric forces binding the electrons to the zinc atoms is somewhat weaker that the forces binding electrons to the copper atoms.
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- At the contact boundary electrons tend to slip across the boundary from zinc to copper. The copper with its stronger grip wrests the electrons from the zinc. Copper gains a negative charge while zinc gains a positive charge. An equilibrium is soon reached and a contact potential difference is set up between the two metals across the contact.
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- If the temperature is changed the force attracting the electrons to the atoms is changed. Now, take a long strip of zinc and a long strip of copper in contact at the two ends only. Heat up one end. Each end is at a different temperature. There is the contact potential at each end, but, the two potentials have different values due to the temperature difference.
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- With the electron concentration at the copper of one end greater than the other end electrons will flow. With electrons lost at the one end the copper can take more electrons from the zinc. This process continues indefinitely with electrons traveling through the copper then back through the zinc as long as the temperature is different between the two ends.
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- This thermoelectric effect was first discovered by Thomas Seebeck (1770-1831). He discovered it in 1921 while making a pot of tea. Thomas got the name the “Seebeck effect” for thermoelectricity but he did not really understand what was going on.
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- Thomas Seebeck was born is 1770. His father was a well-to-do German merchant living in Tallum, Estonia. He observed the thermocouple effect in 1921 while heating up a pot or tea.
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- It was not until William Shockley of transistor fame in 1948 that the correct explanation was developed. Once he understood it Shockley invented solid state rectifiers and then transistors to replace the vacuum tubes in radios.
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- Today a practical application for thermocouples is to be used as thermometers. They can work at very high temperatures with metals such as platinum and they can be accurate to with one degree C. A future application would be to run the thermocouple backwards using an electric current to move the heat from one contact to the other.
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- This was discovered in 1934 by Peltier, known as the “Peltier effect”, that current flowing across a junction of dissimilar metals causes heat to be absorbed. The direction of the heat flow reverses if the current flow is reversed.
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- You will find the heat pump or refrigerator pumps used in car seats. Some day maybe a thermocouple exposed to sunlight and producing electricity with no moving parts.
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September 5, 2022 THERMOCOUPLES AND HEAT PUMPS 966 3669
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--------------------- --- Tuesday, September 13, 2022 ---------------------------
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