4.2 SCIENTIFIC THEORIES OF HEAT

Early Theories of Heat 1-2 million years ago the first hominid (human- like) species was called Homo erectus. 1-2 million years ago the first hominid (human- like) species was called Homo erectus. These early humans lived in Africa and were thought to be the first humans to use fire to cook their food and keep warm. These early humans lived in Africa and were thought to be the first humans to use fire to cook their food and keep warm. They did not understand the science of heat and energy. They did not understand the science of heat and energy.

Empedocles – BC Empedocles’ theory consisted of the following elements: Empedocles’ theory consisted of the following elements: All matter was made from a combination of the 4 elements: EARTH, AIR, FIRE and WATER All matter was made from a combination of the 4 elements: EARTH, AIR, FIRE and WATER He thought that when objects burned, the fire within them was released He thought that when objects burned, the fire within them was released

The Phlogiston Theory Scientists believed that substances that could burn contained an invisible fluid called Scientists believed that substances that could burn contained an invisible fluid called. They believed that phlogiston flowed out of an object when the object burned. They believed that phlogiston flowed out of an object when the object burned. When wood burned, the phlogiston flowed out, leaving only ashes (therefore less mass?) When wood burned, the phlogiston flowed out, leaving only ashes (therefore less mass?) An example that would dispute this theory: An example that would dispute this theory:

The Phlogiston Theory

The Caloric Theory In the late 1700s, it was believed that “caloric” or “heat” was a massless fluid that was found in all substances. In the late 1700s, it was believed that “caloric” or “heat” was a massless fluid that was found in all substances. This theory was called the This theory was called the Scientists believed that this “caloric” couldn’t be created or destroyed, but could flow from one substance to another. In fact, many scientists believed it flowed from warm objects to cooler ones. Scientists believed that this “caloric” couldn’t be created or destroyed, but could flow from one substance to another. In fact, many scientists believed it flowed from warm objects to cooler ones.

Joseph Black Joseph Black defined the calorie as: Joseph Black defined the calorie as:

The Caloric Theory Can you see any problems/limitations with the “Caloric Theory”? Can you see any problems/limitations with the “Caloric Theory”? Caloric Theory = Caloric is a massless fluid that flows from one object (warmer) to another (cooler) Caloric Theory = Caloric is a massless fluid that flows from one object (warmer) to another (cooler)

Benjamin Thompson (aka Count Rumford) Count Rumford lived in Bavaria, Germany and was in charge of looking after the military cannons. He noticed that after boring a hole into the metal cannon, the tools and metal became very hot – but weren’t hot previously. How would the Caloric Theory explain this?

Count Rumford The count discovered that the “caloric” or “heat” as he called it, was not transferred from inside a hotter object to a colder one – instead the “heat” was a type of energy that was transferred from the mechanical energy due to friction.

Count Rumford Count Rumford made the conclusion: Count Rumford made the conclusion: HEAT = ENERGY HEAT = ENERGY

Relationship Between Energy and Heat Scientists now knew that heat and energy were related – but wanted to find a mechanical equivalent of heat – in other words, they wanted to quantify or measure heat in some way. Scientists now knew that heat and energy were related – but wanted to find a mechanical equivalent of heat – in other words, they wanted to quantify or measure heat in some way. Scientists competed to be the first to come up with a measurable quantity for heat – which was finally discovered in the 1800s. Scientists competed to be the first to come up with a measurable quantity for heat – which was finally discovered in the 1800s.

Julius Mayer – 1800s Julius Mayer actually was the scientist who found real evidence of a relationship between energy and heat. Julius Mayer actually was the scientist who found real evidence of a relationship between energy and heat. How did he do this? How did he do this?

Julius Mayer – An important discovery Julius Mayer served as a ship’s doctor on a voyage to the East Indies. Julius Mayer served as a ship’s doctor on a voyage to the East Indies. Many sailors would get sick – and “blood-letting” was a common practice to rid the sailor of “toxins”. The doctor would puncture the sailor’s vein and allow some blood to escape – believing it would cure the sailor. Many sailors would get sick – and “blood-letting” was a common practice to rid the sailor of “toxins”. The doctor would puncture the sailor’s vein and allow some blood to escape – believing it would cure the sailor.

Mayer’s findings: After performing many blood-letting “operations”, Mayer noticed that the blood of the sailors (who lived predominantly in Northern climates) was darker red than the blood of the natives (who lived in tropical climates) After performing many blood-letting “operations”, Mayer noticed that the blood of the sailors (who lived predominantly in Northern climates) was darker red than the blood of the natives (who lived in tropical climates) Mayer knew that darker blood meant less oxygen whereas lighter blood meant more oxygen. Mayer knew that darker blood meant less oxygen whereas lighter blood meant more oxygen. Native’s blood Sailor’s blood

Mayer’s findings con’t Mayer concluded that lighter blood meant more O 2 and therefore less work done by the body (Mayer knew that work done by the body = less oxygen in blood = darker blood) Mayer concluded that lighter blood meant more O 2 and therefore less work done by the body (Mayer knew that work done by the body = less oxygen in blood = darker blood) Mayer determined that the Northern sailor’s used quite a bit of body energy to stay warm – therefore there was less oxygen in the blood which meant darker blood overall. Mayer determined that the Northern sailor’s used quite a bit of body energy to stay warm – therefore there was less oxygen in the blood which meant darker blood overall.

Mayer’s Downfall… Finally! Someone could quantify heat – possibly even measure it! If Julius Mayer could figure out how to calculate how much energy the body expended when staying warm – he could put a quantity or unit on heat. Finally! Someone could quantify heat – possibly even measure it! If Julius Mayer could figure out how to calculate how much energy the body expended when staying warm – he could put a quantity or unit on heat. Sounds good! Why didn’t he publish this and get really famous? (Obviously he didn’t – have you ever even heard of Julius Mayer??) Sounds good! Why didn’t he publish this and get really famous? (Obviously he didn’t – have you ever even heard of Julius Mayer??)

Julius Mayer tried to write a scientific paper outlining his theories on heat and energy but it was overlooked by the scientific community because he lacked the formal education necessary to write a good paper. By the time he had gone back to school to learn how to write a paper… another scientist had quickly published all of these ideas! He became QUITE famous…

James Prescott Joule Joule was a highly trained scientist who performed many experiments. Joule was a highly trained scientist who performed many experiments. He came up with a mechanical equivalent of heat – called a JOULE (J). He came up with a mechanical equivalent of heat – called a JOULE (J). The Joule is now used to measure energy. The Joule is now used to measure energy.

Joule’s Experiment support.co.uk/support/msim/website/experiment/expA_hi- bandwwidth_version/screen_2.htm

Joule's apparatus for measuring the mechanical equivalent of heat. A descending weight attached to a string causes a paddle immersed in water to rotate and the "work" of the falling weight is converted into "heat" by agitating the water and raising its temperature. Joule’s Experiment support.co.uk/support/ msim/website/experim ent/exp_B_virtual_exp_f lash_1.htm

What is work ? Work = Work = W = FΔd W = FΔd W = work (J) (Joules) F = force (N) (Newtons) (F = mg) Δd = change in distance (m) (metres)

Work Problems F = 1.0 N F = 1.0 N Δd = 0.35 m Δd = 0.35 m W = ? W = ? W = FΔd = Assignment: Do problems on page 154 #2-9.