1. 4.2 SCIENTIFIC THEORIES OF HEAT

2. Early Theories of Heat
1-2 million years ago the first hominid (human- like) species was called Homo erectus.
They 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.

3. Empedocles – BC
Empedocles’ theory consisted of the following elements:
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

4. The Phlogiston Theory early 1700s
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.
When wood burned, the phlogiston flowed out, leaving only ashes (therefore ashes had less mass than original wood)

5. The Phlogiston Theory

6. An example that would dispute this theory:
2 Mg (s) + O2 (g)  2 MgO (s)
When Mg burned the ashes weighed MORE!

7. So maybe phlogiston didn’t always leave when substances were burned
Or maybe….phlogiston didn’t exist!

8. The Caloric Theory
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
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.

9. Joseph Black
Joseph Black defined the
calorie as:
A unit of heat

10. 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)

11. 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?

12. 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.

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

14. 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 competed to be the first to come up with a measurable quantity for heat – which was finally discovered in the 1800s.

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

16. Julius Mayer – An important discovery
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.

17. 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)
Mayer knew that darker blood meant less oxygen whereas lighter blood meant more oxygen.
Native’s blood Sailors’ blood

18. Mayer’s findings con’t
Mayer concluded that lighter blood meant more O2 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.

19. 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.
Sounds good! Why didn’t he publish this and get really famous? (Obviously he didn’t – have you ever even heard of Julius Mayer??)

20. 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…

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

22. Joule’s Experiment

23. http://www. einstein-support. co
Joule’s Experiment
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.

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

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