How Invention Begins ch. 6
Now we get to the part of the story where scientists try to understand how steam engines work.
- You might think that Leinhard has showed that scientific
knowledge led to the invention of steam engines.
- It is true that people trying to understand air pressure and whether it is possible to create a vacuum made scientific discoveries that led to to the invention of the steam engine
- But while they understood that when they condensed steam atmospheric pressure would press on the piston, they couldn't explain how a steam engine worked in another sense. Where did the energy come from? From the coal that was burned to boil the water to make steam. But how was that energy hidden in the coal?
- Thermodynamics is the science of energy on a large scale--not what the atoms are doing but what happens in machines
- Chemistry has to explain what happens when something burns
- heat was thought to be a substance, called phlogiston, which
takes part in chemical reactions (the understanding of chemical
reactions was like a mirror image of what modern chemistry understands,
phlogiston was in effect the opposite of oxygen)
- coal has phlogiston in it, and when you burn the coal the phlogiston is released
- phlogiston was an element and was understood mostly in terms
of chemical reactions
- there were people already promoting the idea that heat was the motion of atoms, not a thing in itself, but they couldn't yet use that idea to explain how an engine works
- the phlogiston theory was more practical--if you measure the
quantity of phlogiston needed to do different things you can design
better steam engines
- but it clearly didn't always work, as Joseph Black realized
- so it was changed slightly into the idea of caloric--heat is
still a substance but separate from chemical reactions--developed
particularly by William Cleghorn
- how do people decide the old theory is wrong?
- problems--places where the theory doesn't work--gradually accumulate until people realize they need a new theory
- someone comes up with a new theory, but will it explain everything--it takes a while for the new theory to be worked out
- some scientists resist the new theory, others have a feeling it is the right way even before they can prove it
- a paradigm is an overall theory that is pretty well
accepted--what scientists work with between revolutions, most of the
time
- this is called a scientific revolution: The Structure of
Scientific Revolutions, T. S. Kuhn
- the first law of thermodynamics--conservation of energy--says the amount of energy stays the same when you change its form. It can be in the form of heat or mechanical work or electricity or it can be locked up in matter to be released by a chemical (or nuclear) reaction
- Benjamin Thompson, Count Rumford, proved that heat was not a substance--you can generate it almost indefinitely out of mechanical work by friction
- Rumford's experiment:
- heat is motion (how much the molecules are moving), not a material substance
Law of conservation of energy: energy is not created or destroyed, you can just change it from one form to another
Discovery of conservation of energy:
- Robert Mayer, from physiology
- James Prescott Joule, from research on electricity
- Clausius put the pieces together
- Helmholtz gave the mathematical description
- you can't build a perpetual motion machine that will run forever without fuel
What is
wrong with this ad?
- if energy is conserved then why is perpetual motion impossible?
- Sadi Carnot--second law of thermodynamics: energy moves from
denser to less dense forms unless you do work to change it to a denser
form again (a more technical way of saying this is that entropy stays
the same or increases)
(another science joke) The inventor
of the third law of thermodynamics said:
- three people discovered the first law of thermodynamics (Mayer, Joule and Helmholtz)
- two people discovered the second law (Carnot and Clausius)
- one person discovered the third law (Nernst)
- therefore there is no fourth law