Richard Feynman, a well known physicist of the 20th century once said that if some cataclysm was to destroy all scientific knowledge, the statement that carries the most information in the least words that we would want send to the next generation might be.
"All things are made of atoms--little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another”
The idea of atoms goes back to the greek (Democratus). It took a really long time before the atomic theory was accepted. Now we are able to move and see single atoms at a time.
Here are a few important things to remember about atoms.
We have already discussed how small atoms are. One atom is about 0.1
nanometer in size
There are about 100,000,000,000,000,000,000,000 atoms in twenty grams of
water (each gram is a drop of water). In scientific notation this is
Some atoms in your body are as old as the Universe, others were created in large supernovae explosions way before our sun even existed. You continuously exchange atoms with the outside world (while you breath for example). Since there are as many atoms in your lungs as there are breaths of air in the atmosphere, it is almost certain that you have atoms inside your body right now that once belonged to Einstein, Gandhi or Newton. Atoms last forever, you don’t own them.
This begs the question - where do babies get their atoms to grow? They are not created in the womb, but come from the food the mother eats. Those atoms, are the same ones from the beginning of the universe!
Now, it is technically possible to create new atoms out of old ones by nuclear fusion (the sun transforms hydrogen into helium this way) or we can also destroy them via nuclear fission. For example, in nuclear reactors we break uranium into smaller parts. But both processes are hard to do and most atoms around you have been unchanged since the very beginning of the Universe! We will learn more about nuclear fission and fusion later in this class.
Atoms are constantly in motion. As we will see, just how much in motion is a measure of the temperature of the material. This constant jiggling that atoms do is how we first detected them. This is called Brownian motion and you can see this in your home! If you look at a small piece of dust (itself made of numerous atoms) in the air (assuming no wind), you will notice that it just wanders around sometimes changing completely its direction.
This is because the atoms in the air are continuously bouncing off the dust particles. The net effect is that the dust particle ends up moving randomly. This is shown in Fig 3 and we call this random motion, Brownian motion.
The movie shows Brownian motion of a dust particle (itself made of many atoms) undergoing random movement because it is being hit constantly by jiggling atoms
Atoms move, and their speed varies upon the state of matter (liquid, solid or gas) and on the amount of energy in the system. We will discuss in more details the various states of matter and the notion of energy and temperature later, but let us first get an idea of the approximate speed of atoms in the air around you.
This is easy to figure out. When you see a lightning outside. It often takes a while to hear a thunder.
Sophie (S): I know my parents told me when I was young. For every 5 seconds
between the lightning flash and the thunder, the storm is 1 mile away. 10
seconds delay, the storm is 2 miles away.
Marie (M): That's right. This is because light travels super fast while
sound is slower. From what your parents told you, you can determine the
speed of sound.
The last number is important to remember. And note that we approximate, we
don’t care about being precise. There is approximately 5000 feet in a mile
(really 5280 ) and approximately 3 feet in a meter (really 3.3 feet) .
So if sound does 1 mile in 5 second. This is like 1000 feet in 1 second. About 330 meters per second.
This is the speed of sound: 330 meters/s. We denote this to be 330 m/s
In daily life you often don’t need the precise numbers so we can approximate like this.
S: Ok that's all great but how does it have to do with the speed of molecules or atoms?? M: Well, as you may know, and as we will learn later on, sound is a wave traveling through the air. In order for sound to go from one place to another, molecules of air need to go touch another one (to transmit the message if you like). Since sound move at 330 meters per second the same must be true for the molecules. Molecules move at the speed of sound! Note that sounds travel faster in liquids and solids since the atoms/molecules already are in contact.