3.5. Matter#
You’ll remember that what we call matter is made up of molecules, molecules are particular combinations of atoms, and that atoms are made of protons, neutrons, and electrons. These latter objects might be referred to as “elementary particles.”
Wait. What’s “elementary” mean in this context?
Glad you asked. The simplest way to think of “elementary” is an object that cannot be divided into parts. So an atom is not elementary since it’s made up of other things. As far as we know, and electron is elementary. But we now know that protons and neutrons are not elementary since we know them to be comprised of other particles called quarks and gluons. We’ll get into that.
It’s a lot more complicated than that and while the “planetary model” of electrons circling the nucleus (of protons and neutrons) is evocative, the actual situation is really very abstract. This will occupy our attention when we learn about quantum mechanics and just what the elementary partiles are is the subject of particle physics
3.5.1. Electric Charges#
If you live in the midwest in the winter, then you’re familiar with electrial charge since you can create sparks that you can feel, and even sometimes see when you stand on a carpet — or better, rub your feet on a carpet — and then touch something metalic. What you feel, and maybe hear or see, are electrons jumping from your skin to the metal object. That’s a spark.
Electricity is the subject of electric charges which come in three kinds: positive, negative, and neutral. The names don’t matter and there’s a Ben Franklin story there, but for our introductory purposes we’ll keep track by just remembering the following:
Like electric charges repell one another. Unlike electric charges attact one another. Neutral electric charges feel no electrical force between them.
“Repell” and “attract” are words that indicate that forces are applied, one charge to another. Just how much force and what direction the net force is depends on the amount of charge and how far away the charges are from one another. The fact that you’re sitting on a chair and not falling through it or the earth is because the negative charges at the back of your lap repell the negative charges on the surface of your chair and so you’re suspended between them. The unit of electrical charge is the Coulomb (C), which is pretty enormous and comes from 18th century measurements and definitions, so we’re stuck with it.
The quantum mechanics and particle physics of electricity (and magnetism) are a part of our story and we have explanations for what these phenomena are that is very precise. And pretty.
3.5.2. Electric Currents#
You also all know about electrical currents:
An electric current is charges flowing in time.
We tend to think of currents as charges flowing in metalic wires, but even that spark from your finger to the doorknob are an electric current, unconstrained by the volume of a wire.
We measure currents in Amperes (who we’ll learn about), “A.” 1 A is 1 C of charge flowing past a point per second. And yes, Mr Google knows electricity.
Type “(10 C) per (1 second)” into:
Calculate it:
3.5.3. Radioactivity#
Electric charges have to come from somewhere and one of the surprises in the late 19th century was that some nuclei just sitting around actually spontaneously emit electric charges, electrons and “alpha” particles (which were found to be two protons and two neutrons bound together, a helium nucleus). That was odd. Matter just seemed to fall apart. Radioactivity is the name for this phenomonon and is particularly of concern for radioactive waste from nuclear power or tragically, from the debris of nuclear weapons.
These are the things that I suspect you already know (or maybe you didn’t even realize you knew) and so we can dig into each of these subjects where we’ll find that the 20th century changed everything.