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February 17, 1888: Birthday of Otto Stern more: Today in Physics  





What is electric Charge Made Of?

August 2001

"What is stuff made of?" - molecules
"What are molecules made of?" - atoms
"What are atoms made of?" - protons, electrons and neutrons
"What are protons made of?"...

... this much most people have heard at some point in their schooling, and the pattern seems clear - you can keep finding smaller and smaller pieces to make up whatever you used to say everything is made of. Maybe I'll tackle that another month, for now I want to tackle a common mistake that many think is just an extension of the above discussion - asking what something is made of when it isn't a thing.

Put it this way, there are things and there are properties of things. If I was to ask "what smell is blue?", you would be stuck to even begin an answer that didn't suggest I was a moron. You might say "a blue what?". Some specific thing might be blue and have any smell. Blueness and smell are not things - you cannot have "a blue" without there being something which is blue - these are properties of things. If you want to make something blue, you can do it by adding something else that has the property of being blue, or you can change its color property. For example, toast your bread - as the bread changes the color property of the surface changes.

Electric charge is not a thing, it is a property of things. To try avoiding semantic pitfalls, let me explain how I find the word "property" to be normally used. Properties are how we experience and categorize things. When we say something has a property, it suggests that we could name the property and give it a value. For example, my height property is 6'4", my hair color property is "brown" etc. Obviously we haven't given a name to the property whose value is my height times my weight. That is a property of my body, it just isn't one that has been deemed useful enough to be given a name. (If you've done "object oriented programming", you'll be familiar with the idea that an object is something which is described by the values of some set of properties.)

In physics we recognize many properties of things: length/height/width/radius..., mass, momentum, energy --- and the list includes electric charge. Many small things have the property of having some specific charge. You can have charged particles - which have other properties of size, mass... as well - but there has to be something to have charge. The charge is one of the properties that defines a type of particle.

Basically, to ask "is charge made out of energy?" is like asking "is height made out red?".

So what do we find if we if we continue the sequence I outline above? In the everyday world around us we experience charge by such effects as rubbing two balloons on our hair - they then push away from each other, but are each attracted to a wall. We experience movement of charge in the electric current in wires. We experience charge in many ways. In a physics class you can learn how to make predictions for different situations based on the value of the charge property that something possesses. Where does this charge come from? Everyday matter is made of electrons, protons and neutrons - which in turn make up atoms, molecules, solids, liquids, gases, plasmas, jellies... Electrons and protons have identical and opposite value charges (positive for protons, negative for electrons), neutrons have charge - but its value is zero. Most matter has approximately the same number of electrons and protons, and the number of neutrons is unimportant to the charge value, so there is zero total charge to most of the world around us (or little enough total charge that we don't notice it). What we mean when we say something has a charge is that in total it has more electrons than protons - or vice-versa. What we are doing when we "charge" something is transfering a small amount of material to leave an imbalance in the number of electrons and protons.

Can we divide up any further? In the case of neutrons and protons we can divide up into smaller particles called "quarks". Quarks are not "pure charge" either, they too have mass, size, and a variety of properties that require more study to understand - "up", "charm", "spin", "strangeness"... What about quarks? What are they made of? ...We can fundamentally never find something which is only charge, it will always be a thing with properties including charge that we find - even if all other property values were zero somehow.

For a relatively easy explanation of where we go from here, try M-theory, the theory formerly known as strings. Here particles such as electrons are seen as vibration modes on strings. This theory strives to describe everything observed, from electrons to black holes. For more information, search for "string theory", "theory of everything", "M-branes"... you'll get to recognize some of the basic search phrases as you go.

What about conservation?

I've received an objection to the above explanation that charge and energy aren't "just" properties, they are "conserved properties" and this is supposed to make the difference as compared to properties such as color. What this means is that charge is a mathematical property such that if you include everything that comes into a given "system", then no matter what happens in the system the total value obtained by adding the value of charge of each part will add up to the same number. Doesn't suggest that they are the "things" that are real? Doesn't continued existence suggest something is really there?

Be careful about making logical leaps. I may write about the many ways of classifying properties another time, but I wouldn't use a term such as "just properties". Properties are how we are able to observe/distinguish the things around us - nothing minor about that. Furthermore, properties such as charge and mass (which isn't conserved) are about the most "real" things that exist, in the sense that these properties are what we build our world out of in elementary Physics classes. Story problems like "a 1 gram, 1 microCoulomb charge..." mean that this is so fundamental that it doesn't even matter what it is as long as it has these two property values. However, the fact that we find something to be conserved doesn't suggest it is more "thing-like" than something else. There are many other conserved properties that I never hear people call a "thing" - momentum and angular momentum for starters. I believe the reason for thinking of charge as a "thing" is historic. Charge used to be thought about in models such as a flowing fluid. From the scale of the world we live in this works pretty well, but when examined microscopically we find what I describe above and there is no reason left to think of it as a "thing".


Innocent Misadventure

My inspiration for the month comes from an online physics discussion group. Somebody had come in to ask a series of questions, including what charge is made out of and wondered whether it was made of energy. To a non-scientist, and possibly to many people who've taken a class or two in physics, this is a perfectly valid question. To a group of physicists however, to even ask the question shows that one is quite ignorant of basic physics, and I'm afraid the initial responses were rather rude.

In fairness to the members of this particular group their mandate has nothing to do with helping curious seekers (something to learn before entering any discussion group... what is the purpose of the group). I am partly writing this to demonstrate why the first (and therefore the gut-level) responses were along the lines "start by learning some physics". Here I offer a simple explanation that might help other beginners with a similar question.

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