[Eli5] What is electrical charge? Like, how do we know that electrons have a negative charge and what does that actually mean?
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Electrical charge is a description of how particles interact with each other electrically. The assignment of a negative charge to electrons is a human construct, and means noting in an absolute sense. They could just have easily been assigned positive charge, and nothing would change. Just like temperature exists independent of our measurement conventions, electrical charges exist independent of our naming conventions.
These charges can be thought of as an innate property of the particles, wherein like-charged particles repel each other, and oppositely-charged particles attract. This is similar to magnetic fields, which attract or repel based on their polarity. In addition to the charges that we call positive and negative, there are also particles that remain neutral, and do not interact with electrical fields. For example: wood doesn’t carry or react to electricity (even if the water inside it can), just like wood does not attract or repel magnets.
Hope this helps.
Electrons have a property such that they are attracted to protons and some other things, and are repelled by other electrons and some other things. As well, they move in magnetic fields, but in the opposite direction that a proton would move.
So we define one of the charges, the proton’s, to be positive, and the electron’s to be negative. Anything that doesn’t get either attracted or repelled is considered neutral.
We made up the words we use for it, so that’s how we “know” an electron is negatively charged, we defined it to be that way, because the words negatively charged are what we have said describes the motion of electrons.
We discovered the electromagnetic force rather early on, though we didn’t know what it was that we were observing. Ancient Greeks noted that amber could attract small objects when rubbed with fir. We would later classify other objects with a similar attractive property as “electrica” based on the greek word for amber (elektron).
We then discovered the repulsive side. For example, if you rub glass with silk, then rub amber with wool, the glass will repel a charged gold leaf but the amber will attract it.
Ultimately, we would posit that all this rubbing action was causing a transfer of *something* from one object to another, and the attraction/repulsion was caused be a lack/excess of that something, respectively. Ben Franklin labeled the excess as being positive and the lack being negative, and posited that this substance, having a positive charge, is what was being transferred. However, others thought that there were two kinds of things involved, one wholly positive and one wholly negative.
We would eventually discover the electron. When you energized some cases, they would emit a stream of particles that could be influenced by a magnetic field. The manner that they were influenced was the same way as things which we had come to call as being negatively charged, based on Ben Franklin’s earlier categorization.
So, in short, with incomplete knowledge, we labeled things as being negatively or positively charged based on attraction or repulsion. When we managed to isolate electrons, we found that they behaved the same as the things we labeled as being negatively charged.
The simplest answer is “we made it all up so we can call it what we like.” Positive and negative charge are just terms we gave to the behavior of particles under magnetic fields and when interacting with each other. Positive and negative repel like and attract opposite particles. If it doesn’t do either we call it neutral.
There is this one thing that attracts some stuff with a strength of 5. There is thing other thing that attracts other stuff with a strength of 5. When we put them together, they stop attracting stuff (i.e. attract things with a strength of 0). Things like the first thing push each other away though. Things like the second thing push each other away too.
With math… this looks like negative and positive numbers… putting together 5 and -5 gives you 0. Putting two negatives or two positives together gives you a bigger negative or a bigger positive (as in, farther from 0). Putting a negative and a positive together gives you a number closer to zero. So, we’ve established that there is a “positive” and “negative” property of these things.
From there, we chose arbitrarily which is which. I’ve actually heard arguments that it would have been better to do it the other way around since in practice a lot of electrical stuff we do involves moving electrons around, so it’d make sense that taking them away led to a smaller number and adding them led to a bigger number. But ultimately, it works fine either way.
Electrons being negative is just an arbitrary decision. All we know for sure is that they have the opposite charge of protons. The reason electrons were chosen as the negative charge was because during the cathode ray tube experiment when they were discovered, the side they came from was labeled negative. The ends labeled positive and negative were arbitrary, they just needed to be consistent with each other so we know they are different.
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