While Classical Mechanics, or Physics 1, deals with quantities that we all have direct, visceral experience with--masses, gravity, acceleration, velocity-- electromagnetism, or Physics 2, deals with things that, while technically we have experience with, we don't have a good intuition for. Even if you are a little shaky on the difference between mass and weight, or the difference between velocity and speed, you have an intuition that if something with more mass drops on your head, your head is going to experience more force. You know from driving in a car and stopping short for something that anything not tied down to the car will keep going forward until it hits something--Newton's 3rd law of motion. If you've ever played pool you've used conservation of momentum.
But if something has a lot of charge, what does that mean? What even qualifies as 'a lot' of charge? Something has mass if it's made of 'stuff'. What gives somethings charge and other's not? We use refrigerator magnets and electricity every day of our lives, but its actions are for the most part mysterious.
This post is going to try to cover, in a basic, plain-English kind of way, to describe the quantity that is at the heart of all electromagnetism--charge.
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| Select Field-lines between positive and negative charges. |
So, what is charge?
There's two ways to answer that question. I'm going to use the answer that avoids quantum field theory, and give a mostly practical answer.
Put simply, charge is an inherent property of certain subatomic particles, namely the electron and the proton. In much the same way that these particles have mass as part of their being, they also have charge. You could talk about their component quarks, but then you are down the rabbit hole, and for classical electromagnetic theory you don't really have to go any deeper than electrons and protons. Since electrons are the only subatomic particles that are free to move under normal circumstances, we're going to focus on them.
Electrons are the tiniest of the three particles that compose atoms. An electron has a mass of 9.11 x 10^-31 kg, which is about 1/1,000,000,000,000,000,000,000,000,000,000 of a kilogram, which is about 2 lbs on earth. Put another way, the difference in mass between an electron and a quart of milk is roughly the same as the difference in mass between that same quart of milk and the sun. They also have a charge of 1.6*10^-19 C, which is also defined as the elemental charge e, or 1 electron-Volt. This is the the base unit of charge. Until you start talking subatomic physics and the Standard Model, this is the smallest charge you can get, and all amounts of charge in the universe, from Quantum Mechanic level to General Relativity level, are integer multiples of this amount.
Nearly everything in the electric portion of electromagnetism comes down to the presence and movement of electrons, either in free-space or in matter. Most of magnetism can also be blamed electrons moving in an organized fashion. So when you hear a professor of electromagnetism talking about 'charge' what we mean is 'the measurable/observable effect of having an excess of electrons in one place, or moving from one place to another'. "Charge" in this context is essentially a short hand. "Positive charge" is a dearth of electrons, thus leaving a net excess of protons behind. "Negative charge" is an over abundance of electrons. Current is the flow of electrons in time (similar to water flow).
The greater the difference between the number of protons and electrons in an object, the more charged that object is. And just like an object with a larger mass has a greater gravitational force, the more charged an object, the greater its electromagnetic force. But that is a topic for another post.
~AMPH
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