Foundations — Define electric charge, electron, proton, and the coulomb

This page assumes you have seen nothing. Before you read the parent topic, we build every symbol, word, and picture it quietly leans on. Read top to bottom — each block is a rung on a ladder.

0. What is a "symbol" doing here at all?

In physics a symbol is just a short nickname for a number or an idea, so we don't have to write the whole sentence every time. When you see $Q$, read it as the words "the total amount of charge". When you see $e$, read "the charge of one single electron". Nothing is magic — a letter is a labelled box that holds a value.

We will meet these boxes, in this order:

1. charge (the stuff)
2. positive / negative (the two flavours, and the $+/-$ signs)
3. electron and proton (the particles that carry charge)
4. $e$ — the elementary charge (the size of one lump)
5. coulomb and its symbol C (the unit = the bucket)
6. $N$ — a count of lumps (a plain whole number)
7. $Q=Ne$ — how counting becomes charge
8. scientific notation ($10^{-19}$, $\times$) — how we write absurdly big/small numbers

Each new symbol is only allowed to appear once the ones before it are built.

1. Charge — the stuff itself

The picture: imagine two beads floating in space. Nothing connects them, yet one slides toward the other, or away from it. That invisible reason-to-move is charge.

Why the topic needs it: every later word — electron, current, voltage — is a story about charge moving. If you don't have "the stuff", there is nothing to move.

2. Positive and negative — the two flavours and the signs

Experiments show only two behaviours: some pairs of beads push apart, some pull together. There is no third behaviour. So we invented exactly two labels.

The picture: think of a number line. Right of zero is "$+$", left of zero is "$-$". These aren't "more" or "less" of the same thing — they are two directions of the same property.

3. Electron and proton — who carries the charge

Charge never floats around loose in everyday matter; it rides on particles.

The picture: an atom is like a tiny solar system — a dense central nucleus (protons, glued in place) surrounded by orbiting electrons (light, free to wander). See Atomic Structure for where these live.

Why the topic needs it: the parent says "in wires, charge moves because electrons move." That sentence is meaningless until you know an electron is (a) charged and (b) free to move while protons are stuck. Now it makes sense.

4. $e$ — the elementary charge (the size of one lump)

Here is the key experimental surprise: every electron carries exactly the same amount of charge as every other electron. Charge comes in identical lumps, like eggs come only in whole eggs.

• An electron carries $-e$ (one negative lump).
• A proton carries $+e$ (one positive lump).

The picture: a tray of identical marbles — you can have 1, 2, or a trillion, but never $1.5$. This "whole-lumps-only" fact is called quantisation.

5. The coulomb (C) — the bucket we measure in

One lump $e$ is a ludicrously tiny amount of charge. Measuring charge in lumps is like measuring an ocean in raindrops. So we bundle a huge, fixed number of lumps and call that bundle one coulomb.

The picture: a giant bucket labelled "1 C". You keep dropping identical marbles ($e$ each) into it; when it holds about six-billion-billion of them, it is exactly full.

6. $N$ — a plain count

The picture: the number of marbles in your hand before you know their total charge.

7. $Q=Ne$ — turning a count into a charge

Now every ingredient exists, we can assemble the one formula the whole topic rests on. We ask: "if I have $N$ identical lumps, each of size $e$, what is the total charge $Q$?"

Because charge simply adds up (two lumps side by side make double the push), totalling $N$ equal lumps is the same as multiplying:

$Q=\underset{N\text{ times}}{\underbrace{e+e+\cdots +e}}=Ne$

The picture: stack $N$ identical marbles; the height of the stack is $Q$. Read the arrow forward ($Q=Ne$) to go count → charge; read it backward ($N=Q/e$) to go charge → count. The division sign just undoes the multiplication.

8. Scientific notation — reading $10^{-19}$ and $\times$

You keep seeing numbers like $1.602\times 10^{-19}$. This is not a new idea, just shorthand for very big or very small numbers so we don't write dozens of zeros.

Putting it together — a tiny worked check

Prerequisite map

Equipment checklist

Test yourself — say each answer aloud before revealing.

Connections

• Parent topic — the full definition
• Atomic Structure — where electrons and protons live.
• Electric Current and the Ampere — charge counted per second.
• Voltage and Potential Difference — the push that moves these charges.
• Conductors and Insulators — whether electrons are free to move.
• Coulomb's Law — the force between two charges.