Electric charge, conductors & insulators
The starting point of the whole course: what electric charge is, the rule that governs it, and how charge moves through different materials.
Source: Serway & Jewett, Physics for Scientists and Engineers, 7th ed., §23.1–23.2.
Before you start
What you need first
- The atom — that matter is made of atoms with a central nucleus and electrons around it (basic chemistry).
- Powers of ten — reading and multiplying numbers like \(1.6\times10^{-19}\) (scientific notation).
This is the entry topic of the course — no earlier physics topic is needed.
What you'll be able to do
- Say what electric charge is and name its two kinds (+ / −).
- Apply the rule like repels, unlike attracts.
- Count charge from a number of electrons with \(q = ne\).
- Tell a conductor from an insulator and say why.
- Explain charging by friction, contact, and induction, and why a charged object attracts neutral things.
The idea
What is electric charge?
Electric charge is a basic property of matter — like mass — that makes an object push or pull on other charged objects. That push or pull is the electric force.
Mass causes gravity, which is always a pull. Charge is different: it can cause a push or a pull. Charge comes in two kinds, which we call positive (+) and negative (−).
We measure the amount of charge in coulombs, with the symbol C.
The golden rule: like repels, unlike attracts
There is one rule that decides the direction of every electric force:
- Two charges of the same sign (+ and +, or − and −) repel — they push apart.
- Two charges of opposite sign (+ and −) attract — they pull together.
This is why a balloon rubbed on your hair sticks to a wall, and why the small parts inside every atom hold together.
Where does charge come from? The atom
Every object is made of atoms, and every atom carries charge in three kinds of particle:
- Protons — positive (+), packed in the central nucleus.
- Electrons — negative (−), moving in the space around the nucleus.
- Neutrons — no charge, also in the nucleus.
A normal atom has an equal number of protons and electrons, so the + and − exactly cancel and the atom is neutral (no net charge).
Charge is never created or destroyed
When you charge something, you are not making charge — you are only moving it from one object to another.
Rub a balloon on your hair: electrons move from your hair onto the balloon. The balloon gains electrons and becomes negative; your hair loses the same number and becomes positive by exactly the same amount. Nothing is lost — charge is just poured from one place to another, like water between two cups.
Measuring charge: the coulomb and the elementary charge
Charge is measured in coulombs (C). The smallest amount of free charge that exists is the charge on a single proton or electron. We call it the elementary charge, symbol \(e\):
| Particle | Charge |
|---|---|
| Proton | \(+e = +1.60\times10^{-19}\) C |
| Electron | \(-e = -1.60\times10^{-19}\) C |
| Neutron | \(0\) |
One coulomb is a huge amount of charge — it is the charge of about \(6.25\times10^{18}\) electrons. That is why real charged objects usually carry only microcoulombs (\(1\ \mu\text{C}=10^{-6}\) C) or nanocoulombs (\(1\ \text{nC}=10^{-9}\) C).
The equation
Charge comes in whole steps: \(q = ne\)
Because every electron and proton carries exactly \(e\), the charge of any object is just a whole number of these elementary charges. We say charge is quantized.
| Symbol | Meaning | SI unit |
|---|---|---|
| q | total (net) charge of the object — how much + or − it carries | C |
| n | number of extra electrons or protons — a whole number (1, 2, 3, …) | — (none) |
| e | the elementary charge, the fixed charge on one proton \(=1.60\times10^{-19}\) C | C |
Charge from a number of electrons
A small metal ball has gained \(n = 5.0\times10^{12}\) extra electrons. What is its total charge \(q\)?
✏️ Try it yourself
A plastic rod is rubbed with cloth and loses \(n = 2.5\times10^{13}\) electrons. What is the rod's charge — its size and its sign?
Hint: losing electrons leaves an object positive. Use \(q = ne\).
Conductors and insulators
Materials differ in how easily charge can move through them. This single difference decides almost everything about how we build electrical things.
| Conductor | Insulator | |
|---|---|---|
| Charge can… | move freely through it | stay stuck where it is put |
| Why | some electrons are loose and free to roam | every electron is held tightly to its atom |
| Examples | copper, gold, most metals, salt water | plastic, glass, rubber, dry air |
Three ways to charge an object
All three methods do the same thing — move electrons — but in different ways.
1. By friction (rubbing)
Rub two different materials together and electrons rub off one onto the other. The balloon on your hair works this way: the balloon ends up negative, your hair positive.
2. By contact (conduction)
Touch a charged object to a neutral conductor. Charge spreads across both until they share it. Both objects end up with the same sign as the original charge.
3. By induction (no contact)
You can charge a conductor without touching it, using only the field of a nearby charge. This is the clever one, so we draw it step by step.
Why a charged object attracts neutral things
Hold a charged rod near small pieces of paper and they jump to it — even though the paper has no net charge. Why?
Inside the neutral insulator, the rod's field pulls the + and − parts of each atom slightly in opposite directions. The atoms become tiny stretched pairs called dipoles; we say the material is polarized.
Now the side of the paper facing the rod has the opposite sign to the rod. Opposite charges are closer than like charges, so the pull wins over the push and the paper is attracted.
✏️ Try it yourself
You bring a positively charged rod near (but not touching) a metal sphere, connect the far side of the sphere to the ground for a moment, then disconnect the ground and finally remove the rod. What is the final sign of the sphere's charge?
Common mistakes
| Mistake | Fix |
|---|---|
| Saying protons move when an object charges. | Only electrons move. Lose electrons → +, gain electrons → −. |
| Thinking charging "creates" charge. | Charge is conserved — it is only moved. What one object gains, another loses. |
| Expecting induction to give the same sign as the rod. | Induction leaves the opposite sign to the rod; contact gives the same sign. |
| Saying a neutral object cannot be attracted. | It can — polarization puts the unlike charge nearest, and nearest wins. |
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| Two kinds | Charge is + or −; like repels, unlike attracts. |
| Source of charge | Atoms; moving electrons charges objects (lose → +, gain → −). |
| Conservation | Total charge never changes — it is only moved around. |
| Quantization | \(q = ne\), with \(e = 1.60\times10^{-19}\) C; charge comes in whole steps. |
| Materials | Conductor: charge moves freely. Insulator: charge stays put. |
| Charging | Friction, contact (same sign), induction (opposite sign, no touch). |
| Polarization | A field separates charge in a neutral object, so it is still attracted. |