LessonWhat electricity is · 12 min
Charge, electrons and conductors
Every phone, solar charge controller and motorbike ignition works by moving one thing around a loop: electric charge. Before any formula, it pays to know what that charge is and why it moves easily through some materials and not at all through others.
Atoms, electrons and charge
All matter is made of atoms. Each atom has a small, heavy nucleus of protons (positive charge) and neutrons (no charge), surrounded by much lighter electrons (negative charge). A normal atom has as many electrons as protons, so it is neutral. Remove some electrons and it is positively charged; add some and it is negative.
- Like charges repel (− pushes −), unlike charges attract (− pulls +).
- Charge is measured in coulombs (C). One electron carries 1.6 × 10−19 C, so one coulomb is about 6.24 × 1018 electrons - six billion billion.
- Charge is never created or destroyed, only moved. This is why current that goes out of a battery must come back to it.
- Q
- the charge, in coulombs (C)
- n
- the number of electrons
- e
- the charge of one electron, 1.6 × 10−19 C
Conductors and insulators
In a metal, the outermost electron of each atom is not tied to its atom: it wanders freely through the metal. A piece of copper is full of these free electrons. Push on them from one end and they all shuffle along together - that is an electric current. A material like this is a conductor.
In plastic, rubber, glass or dry wood every electron is held tightly by its atom. Almost nothing can flow: these are insulators. Between the two are semiconductors such as silicon, whose ability to conduct can be controlled - the basis of diodes, transistors and every chip (later courses).
| Material | Type | Where you meet it |
|---|---|---|
| Copper | Excellent conductor | Almost every wire, PCB track and motor winding |
| Aluminium | Good conductor (lighter, cheaper) | Overhead power lines, large cables |
| Silver, gold | Excellent conductors | Plating on connectors and switch contacts |
| Nichrome | Poor conductor (on purpose) | Heating elements in irons, kettles, hair dryers |
| Silicon | Semiconductor | Diodes, transistors, solar cells, chips |
| PVC, rubber, glass, ceramic | Insulators | Cable sheaths, switch bodies, insulators on poles |
| Air (dry) | Insulator | The gap in an open switch - until lightning-level voltages |
Where you meet it
A copper core for the current, a PVC sheath so it cannot leak out or touch you.
Thin copper tracks on an insulating fibreglass board (FR-4) connect every part.
Rubbing moves electrons from one material to another: the spark when you touch a car door, or lightning on a much bigger scale. Static can destroy chips - hence anti-static bags and wrist straps.
Practice 1How much charge do 2 × 1019 electrons carry?
Hint
Q = n × e, with e = 1.6 × 10−19 C.
Show the answer
Answer: 3.2 C
Q = 2 × 1019 × 1.6 × 10−19 = 3.2 C.
Practice 2Which material would you use for a switch's contacts?
- Rubber
- Silver-plated copper
- Glass
- Dry wood
Show the answer
Answer: B. Silver-plated copper
Contacts must conduct: silver-plated copper. The other three are insulators - good for the switch's body and button, not its contacts.
Practice 3A plastic comb rubbed on dry hair picks up small bits of paper. What has happened?
- The comb has become a conductor
- Electrons moved between hair and comb, leaving the comb charged
- The comb has made new charge
- The paper is magnetic
Show the answer
Answer: B. Electrons moved between hair and comb, leaving the comb charged
Rubbing moves electrons from one material to the other. The comb is now charged (charge was moved, not created) and attracts the neutral paper.
Challenge 4A lightning stroke moves about 5 C of charge. How many electrons is that, in units of 1018 (so 1 × 1018 is answered as 1)?
Hint
n = Q / e.
Show the answer
Answer: 31.25 × 10¹⁸ electrons
n = 5 / 1.6 × 10−19 = 3.125 × 1019 = 31.25 × 1018 electrons. Huge numbers of electrons are behind even everyday amounts of charge.