---
title: "Conductors and Insulators"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 22
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/22-conductors-and-insulators
---

# Chapter 22 — Conductors and Insulators

Why are electric wires made of metal — but dressed in plastic? Why does the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch)’s little bridge conduct the current, while its casing keeps your fingers safe? Last year we closed loops; this year we ask which materials electricity can travel through at all. The answer sorts every material into two kinds: [conductors](#def-g4-conductors-insulators-conductor) and [insulators](#def-g4-conductors-insulators-insulator).

## 22.1 The tester

**Method 22.1 (Building a tester).**

Build the loop of last year — [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), wires, [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) — but leave one deliberate gap: two free wire ends, a finger’s width apart.

1. touch the two free ends together: the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — the tester works;
2. bridge the gap with the object on trial, one metal end pressed firmly to each wire;
3. watch the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) : [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) means electricity crossed the object; darkness means it could not.

The [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) gives the answer: it [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) exactly when the tested object completes the loop.

![The tester: a healthy loop with one gap. Whatever bridges the gap is being tested — the bulb shows whether current crosses it.](https://one-course.com/images/onecourse/chapters/physics-1/g4-conductors-insulators/fig-b8d9481c1123.svg)

*The tester: a healthy loop with one gap. Whatever bridges the gap is being tested — the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) shows whether current crosses it.*

## 22.2 Conductors and insulators

**Definition 22.2 (Conductor).**

A *conductor* is a material that lets electricity travel through it. In the tester, a conductor bridging the gap closes the loop and [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb).

**Definition 22.3 (Insulator).**

An *insulator* is a material that blocks electricity. Bridging the gap with an insulator leaves the loop as open as before: the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) stays dark.

**Example 22.4 (A morning of tests).**

Results from a classroom tester: a steel paperclip — [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). A coin — [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). Aluminium kitchen foil — [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). A key — [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). A plastic ruler — dark. A wooden pencil — dark. A glass marble — dark. A rubber eraser — dark. A strip of paper — dark. The pattern jumps out: the objects that [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) are all made of metal.

**Proposition 22.5 (Metals conduct).**

All metals conduct electricity — iron, copper, aluminium, gold, every one, which is why the [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet)’s fussiness about iron surprised us but the tester never will. Most other everyday [solids](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-solid) — plastic, wood, glass, rubber, paper, wax — are [insulators](#def-g4-conductors-insulators-insulator).

**Remark 22.6 (Conductor is not magnetic).**

Do not mix up the two metal talents. The [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) chooses only iron and its family; the electric current is far less picky and travels through *every* metal. The aluminium foil that ignored your [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) carries current beautifully — two different games, two different rules.

## 22.3 Conductors and insulators work together

**Example 22.7 (Anatomy of a wire).**

Look at the end of a lamp’s cable where it enters the plug: inside runs shiny copper — a [conductor](#def-g4-conductors-insulators-conductor), the current’s road; around it wraps colored plastic — an [insulator](#def-g4-conductors-insulators-insulator), the road’s fence. The copper does the work; the plastic keeps the current in and your fingers out. Every cable in your house pairs the two: a [conductor](#def-g4-conductors-insulators-conductor) inside an [insulator](#def-g4-conductors-insulators-insulator).

![An electric cable undressed: a copper road for the current, inside a plastic fence for safety.](https://one-course.com/images/onecourse/chapters/physics-1/g4-conductors-insulators/fig-64a469337ee1.svg)

*An electric cable undressed: a copper road for the current, inside a plastic fence for safety.*

![A cable’s cut end, for real: shiny copper strands — the conductor — inside their colored plastic sleeve — the insulator.](https://one-course.com/images/onecourse/chapters/physics-1/g4-conductors-insulators/img-8d572e6def44.jpg)

*A cable’s cut end, for real: shiny copper strands — the [conductor](#def-g4-conductors-insulators-conductor) — inside their colored plastic sleeve — the [insulator](#def-g4-conductors-insulators-insulator).*

**Example 22.8 (Reading everyday objects).**

Now many designs explain themselves. The [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): a metal bridge (it must carry current) in a plastic case (your finger must not). The electrician’s screwdriver: steel blade, thick insulating handle. The socket: metal deep inside, plastic face outside. Pylons carry bare metal cables high out of reach, hanging from long insulating discs so the current cannot leak into the tower.

**Remark 22.9 (Water, people, and why rules exist).**

One more [conductor](#def-g4-conductors-insulators-conductor) must be named: *damp things* — including damp human skin. We conduct badly, but the wall socket pushes so mightily that badly is more than enough. This is the deep reason behind the safety rules you know: never poke sockets, never touch anything electric with wet hands, keep hair dryers far from the bath. The [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) tester is safe *because* a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)’s push is tiny; the socket forgives nothing.

## 22.4 Exercises

**Exercise 22.1 ★.**

What is a [conductor](#def-g4-conductors-insulators-conductor)? An [insulator](#def-g4-conductors-insulators-insulator)? Which one [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the tester’s [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb)?

**Solution of Exercise 22.1.**

A [conductor](#def-g4-conductors-insulators-conductor) lets electricity travel through it; an [insulator](#def-g4-conductors-insulators-insulator) blocks it. The [conductor](#def-g4-conductors-insulators-conductor) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) — it closes the loop.

**Exercise 22.2 ★.**

Sort into [conductors](#def-g4-conductors-insulators-conductor) and [insulators](#def-g4-conductors-insulators-insulator): a steel spoon; a cork; kitchen foil; a glass; a copper coin; a plastic brick.

**Solution of Exercise 22.2.**

[Conductors](#def-g4-conductors-insulators-conductor): the steel spoon, the kitchen foil, the copper coin. [Insulators](#def-g4-conductors-insulators-insulator): the cork, the glass, the plastic brick.

**Exercise 22.3 ★.**

Before any trial, how do you check that the tester itself works? Why is this check important?

**Solution of Exercise 22.3.**

Touch the two free ends directly together: the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) must [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). If the tester itself is faulty, every object would seem to be an [insulator](#def-g4-conductors-insulators-insulator) — the check protects against blaming the innocent.

**Exercise 22.4 ★.**

A trial object leaves the [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) dark. Give two possible reasons — one about the object, one about sloppy testing.

**Solution of Exercise 22.4.**

Either the object is an [insulator](#def-g4-conductors-insulators-insulator) — or the testing was sloppy: a wire end touching paint, varnish or dirt instead of pressing firmly on the object’s bare stuff (or a connection came loose).

**Exercise 22.5 ★.**

Why is a lamp cable copper on the inside and plastic on the outside? What job does each material do?

**Solution of Exercise 22.5.**

The copper conducts: it is the current’s road to the lamp. The plastic insulates: it keeps the current in the wire and fingers safely out.

**Exercise 22.6 ★.**

In a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch), which part is a [conductor](#def-g4-conductors-insulators-conductor) and which an [insulator](#def-g4-conductors-insulators-insulator)? Why does it need both?

**Solution of Exercise 22.6.**

The little metal bridge conducts — it must close the loop when the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) is on. The case insulates — fingers press it all day. Without the bridge, no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source); without the case, no safety.

**Exercise 22.7 ★.**

Aluminium foil ignores a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) completely. Will it [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the tester’s [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb)? Which rule answers, and what mix-up does [Remark 22.6](#rem-g4-conductors-insulators-notmagnet) warn against?

**Solution of Exercise 22.7.**

Yes — aluminium is a metal, and all metals conduct. The warning: magnetic and conducting are different talents; the [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet)’s iron-only rule says nothing about whether it conducts.

**Exercise 22.8 ★.**

A pencil is wood with a graphite lead. Nino’s tester stays dark across the painted wood but [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) faintly tip-to-tip through the sharpened graphite. What did Nino discover about the pencil’s two materials?

**Solution of Exercise 22.8.**

The wood (and paint) is an [insulator](#def-g4-conductors-insulators-insulator), but the graphite lead inside is a [conductor](#def-g4-conductors-insulators-conductor) — a rare non-metal [conductor](#def-g4-conductors-insulators-conductor), running hidden through the pencil from tip to tip.

**Exercise 22.9 ★.**

Why do electricians’ screwdrivers and pliers have thick plastic handles? Which material is protecting whom?

**Solution of Exercise 22.9.**

The [insulator](#def-g4-conductors-insulators-insulator) is protecting the human: if the tool’s metal ever touches something electric, the thick plastic handle keeps the current from crossing into the hand.

**Exercise 22.10 ★★.**

A necklace chain gives “[light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)” in the tester; a wooden bead necklace gives “dark”; a necklace of metal beads knotted on a thick cotton string gives “dark” too — although every bead conducts! Explain the third result by following the current’s road bead to bead.

**Solution of Exercise 22.10.**

Current must travel the whole road. In the chain, metal link touches metal link: a complete metal road. In the beaded necklace, between one metal bead and the next lies a stretch of cotton knot — an [insulator](#def-g4-conductors-insulators-insulator) — so the road is broken at every bead: dark, despite all those [conductors](#def-g4-conductors-insulators-conductor).

**Exercise 22.11 ★★.**

Why is it far more dangerous to touch electrical things with wet hands than with dry ones? And why is the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) tester safe for your experiments while the socket never is? Use [Remark 22.9](#rem-g4-conductors-insulators-safety).

**Solution of Exercise 22.11.**

Damp skin conducts much better than dry skin, so wet hands offer the current an easy road through the body. The [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) tester is safe because a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)’s push is tiny; the wall socket’s push is hundreds of times mightier — through damp skin it can be deadly, which is why the rules allow [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) games and forbid sockets, always.