D. Physics: Principles and Applications of Electricity · Electricity: Charges, Circuits and Energy

Conductors, Insulators and Testing Conductivity

D2.2 D2.1 A1.2 A1.5About 70 minutes

Learning goals

I can explain the difference between conductors and insulators using the movement of electrons.
I can explain why insulators hold static charge and conductors transfer it.
I can test the conductivity of materials using an electroscope and a continuity tester, controlling variables.
I can describe semiconductors and give examples of how conductors and insulators keep us safe.

Words to know

conductor
A material that lets electric charge move through it easily, such as copper or aluminium.

insulator
A material that does not let electric charge move through it easily, such as rubber, plastic or glass.

conductivity
How well a material allows electric charge to move through it.

free electrons
Electrons in a metal that are not tied to one atom and can drift through the metal.

semiconductor
A material, such as silicon, whose conductivity is between that of conductors and insulators and can be controlled. Computer chips are made from semiconductors.

continuity tester
A simple circuit with a gap. If a material placed across the gap lets current through, the lamp or LED lights.

electrical insulation
An insulating covering, like the plastic around a wire, that keeps charge in the conductor and protects people.

A puzzle

You can easily charge a plastic ruler by rubbing it with wool. But try rubbing a metal spoon while holding it in your hand, and it will not pick up a single bit of paper. Is metal impossible to charge?

No! The metal does get charged for an instant, but the charge escapes straight away through the metal, through your hand, and into the ground. The difference between the ruler and the spoon is conductivity.

Conductors and insulators at the atomic level

In metals, each atom lets go of one or more outer electrons. These free electrons are not tied to any one atom. They can drift through the whole piece of metal. So when extra charge is added to a metal, it moves and spreads out quickly. Metals are conductors.

In insulators like plastic, rubber and glass, electrons are held tightly by their own atoms. Extra charge placed on an insulator stays where it was put. That is why insulators are the best materials for building up static charge.

A model of a conductor and an insulator. In a metal, positive ions sit in a regular pattern and free electrons (blue) drift between them. In an insulator, each atom keeps its electrons (green) close, so charge cannot travel.

	Conductor	Insulator
Electrons	Free to move through the material	Held tightly by atoms
Static charge	Spreads over the whole surface; escapes quickly if grounded	Stays where it is placed; good for holding static charge
Current	Lets current flow through it	Blocks current
Examples	Silver, copper, gold, aluminium, graphite, salt water	Rubber, plastic, glass, dry wood, dry air, pure (distilled) water
Uses	Wires, lightning rods, cooking-pot bottoms	Wire coatings, tool handles, safety gloves

Comparing conductors and insulators.

It is a range, not two boxes

Conductivity is a scale. Silver and copper are excellent conductors. Plastic and rubber are excellent insulators. In between are semiconductors such as silicon. Engineers can control how well a semiconductor conducts by adding tiny amounts of other elements, which is why semiconductors are used to make computer chips, LEDs and solar cells.

Some materials change. Pure water is a poor conductor, but tap water, lake water and sweat contain dissolved substances that form ions, so they conduct. That is why electrical devices and water are a dangerous mix. Even air, normally an insulator, can conduct for a moment when the charge is huge, as in a spark or lightning.

Conductivity is a range. Semiconductors such as silicon sit between insulators and conductors.

Two ways to test conductivity

1. Static test (hold or transfer charge?). Charge an electroscope so its leaves spread. Hold the test material in your hand and touch it to the knob.

If the leaves fall, charge flowed through the material and your body to the ground: the material is a conductor.
If the leaves stay apart, charge could not move through it: the material is an insulator.

2. Current test (continuity tester). Build a circuit with a cell, a lamp or LED and two probes with a gap. Place the material across the gap.

The lamp lights: current flows, so the material is a conductor.
The lamp stays off: the material is an insulator (or a very poor conductor).

A continuity tester as a circuit diagram. The circuit is complete only if the sample between the two test probes lets charge through.

Worked example

Example 1: Planning a fair test

Amira wants to compare how well 6 materials conduct using a continuity tester. Name the independent variable, the dependent variable and three controlled variables.

Independent variable (what she changes on purpose): the material being tested.
Dependent variable (what she observes): whether the LED lights, and how brightly.
Controlled variables (kept the same): the same cell and LED, the same distance between the probes, and samples of similar size and thickness, all clean and dry.

Changing only the material makes it a fair test, so any difference in the LED must be caused by the material.

Worked example

Example 2: Explaining an observation

A student rubs a copper rod held in her bare hand with wool. It does not attract paper. She then holds the copper rod by a plastic handle and rubs again. Now it attracts paper. Explain.

Copper is a conductor. When it was held by hand, the charge from rubbing flowed through the copper and through her body to the ground.
Plastic is an insulator. The plastic handle stops the charge from escaping.
The charge stays on the copper, so it attracts the neutral paper (induced charge separation).

The plastic handle insulates the conductor from the ground, so the copper can keep its charge.

Safety in the real world

Wires are copper (a conductor) wrapped in plastic (an insulator). Never use a cord with cracked or missing insulation.
The round third pin on a Canadian plug is the ground connection. If a fault happens, it gives the current a safe path to the ground.
Never touch switches or devices with wet hands: water with dissolved substances conducts.
Lineworkers and electricians (skilled trades) use insulated gloves and tools rated for the job.

Hands-on activity

Build a continuity tester and test household materials

You need: one 3 V coin cell (size CR2032), one LED (from an electronics kit or an old light-up toy), two strips of aluminium foil about 2 cm × 15 cm, tape, and test items: a paper clip, a coin, a key, a rubber band, a plastic spoon, a wooden craft stick, an eraser, a small glass jar, a pencil with a sharpened point, paper.

Fold each foil strip lengthwise into a thin, flat "wire".
LEDs only work one way. Tape one foil wire to the longer LED leg and the other foil wire to the shorter leg.
Touch the foil wire from the long leg to the + side of the coin cell and tape it there. Leave the other foil wire free: its end is one probe.
Tape a third short foil strip to the − side of the cell as your second probe. Touch the two probes together: the LED should light. If not, flip the cell.
Predict (conductor or insulator?) for each item, then touch both probes to it, 2 cm apart. Record on, dim or off in a table.

What to look for: metal items (paper clip, coin, key) light the LED. Rubber, plastic, wood, paper and glass do not. Pencil graphite may make a very dim glow or none at all: graphite is a conductor, but a weak one.

Safety (grown-up note): Coin cells are very dangerous if swallowed. Keep the cell and LED away from young children and pets and put them away right after the activity. Never use wall outlets for any test.

Just for fun

Pencil "lead" contains no lead at all. It is mostly graphite, a form of carbon, and graphite conducts electricity. A thick, dark line drawn with a soft pencil can act as a very weak "wire". Your math homework might be more electric than you thought.

Your turn

Explain, in terms of electrons, why copper is a conductor.

Why are balloons and plastic rulers good for static electricity experiments?

A charged electroscope is touched with a wooden pencil's metal end-band held in your fingers. The leaves fall. What does this tell you?

Give two examples of semiconductors and one use.

Why are electrical cords made of copper wire covered in plastic?

Is tap water a conductor or an insulator? Explain.

More practice: warm-up

Sort: aluminium foil, rubber band, silver ring, glass marble.

In a continuity tester, what does it mean if the lamp lights?

Which particles move through a metal wire?

What is the third round pin on a Canadian plug for?

Name a material used for safety gloves for electrical work.

More practice: on your own

A metal sphere and a plastic sphere are each touched at one point by a negative rod. Describe where the charge goes on each.

In a conductivity test, one student uses a 10 cm long sample and another uses a 2 cm sample. Why could this make the comparison unfair?

Why is it dangerous to fly a kite near power lines, even if the string is dry?

A student says, "Insulators cannot be charged." Correct the statement.

Why are computer chips made of semiconductors and not of copper or plastic?

Challenge

Design an investigation to rank 5 metals (copper, aluminium, steel, brass, nickel) by conductivity. A simple lamp tester will light for all of them. What could you measure instead, and what must you control?

Explain why a bird can sit on a bare power line safely, but a person on a ladder touching the same line could be hurt.

Static charge on clothes can be reduced with dryer sheets or by adding moisture to the air in winter. Use the idea of conductivity to explain why these work.

Check your understanding

Quiz

Why is copper a good conductor?

Which is the best insulator?

A charged electroscope's leaves stay apart when touched with a material held in your hand. The material is:

Silicon is an example of a:

Which is a controlled variable when testing conductivity of different materials?

Why can a rubbed balloon keep its charge for a long time?

For parents and teachers

Big idea: Expectation D2.2 asks students to determine the conductivity of materials by investigating their ability to hold or transfer charge. Insulators hold charge; conductors transfer it. The activity also practises fair testing (A1.2) and safe procedures (A1.5).

Common difficulties:

"Insulators can't be charged." They can, and they hold charge best.
Forgetting controlled variables, such as sample size.
Water: pure water is a poor conductor, but the water we meet every day (tap water, sweat) conducts.

Safety: Coin cells are extremely dangerous if swallowed; store them out of reach of young children and pets. Never experiment with wall outlets.