Electrical Conductivity

Introduction

Every electrical device depends on materials that let electricity flow and materials that stop it. Copper wires carry current into your home; plastic coatings on those wires keep you safe from shocks. Electrical conductivity is the physical property that describes how easily an electric current passes through a material. Testing it is one of the quickest ways to tell a metal from a non-metal.

Core explanation

Conductors and insulators. An electrical conductor allows current to flow easily. All metals are conductors: silver is the best, followed closely by copper, gold and aluminium. Graphite, a form of carbon, is the notable non-metal conductor. Electrical insulators — plastics, rubber, glass, dry wood, ceramics, sulfur and pure water — allow almost no current to flow.

What current is. An electric current is a flow of charged particles. In metals and graphite, the moving particles are electrons that are free to move through the structure. Insulators have no free charged particles, so no current flows even when a voltage is applied.

Testing conductivity. A simple test circuit contains a cell (battery), a lamp or ammeter, connecting wires and two contacts. The sample is placed between the contacts to complete the circuit. If the lamp lights (or the ammeter shows a current), the sample conducts. If not, it is an insulator. For safety, only low-voltage cells are used.

Solids, liquids and solutions. Some substances conduct only in certain states. Solid sodium chloride does not conduct, but molten sodium chloride and salt solution do, because their ions are free to move. Pure water barely conducts; tap water conducts slightly because of dissolved ions. You will study these in the bonding and electrolysis units.

Semiconductors. Silicon and germanium conduct electricity poorly at low temperatures but better when warmed or when tiny amounts of other elements are added. These semiconductors are the foundation of computer chips and solar cells.

Step-by-step reasoning

To test whether an unknown solid conducts electricity:

1. Build a circuit with a low-voltage cell, a lamp and two crocodile clips. 2. Touch the clips together to check that the lamp lights (the circuit works). 3. Attach the clips to opposite ends of the sample. 4. Observe the lamp: lit means conductor, unlit means insulator. 5. Record the result and compare with other samples.

Visual explanation

A circuit diagram shows a cell, a lamp and a gap between two contacts. Three versions follow: with a copper strip in the gap, the lamp glows brightly; with a pencil lead (graphite), it glows; with a plastic ruler, it stays dark. A particle diagram shows electrons drifting through a metal lattice from the negative to the positive terminal.

Real-world analogy

A conductor is like a motorway with open lanes: traffic (electrons) flows freely. An insulator is like a road blocked by a locked gate: however many cars are waiting, none can pass. A semiconductor is like a road where the gate opens only under certain conditions.

Real-world example

Electricians use rubber-soled boots and gloves because rubber is an insulator. Cables are made with copper cores, for excellent conductivity, and PVC plastic sheaths, to prevent current from reaching people or other wires. Overhead power lines use bare aluminium wires held on ceramic or glass insulators.

Why?

Why is silver not used for household wiring even though it is the best conductor? Silver is much more expensive than copper and conducts only slightly better. Copper provides almost the same performance at a fraction of the cost, so it is the practical choice; silver is reserved for specialised contacts.

Common misconception

"Water is a good conductor of electricity." Pure water is a very poor conductor. Water in everyday life conducts because of the dissolved salts in it. That is still dangerous — tap water, sweat and sea water conduct well enough to cause electric shocks — so electrical devices must never be handled with wet hands.

Worked example

Question: In a conductivity test, the lamp lights with an iron nail, a graphite rod and an aluminium strip, but not with a glass rod or a piece of sulfur. Which samples are metals?

Reasoning: Metals conduct, but graphite also conducts and is a non-metal. Glass and sulfur are insulators.

Answer: Iron and aluminium are metals; graphite conducts but is a non-metal; glass and sulfur are insulators.

Quick check

1. Name the non-metal that conducts electricity. Answer: Graphite (a form of carbon).

Exam focus

Remember that all metals conduct electricity in the solid and liquid states, and graphite is the key non-metal exception. Explain conduction in terms of free (delocalised) electrons. Describe the circuit test clearly, including checking the circuit first.

Advanced insight

The conductivity of metals decreases as temperature rises, because vibrating atoms scatter the moving electrons more. Semiconductors behave the opposite way: warming frees more electrons, so they conduct better. At very low temperatures, some materials become superconductors with zero electrical resistance — the basis of MRI scanner magnets.

Summary

Electrical conductors allow current to flow; insulators do not. Metals and graphite conduct because they contain free electrons. A simple lamp-and-cell circuit tests conductivity. Ionic substances conduct when molten or dissolved, pure water barely conducts, and semiconductors such as silicon conduct under certain conditions and power modern electronics.

Practice questions

1. Why are electrical wires coated in plastic? Answer: Plastic is an electrical insulator, so it prevents current from passing to people or other wires. 2. What particles carry the current in a metal wire? Answer: Free (delocalised) electrons. 3. A student says "all conductors are metals". Give a counter-example. Answer: Graphite conducts electricity but is a non-metal. 4. Why must you check that the lamp lights before testing a sample? Answer: To make sure the circuit works, so an unlit lamp can be trusted to mean the sample is an insulator.