5.1 Mobile charge carriers in metals

A metal contains positive ions arranged in a lattice and delocalised electrons that can move through the structure. Without an electric field, electron motion is random and there is no net current. When a potential difference is applied, electrons acquire a small drift velocity opposite to conventional current.

Electrical energy is transferred through the circuit much faster than individual electrons travel from source to load. The electric field is established around the circuit rapidly, causing electrons throughout the conductor to begin drifting.

5.2 Insulators

In an insulator, electrons are bound to atoms or molecules and cannot move freely through the material under ordinary fields. Charge placed on an insulating surface tends to remain localised. Plastic cable coatings, glass supports and ceramic insulators prevent unwanted current.

No material is a perfect insulator under every condition. A sufficiently high voltage can cause breakdown, and surface moisture or contamination can provide a conducting path. Statements about conductor and insulator therefore refer to normal conditions.

5.3 Conductivity and resistance

A conductor allows charge to flow readily but still has resistance. Copper is used for connecting wires because it has low resistivity, is ductile and is comparatively economical. Heating elements use materials with higher resistance and high melting point so electrical energy is converted to thermal energy safely.

Graphite conducts because it has mobile electrons within layers, even though it is a non-metal. Semiconductors have conductivity between that of good conductors and insulators, and their resistance can be strongly affected by temperature, light or added impurities.

5.4 Charge on conductors

Excess charge on an isolated conductor moves until electrostatic equilibrium is reached. It lies on the outer surface and is more concentrated at sharp points, where the electric field is strongest. This can encourage corona discharge or sparking.

A hollow conductor can shield its interior from external static electric fields. This principle underlies a Faraday cage. A closed metal car body can offer protection during lightning because charge travels mainly over the outside, though touching external metal and leaving the vehicle can create risks.

5.5 Testing materials

A simple low-voltage circuit containing a cell, lamp or LED, and a gap can test a sample. If the sample completes the circuit and the indicator responds, it conducts. An ammeter gives a more sensitive and quantitative test than a lamp.

The test voltage must be safe and suitable for the component. An LED requires correct polarity and a current-limiting resistor. The same dimensions should be used when comparing materials because resistance depends on length and cross-sectional area as well as material.

5.6 Moisture, safety and everyday examples

Pure water conducts poorly, but tap water and body fluids contain ions and conduct. Wet skin therefore has lower resistance than dry skin, increasing current for a given voltage. This is why electrical equipment must be kept away from water and protected by suitable devices.

Air is normally an insulator, but high electric fields ionise it. Lightning is a large-scale discharge through ionised air. Cable insulation must be chosen for voltage, temperature and environmental conditions rather than merely thickness.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. Why does copper conduct electricity?
2. Why does wet skin increase electrical danger?
3. Where does excess static charge reside on an isolated conductor?