Look at a phone charger cable. Inside is copper metal, and around it is a layer of plastic. Why both? The copper lets charge flow easily to your phone. The plastic stops that charge from flowing into your hand. Copper is a conductor; plastic is an insulator. The same idea explains why a metal doorknob gives you a shock, why lightning strikes tall objects, and why a power line technician wears thick rubber gloves. In this lesson you will see what is different inside these materials and how we use that difference to stay safe.
Why electrons move easily in some materials
An electrical conductor is a material that lets electric charge move through it easily. In a metal, each atom's outermost electrons are held so loosely that they leave their atoms and drift freely among the fixed positive metal ions. These free electrons can carry charge from one end of a wire to the other. An electrical insulator is a material that does not let charge move easily, because its electrons are held tightly by their own atoms. Semiconductors, like silicon, are in between: they conduct a little, and engineers can control how much. That is why silicon is used in computer chips, LEDs and solar panels. The ability of a material to let charge flow is called its conductivity.
| Type | Examples | Where you see it |
|---|---|---|
| Good conductors | Silver, copper, gold, aluminium, other metals | Wires (usually copper), power lines (usually aluminium), connectors |
| Fair conductors | Salt water, tap water, the human body, graphite (pencil lead), damp soil | Why wet hands and electricity are dangerous; why the ground can take charge |
| Semiconductors | Silicon, germanium | Computer chips, LEDs, solar cells |
| Insulators | Rubber, glass, plastic, dry wood, dry air, porcelain | Wire coatings, tool handles, gloves, the glass or ceramic insulators on power poles |
Here is a surprise: insulators are often the objects that hold static charge best. When you rub a plastic comb, the extra electrons cannot flow away through the plastic, so they stay put. If you rub a metal spoon while holding it in your hand, the charge flows through the metal and your body and is lost. A conductor only holds a static charge if it is isolated on an insulator, like the metal parts of an electroscope sitting in an insulating stopper.
Grounding and discharge
Grounding means connecting a charged object to the Earth with a conductor. The Earth is so huge that it can give or take almost any number of electrons without its own charge changing noticeably. A negative object that is grounded loses its extra electrons to the Earth; a positive object that is grounded gains electrons from the Earth. Either way, it becomes neutral. When charge builds up enough, it can even jump through air, which is normally an insulator. The charge tears electrons off air molecules and makes a hot, glowing path for a moment: a spark. The doorknob shock is your charged body discharging to the metal knob.
Lightning
Lightning is static discharge on a giant scale. Inside a thunderstorm cloud, strong air currents make ice crystals and small hail pellets collide, and electrons are transferred. The bottom of the cloud usually becomes negative and the top positive. The negative cloud base repels electrons in the ground below, so by induction the ground's surface becomes positive. When the charge difference is big enough, the air breaks down and a huge current flashes between cloud and ground (or within the cloud). The lightning channel heats the air to about 27 000 °C, roughly five times hotter than the Sun's surface. The air expands so fast that it makes the boom we call thunder. A lightning rod, invented by Benjamin Franklin in the 1750s, is a metal rod on top of a building connected by a thick wire to a metal plate buried in the ground. If lightning strikes, the current follows this easy path into the ground instead of through the building.
Grounding and safety in your home
- The third prong. On a three-prong plug, the round pin is the ground. It connects the metal case of an appliance (like a toaster or washing machine) to the Earth. If a loose live wire touches the case, the charge flows through the ground wire instead of through you, and the large current trips the breaker. Never cut off the ground prong to fit a two-hole outlet.
- GFCI outlets. A ground fault circuit interrupter (the outlet with TEST and RESET buttons) checks that the current going out to a device equals the current coming back. If a little current is missing, about 5 thousandths of an ampere, it may be leaking through a person, so the GFCI cuts the power in a fraction of a second. Building codes require them near water: bathrooms, kitchen counters near sinks and outdoors.
- Gas stations. Touch a metal part of the car before you handle the fuel nozzle, and do not get back into the car while fuelling, so a static spark is less likely to ignite gasoline vapour.
- Electronics. Technicians wear a grounded anti-static wrist strap so a static discharge from their body does not damage computer chips.
- Wet skin. Dry skin resists current; wet or sweaty skin lets far more through. Keep electrical devices away from bathtubs, sinks and pools.