Materials such as copper metal that conduct electricity (allow it to flow freely) are called conductors. Materials that don’t allow electricity to pass through them so readily, such as rubber and plastic, are called insulators. What makes copper a conductor and rubber an insulator?
A current of electricity is a steady flow of electrons. When electrons move from one place to another, round a circuit, they carry electrical energy from place to place like marching ants carrying leaves. Instead of carrying leaves, electrons carry a tiny amount of electric charge.
Electricity can travel through something when its structure allows electrons to move through it easily. Metals like copper have “free” electrons that are not bound tightly to their parent atoms. These electrons flow freely throughout the structure of copper and this is what enables an electric current to flow. In rubber, the electrons are more tightly bound. There are no “free” electrons and, as a result, electricity does not really flow through rubber at all. Conductors that let electricity flow freely are said to have a high conductance and a low resistance; insulators that do not allow electricity to flow are the opposite: they have a low conductance and a high resistance.
For electricity to flow, there has to be something to push the electrons along. This is called an electromotive force (EMF). A battery or power outlet creates the electromotive force that makes a current of electrons flow. An electromotive force is better known as a voltage.
Direct current and alternating current
Electricity can move around a circuit in two different ways. In the big picture up above, you can see electrons racing around a loop like race cars on a track, always going in the same direction. This type of electricity is called direct current (DC) and most toys and small gadgets have circuits that work this way.
Artwork: Top: In a direct current (DC) circuit, electrons always flow in the same direction. Bottom: In an alternating current (AC) circuit, the electrons reverse direction many times each second.
The bigger appliances in your home use a different kind of electricity called alternating current (AC). Instead of always flowing the same way, the electrons constantly reverse direction—about 50–60 times every second. Although you might think that makes it impossible for energy to be carried round a circuit, it doesn’t! Take the flashlight bulb in the circuit above. With direct current, new electrons keep streaming through the filament (a thin piece of wire inside the bulb), making it heat up and give off light. With alternating current, the same old electrons whiz back and forth in the filament. You can think of them running on the spot, heating up the filament so it still makes bright light we can see. So both types of current can make the lamp work even though they flow in different ways. Most other electric appliances can also work using either direct or alternating current, though some circuits do need AC to be changed to DC (or vice versa) to work correctly.