Magnetic and electrostatic force

Does a force always need contact? No. Some forces work across a gap. They are non-contact forces. Think of the difference between a superhero who has to touch or hit to move something, and one who moves it with a wave of the hand from far away.

From Grade 6: a magnet pulls things made of magnetic materials, and between two magnets, like poles repel and unlike poles attract. Attraction and repulsion are pulls and pushes, so they are forces.

Floating magnet (Activity 5.5). Stand a stick upright, slide a ring magnet onto it, then a second ring magnet above it with like poles facing. The top magnet floats in the air. Press it down gently and you feel it push back. Turn both magnets round and it no longer floats.

So a magnet can push or pull another magnet without touching it. The force a magnet puts on another magnet or a magnetic material is the magnetic force, a non-contact force.

Static (Activity 5.6). Rub a plastic scale or straw hard with polythene (don't touch the rubbed part) and hold it near tiny bits of paper. The bits jump up and cling to it.

Rubbing certain materials together builds up electric charges on their surfaces. These charges stay put, so they are called static charges, and the object is said to be charged. A charged object can pull on uncharged things such as paper, without touching them. (Neutral doesn't mean an object has no charge at all: it means its positive and negative charges are equal.)

Two balloons (Activity 5.7). Hang two blown-up balloons so they don't touch, rub both with a woollen cloth, and let go. They move apart. Now bring the woollen cloth near one balloon: they pull together.

So like charges repel and unlike charges attract. The two kinds of charge are called positive and negative. The force a charged body puts on another body, charged or not, is the electrostatic force, also a non-contact force. And when charges move, they make an electric current, the same current that lit lamps and made magnets in the last chapter.