Alice Moran of Queenstown asks :-

Why does my long hair stick up after I have brushed it?

John Campbell, a physicist at the University of Canterbury responded.

Because of a fundamental observation that like electrical charges repel.

As Ernest Rutherford showed, atoms consist of a heavy nucleus surrounded by orbiting electrons. In some materials, electrical insulators, the atoms form combinations (molecules) which lock the electrons to that molecule. In others, electrical conductors, an electron is free to wander away from each atom or molecule.

There are two sorts of electrical charge which historically we call positive (an excess of atoms and/or molecules which have lost an electron or, put another way, a shortage of electrons) and negative (an excess of electrons).

When two non-conductors of electricity are rubbed together, some electrons transfer from one object to the other causing both to become electrically charged, one negative (an excess of electrons) and one positive (a shortage of electrons).

When you brush your dry hair with a plastic comb or brush they each take on an opposite charge. So each hair takes on the same type of charge. Your hair does conduct electricity though weakly, depending on whether or not it has just been cleaned and what residual shampoos etc coat it. You observe your hairs trying to stand on end. What the electrical charges on the hair are trying to do is to get as far away from each other as possible. And that is when the charge has flowed to the ends of the hairs and they are as far apart as it is possible to get.

This demonstrates a fundamental fact about electrical forces - like charges repel.

Electrical insulators can be arranged in a list with one end the one most easily charged positively and the other end the one most easily charged negatively - the triboelectric list. The strongest effect is obtained with two materials on opposite sides of the list, eg wool (positive) and plastic (negative).

You may also see this effect when taking a woollen sweater off whilst wearing a synthetic undershirt such as polyester. You may hear the crack of a spark between the two. If the room is dark you may see the sparks.

Furthermore, unlike charges attract. You can also show this because in giving your hair one type of charge the comb becomes charged with the other type. (Overall your hair and comb are electrically neutral.) So when your hair is charged (ie standing out) watch in a mirror as you bring the comb or brush near. The hair is attracted towards the comb. You can enhance this by charging the comb even more, by rubbing it on wool after combing your hair.