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All matter is made up of atoms. At the centre of each atom is a heavy nucleus. Surrounding the nucleus is a lot of empty space and tiny particles called electrons. Electrons are constantly moving around the nucleus. Each electron carries a negative electric charge.

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Inside the nucleus are two different types of particles. The protons and neutrons inside the nucleus are much heavier than electrons. Each proton carries a positive electric charge. Neutrons are similar to protons but carry no electric charge. The positive electric charge of a proton exactly balances the negative charge of an electron. Atoms usually contain an equal number of electrons and protons.

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Any particle or substance that has more protons than electrons is said to be positively charged. Any particle or substance that has more electrons than protons is said to be negatively charged. Any particle or substance that has equal amounts of positive and negative charge is said to be uncharged or neutral.

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Substances usually become charged by the addition or removal of electrons. This can be done in two ways: by friction or by contact.

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The rubbing of one neutral substance against another adds or removes electrons. When you rub a plastic ruler with wool, electrons are moved from the neutral wool onto the neutral plastic ruler. The wool, having lost electrons, becomes positively charged. The plastic ruler, having gained electrons, becomes negatively charged.

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If a neutral substance is touched by a charged object, electrons can move to or from the charged object. When the charged object is removed, the previously neutral substance has gained or lost electrons.

Electrons are the easiest particles to add to or remove from atoms, because they are not held together in the nucleus as protons are.

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The area around an electrically charged object where objects experience an electric force is called an electric field. The strength of the electric field increases with the size of the charge producing it and decreases with distance. When two charged particles are near one another, their electric fields interact to produce forces of attraction or repulsion.

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The electricity that builds up on plastic rulers, balloons and the Van de Graaff generator is called static electricity. The word ‘static’ means standing still. The charge on the objects is called electrostatic charge (or static charge). Static charge can leak slowly through substances such as rubber and air. Substances through which electric charge cannot move quickly are called insulators. Static charge builds up easily in insulators. Substances through which electric charge flows easily are called conductors. Metals are good conductors of electricity. Static charge doesn’t build up in conductors.

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Objects with the same static charge repel each other while those with opposite charges attract each other. If sufficient charge builds up in oppositely charged objects, the attraction between the electric charges is so great that they can jump across small air gaps. Lightning is caused by the movement of electric charge between a cloud and the ground. However, the clouds and ground are both neutral! Lightning seems to show that electric charge can move between neutral objects as well as between oppositely charged objects.

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Charged objects and neutral objects can be attracted to each other. A charged plastic pen attracts a neutral stream of water. A charged balloon sticks to a neutral wall. A charged comb will make dry hair stand up.

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This effect can be seen when the negatively charged pen is close to the paper, electrons are repelled from the pen, leaving the top surface of the paper with a positive charge. Note that the whole piece of paper is still neutral. If there is enough charge and the pen is close enough to the paper, the attraction is great enough to pull the paper up. Once the paper is touching the pen, the charge moves across and arranges itself so that it is evenly spread out.

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Charged and neutral objects can be attracted to each other, such as a. paper and b. water to a charged pen

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Static electricity is not always a pest. For example, photocopying machines use static electricity to make copies. Electrostatic speakers are used with some stereo equipment. Electrostatic attraction can be used to separate light particles from other substances. In a chimney, smoke particles are charged as they move past a metal grid. They are then attracted to the sides of the chimney and form a layer of soot instead of passing out to pollute the air.