We know all atoms are made up of three kinds of particles, called electrons, protons and neutrons. Electrons are the tiniest of these, and negatively charged. Protons and neutrons have about the same mass, but protons are positively charged, while neutrons have no charge.
In most objects there are same number of electrons as protons. So normally an object has no overall charge, because the positive charge on all the protons is matched by the negative charge on the electrons. If there are more electrons than protons the object carries an overall negative charge. If there are fewer electrons than protons, the object carries an overall positive charge.
Every proton and electron produces an electric field. So around any object in which the charges are not balanced, there is an electric field. When a charged particle moves into the field, it feels a force towards or away from the other particle. The strength of the force depends on how close the particles are and how much electrical charge they carry. If particles are closer then the force between them is larger. Similarly the more charge, the larger the force.
When an unbalanced charge collects on the surface of an object, the charge is called static charge(‘Static’ means ‘not moving’). When electrons move, or flow, from one place to another, they produce an electric current.
As you can see, these forces, which are called electrostatic forces, look rather similar to magnetic forces. They are however completely different. An electric field does not affect a magnet in any special way, and a magnetic field does not affect an electric charge (so long as it is not moving). You can even have a space that contains both types of field in different directions at the same time.
Electric fields and electrostatics feature in our lives. They can be useful: electrostatic scrubbers remove the dust from the smoke of coal power stations, and photocopiers use electrostatics to move the ink powder to the right place on the paper. But they can also be harmful. You may have noticed that you can get a nasty spark from your finger if you touch a metal object after rubbing your feet on a nylon carpet. For this reason, aircraft are connected to the ground by a special wire before refueling; any electrostatic charges can thus flow away safely to earth and not cause a spark. The charges might be built up by friction between the fuel and the fuel pipe.
When you charge an object you are giving or taking away negatively charged electrons, so that the charge on the object overall is unbalanced. For example, when you rub a glass rod with a cloth, electrons from the rod get rubbed onto the cloth (see diagram below). So the cloth becomes negatively charged overall, and the rod is left with an overall positive charge. When you rub a polythene rod with a cloth, electrons from the cloth get transferred to the rod, so the polythene carries a negative charge overall, and the cloth carries a positive charge.
If you suspended charged polythene and glass rods so they could move freely, and brought the two close together, they would attract each other, since unlike charges attract. Both the polythene and the acetate rod would attract small bits of paper or dust, because they give each bit an opposite charge by induction.
Materials like glass, acetate and polythene can only become charged because they are insulators. Electrons do not move easily through insulating materials, so when extra electrons are added, they stay on the surface instead of flowing away, and the surface stays negatively charged.
Similarly, if electrons are removed, electrons from other parts of the material do not flow in to replace them, so the surface stays positively charged. A material through which electrons flow easily is called a conductor. Conductors, such as metals, cannot be charged by rubbing.
You can show how much charge is on an object, and whether it is positive or negative, using a gold leaf electroscope .