Static electricity

​​In a nutshell

When two insulating materials are rubbed together static electricity is produced. Static electricity is the transfer of electrons between two different materials. Positive charges do not move. Electric fields are an invisible force field which causes charged particles to experience a force. 

Static electricity

What is static electricity? 

​​​​Static electricity is to do with electric charges that can't move. Recall that conductors allow charges to flow easily and insulators do not allow charges to move easily. So, in order to stop charges from moving, the materials involved with generating static electricity are all insulators, to stop the charge flowing.

Atoms are made up of a protons, neutrons and electrons. The protons and neutrons are in the centre of the atom in the nucleus and the electrons are orbiting around the nucleus in electron shells. Protons are positively charged, neutrons are neutral and electrons are negatively charged. 

Atoms are neutral, so have an equal number of protons and electrons. Static electricity is the transfer of electrons from one material to another. 

How static electricity is made

When two insulating materials are rubbed together, electrons are transferred from one material to the other. The material that loses the electrons will now have more protons than it does electrons which means it will be positively charged. The material that gains the electrons now has more electrons than it does protons, it will be negatively charged. 

Example

A plastic rod is rubbed with a cloth.

​Initially the rod and the cloth are both neutral and have an equal amount of positive (protons) charge and negative (electrons) charge, so the charges cancel each other out.
​When the cloth is rubbed against the plastic rod, the electrons are transferred from the rod to the cloth. ​
​After the electrons have been transferred, the plastic rod is left positively charged as it now has more protons than electrons. The cloth is negatively charged as it has more electrons than it does protons. ​

If the cloth was used to rub a metal rod, the charges would flow from the rod onto the cloth, but then due to the conductivity of the metal rod, the charges would move straight back onto the rod, so it would not work. 

By using insulators, the charges are now trapped on the rod and cloth. The direction the charges move depends on what the materials are made from. 

For an acetate rod, the electrons move from the rod to the cloth, like in the diagram but for a polythene rod, the electrons move from the cloth to the rod.

Note: Static electricity is always to do with the transfer of electrons. Positive charges are protons, which are in the nucleus and do not move. To make something positive, negative charges are removed.

Electric fields

An electric field is an invisible field which is caused by electric charges. Electric fields are similar to gravitational fields, both being non-contact forces. 

Hold and object at arms length and release it, it will fall the floor. This is due to the gravitational field exerting a force on the mass which has just been dropped. 

Similarly electric fields also exert a force, but instead of exerting a force on objects with mass, electric fields exert a force on charged objects. 

When two charged objects are bought together, one of two things can happen. Either the objects can be attracted to one another or they will repel. 

If they have the same charge then the objects will repel each other. If the objects have the opposite charge, then they will attract one another. Below are electric field diagrams to show the interactions with same or opposite charges.

Unknown charges

By knowing whether charges will attract or repel, the charge of an object can be found.

An object which has an unknown charge is suspended by a piece of string so it is free to rotate. Another rod is charged, however this time the charge is known. If the rod was acetate then the rod would be positively charged as the electrons would be transferred to the cloth.

The charged rod is held near the suspended object and the interaction is observed. If the object is attracted towards the positively charged rod, then the charge of the unknown object must be negative.

If the object moves away from the positively charged rod, then the charge of the unknown object must be positive too as they are repelling.

Curiosity: If a charged balloon was placed on the ceiling, it would stick to the ceiling. However this does not mean that the ceiling is charged. Sometimes charged objects can cause neutral objects to become attractive, like small pieces of paper and a charged rod, the paper will stick to the rod, but the paper isn't charged. The only way to fully tell if an object is charged or not is by repelling another object. 

​​​