Electrical principles - Principles
Electricity
The two forms of electricity are:
- static electricity – an electrical charge builds up on the surface of an object
- current electricity – electricity comes from a power source, such as a battery or generator.
Static electricity
Atoms generally remain in an 'electrically balanced' state, meaning they have an equal number of protons and electrons. If there is friction force between two objects of an opposite charge, then there will be a transfer of electrons. The result of this is that one object becomes negatively charged (more electrons than protons) and the other becomes positively charged (more protons than electrons). This build up of charge has the potential to cause electrons to flow. While objects are charged and there is no current flow, the electricity is static, that is, stationary or not moving. When two objects of opposite charge move close enough, electrons will rapidly discharge between the two objects and cause a spark. An example of this is when you get a shock from touching a metal surface such as a car or door handle.
Laws of static charges:
- like charges repel one another
- unlike charges attract one another.
Dangers of static charge
Friction causes static charges to build up between materials. Many of these materials have the potential to catch fire or explode if a spark from a discharge takes place. Minor shocks can be received from machines and cars, but in the case of lightning, the discharge can kill.
Some of the dangers are described below.
- Liquids flowing through a pipe can build up a static charge. Flammable liquids like petrol have the potential to explode.
- Dusts such as powdered milk, flour, etc, which are are often moved by means of air. The friction of this movement builds up a static charge with the potential to cause explosions.
- Conveyer belts and materials such as fabrics or paper travelling through long processing machinery may acquire large static charges. These charges can cause fires if there is a build up of dust or lint.
Current electricity
Current electricity refers to electricity that is flowing continuously through an electric circuit. This flow of electrons (current flow) through the circuit is maintained by a power source such as a generator or a battery.
Circuits
The path the current flows through is called an electric circuit. An operational circuit needs a voltage supply, conductors and a load.
The diagram below uses a battery as the votage supply, electrical wires as conductors and a globe as the load. This circuit also has a switch to allow current flow to be turned on or off. Try turning off the current flow now by selecting the switch.
Circuit types include:
- series – one path only
- parallel – more than one path
- series/parallel (combination of both).
The following conditions can exist in any given circuit:
- open – no current can flow
- closed – current is flowing
- shorted – current is flowing through a shorter path and the load in the circuit is bypassed. This is a fault condition.
