2.5 Static electricity Flashcards
Charging by friction
When certain insulating materials are rubbed against each other they become electrically charged.
This is called charging by friction.
The charges remain on the insulators and cannot immediately flow away.
One gains a net positive charge and the other gains a net negative charge.
Charging by friction occurs due to the movement of electrons.
When a polyethene rod is rubbed with a cloth, negatively charged electrons are transferred from the rod to the cloth.
The cloth has gained electrons, so it becomes negatively charged.
The rod has lost electrons, so it becomes positively charged.
Static electricity
Electricity is caused by the movement of charge in a circuit.
This occurs in conductors.
Static electricity, on the other hand, is caused by a build up of stationary charge on a surface.
This occurs on the surfaces of insulators.
Some examples of static electricity are:
The accumulation of dust particles on surfaces
Hair sticking up after combing it with a plastic comb or going down a plastic slide
Rubbing a balloon and sticking it to a wall
Sparking
Sparking
The build-up of electrostatic charge can lead to sparking.
A spark occurs between two objects when:
There is a large potential difference between the two objects.
Which causes a current to flow between them.
Sparking often occurs between a charged insulator and an earthed conductor.
An earthed conductor is a wire, usually made from copper, that allows a current to flow to the Earth.
A current will always take the path of lower resistance.
Since copper has a lower resistance than, for example, a person, the current will flow from the insulator to the Earth through the copper wire rather than the person.
Sometimes, when a potential difference between two objects becomes very large, the electric field may become strong enough to cause the breakdown of air.
As a result, an electrical discharge (large spark) can travel through the air to allow the current to flow.
Lightning
An extreme example of sparking is lightning.
In a storm, ice crystals in clouds rub against each other causing a movement of electrons between them .
The top of the cloud becomes positively charged, and the bottom becomes negatively charged.
The electrons on the ground are strongly repelled by the negative charge on the cloud, which causes it to become positively charged.
The potential difference between the cloud and the ground becomes increasingly large (~106 V).
Eventually, the cloud discharges a large spark as the negative charges jump to meet the positive charges on the ground.
Ignition by sparking
A spark may ignite an explosion or fire when close to a flammable gas or liquid.
For example, when refuelling aeroplanes, a build-up of static charge can pose a significant danger.
As the fuel passes through a pipe, the friction between them causes static charge to build up.
If the potential difference becomes too large, it could cause a spark.
A spark could then ignite the fuel and cause an explosion.
The risk can be reduced by connecting the fuel tank to the Earth with a wire called the bonding line.
The conductor earths the aeroplane by carrying excess charge through to the Earth and removes the risk of any sparks.
Electric fields
A charged object creates an electric field around itself.
This can be shown by electric field lines.
Fields lines always point away from positive charges and towards negative charges.
The field is strongest close to the charged object - this is shown by the field lines being closer together.
The field becomes weaker further away from the charged object - this is shown by the field lines becoming further apart.
