Static Electricity (AQA GCSE Physics)

• 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

• An example of this is a plastic or polythene rod being charged by rubbing it with a cloth
  • Both the rod and cloth are insulating materials

A polyethene rod may be given a charge by rubbing it with a cloth

Movement of Electrons

• 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

When electrons are rubbed onto a cloth, it gives it a negative charge and leaves the rod with a positive charge

• 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

Examples of Static Electricity

Static electricity can be seen in everyday scenarios when two or more insulators interact

Demonstrating Forces between Charges

• All objects are initially electrically neutral
  • This means the negative and positive charges are evenly distributed within the object

• When charged objects are brought near to each other, forces of attraction or repulsion can be observed
  • When two objects with different types of charge come together, a force of attraction is observed
  • When two objects of the same type of charge come together, a force of repulsion is observed

• One example is sticking a balloon to the wall:
  • Rubbing a balloon on a woollen jumper transfers electrons onto the balloon by friction
  • The balloon becomes negatively charged and the jumper becomes positively charged
  • The wall is uncharged initially, but when the balloon is placed near the wall, the electrons on the balloon's surface repel the electrons on the wall's surface
  • Furthermore, the positive charges on the wall's surface are attracted to the negatively charged balloon which allows the it to stick to the wall

Sparking

• The build-up of electrostatic charge can lead to sparking
• An example of this is the small electric shock sometimes felt when a person touches a metal door handle

Sparking can occur when a charge builds up on a person which is allowed to flow when they touch an earthed metallic object, such as a door handle

• 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

• Another example of sparking is the small electric shock felt from touching another person after walking on a vinyl floor or nylon carpet with rubber shoes

Sparking can occur when a person with a large build up of charge comes into contact with an uncharged person as there is a large potential difference 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
  • This can be seen in a Van de Graff generator or a lightning strike 

Dangers of Sparking

• Sparking can become dangerous in certain situations such as:
  • Electrocution e.g. by lightning
  • Ignition of a fire or explosion by a spark

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

Lightning is caused by the build-up of charge in clouds

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

Sparking can be reduced when refuelling a plane by connecting a bonding line to the Earth to allow any excess build up of charge to dissipate and travel to the ground 

• 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