Static Electricity (SL IB Physics)

• There are several methods by which electric charge can be transferred, such as
  • charging by friction
  • charging by electrostatic induction
  • charging by contact

Charging by Friction

• When two insulators are rubbed together, electrons are transferred through friction
• Depending on the materials, one insulator will become negatively charged and the other positively charged
• For example, when a cloth and rod are rubbed together, electrons are transferred from the rod to the cloth
  • This occurs because negatively charged electrons are transferred from one material to the other
  • The material, in this case, the rod, loses electrons
  • Since electrons are negatively charged, the rod becomes positively charged
  • As a result, the cloth has gained electrons and therefore is left with an equal negative charge

Electrons are transferred onto the cloth by friction. The cloth becomes negatively charged and the rod becomes positively charged

• Charging by friction is not limited to solid insulators, it can occur between any two substances e.g. liquid flowing in a pipe

Earthing

• To prevent a transfer of charge through contact, both bodies can be grounded
• This means they are connected electrically to the earth
• If a charged body is grounded (earthed), it will discharge until it has a potential of 0 V

Earth Circuit Symbol

An Earth symbol in a circuit indicates a point that is kept at 0 V

• Electrical appliances are kept safely at 0 V by connection to an earthed conductor, usually a wire made from copper, that allows a current to flow to the Earth
  • This is because a current will always take the path of lower resistance
  • Since copper has a lower resistance than, for example, a person, any build-up of charge will flow to the Earth through the copper wire rather than the person

Charging by Electrostatic Induction

• Electrostatic induction is the separation of charge caused by a nearby charged object without any physical contact
  • Note: this is not the same as electromagnetic induction
• When a charged object is placed near a material, electrons in the material move towards or away from the surface
• This causes the charges within the material to be redistributed
• As a result, one side of the material gains an excess of either positive or negative charges

Electrostatic induction can be observed using a comb and small pieces of paper

• An everyday example of electrostatic induction is when a comb, previously charged by friction, is placed near small uncharged pieces of paper
  • The negative charge on the comb repels electrons away from the top of the paper, leaving the bottom negatively charged
  • The top of the paper is attracted towards the comb and the bottom of the paper is repelled
  • As the top of the paper is closer to the comb, the attractive force is larger than the repulsive force, so there is a resultant upward force 

Charging a conducting sphere by induction

• An initially neutral conducting sphere can become charged by induction
  • A charged rod is brought near the sphere without touching it and causes the charges on it to separate
  • The sphere is grounded to allow electrons to move onto, or away from the sphere
  • When the charged rod and earth connection are removed, the excess charge remains

When a charged rod is brought near a conducting sphere without touching it, a charge can be induced when the sphere is connected and then disconnected to the Earth

Charging by Contact

• Charge can also be transferred when there is physical contact between two objects
• It often occurs between a charged insulator and an earthed conductor, when
  • There is a large potential difference between the two objects
  • The insulator prevents the charge from flowing out into a neighbouring object

• When the two objects touch, electrons flow from one to the other to reduce the potential difference between them
• An example of charge transfer via contact is a 'shock' felt when touching a doorknob

Charge transfer by contact can be felt as a 'shock' 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   

Charging a conducting sphere by contact

• An initially neutral conducting sphere can become charged by contact with a charged object
  • A charged rod is brought into contact with the sphere
  • Electrons are transferred from the rod onto the sphere 
  • When the rod is removed, the excess charge remains 

When a charged rod comes into contact with a conducting sphere, the charge flows between them

Dangers of Static Electricity

• When the potential difference between two objects becomes very large
  • the electric field between them becomes strong enough to cause the breakdown of air
  • a current can flow as an electrical discharge (spark) through the air

• This can be dangerous in certain situations, such as
  • electrocution e.g. by lightning
  • ignition of a fire or explosion by a spark

• A spark may ignite an explosion or fire when close to a flammable gas or liquid, for example, when refuelling aeroplanes
• The risk can be reduced by connecting the fuel tank to the Earth with a wire called the bonding line

The risks of sparks 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