Static Electricity (HL IB Physics)

• There are several methods by which electric charge can be transferred
  • Charging by friction
  • Charging by electrostatic induction
  • Charging by contact

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 becomes positive and the other negative

• 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 polythene rod may be given a charge by rubbing it with a 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

Charging by Electrostatic Induction

• Electrostatic Induction is defined as:

A process which generates an electrostatic force in a material by bringing an electrically charged object near it

• This causes the electrical charges within the material to be redistributed
  • So, one side has an excess of either positive or negative charges

• When a charged object is held close to a conductor, electrons in the conductor are able to move towards (or away from) the charged object:

Negative electrons in the aluminium are attracted to the positive rod 

• In the diagram above, electrons in the aluminium foil are attracted to the positively charged rod
• This causes the top of the foil to become negatively charged, whilst the bottom edge of it will be left with a positive charge
• The electrostatic attraction between the positive rod and the negative charges on the top surface of the foil causes the foil to be attracted to the rod

Charging by Contact

• Charge can also be transferred via contact
• For this to happen one of the two bodies involved must have a considerably higher net positive or negative charge build-up
• An insulator must prevent this charge from flowing out into neighbouring bodies
  • When the two bodies touch electrons flow from one to the other to reduce the difference between the charges of the two objects

• An example of charge transfer via contact is a 'shock' from a doorknob
• As you walk across a carpet the friction creates a build-up of negative charge on your shoes
  • Now you have a high net negative charge

A negative charge builds up on the bottom of your shoes as you walk across the carpet to the neutrally charged door

• During this process, you are insulated, as you are not in contact with anything that can conduct electricity
  • However, when you touch a metal doorknob, electrons flow from you to the doorknob to reduce the difference between your charge and the doorknob's charge

• This is because metal conducts electricity, and you are no longer insulated

Upon contact with the metal door knob, the negative charge is transferred to the door and the charge balances out

Earthing

• To prevent a transfer of charge through contact, both bodies can be grounded
• This means they are connected electrically to the earth
  • This is called earthing 

The electrical symbol for Earth

• Many electrical appliances have metal cases
  • If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted

• The electrical appliance can be connected to an earthing system via an earth wire

A problem in the electric appliance causes a surge of current which passes through the earth wire and is grounded into the earth. The person in contact with the appliance does not get an electric shock.

• If there is a sudden surge of current within the system:
  • The earth wire provides a direct low resistance path to the earth
  • So the current flows through the earth wire 
  • Into the ground
• It makes the appliance safe to touch and fix