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• All of us have the experience of seeing a spark or hearing a crackle when we take off our synthetic clothes or sweater, particularly in dry weather
• Another common example of electric discharge is the lightning that we see in the sky during thunderstorms
• We also experience a sensation of an electric shock either while opening the door of a car or holding the iron bar of a bus after sliding from our seat
• Static electricity


  Electricity that does not move or change with time
• Electrostatics


  The study of forces, fields and potentials arising from static charges
• Thales of Miletus, Greece, around 600 BC discovered that amber rubbed with wool or silk cloth attracts light objects
• The name electricity is coined from the Greek word elektron meaning amber
• Rubbing materials


  • Glass rods with wool or silk cloth
  • Plastic rods with cat's fur
• When glass rods rubbed with wool or silk cloth are brought close


  They repel each other
• When plastic rods rubbed with cat's fur are brought close


  They repel each other
• Glass rod and wool


  Attract each other
• Plastic rod and glass rod


  Attract each other
• Plastic rod and silk/wool


  Repel each other
• Glass rod and fur


  Repel each other
• Electric charge


  There are two kinds of electrification, like charges repel and unlike charges attract each other
• Polarity of charge


  The property that differentiates the two kinds of charges
• When a glass rod is rubbed with silk, the rod acquires one kind of charge and the silk acquires the second kind of charge
• When the electrified glass rod is brought in contact with silk, they no longer attract each other
• Charges acquired after rubbing are lost when the charged bodies are brought in contact
• Positive and negative charges


  The names given to the two kinds of charges by Benjamin Franklin
• Electrified or charged


  When an object possesses an electric charge
• Electrically neutral


  When an object has no charge
• Gold-leaf electroscope


  • A simple apparatus to detect charge on a body
• All matter is made up of atoms and/or molecules, which contain charges that are normally balanced
• Electrifying a neutral body


  Adding or removing one kind of charge
• Charging a body positively


  Losing some of its electrons
• Charging a body negatively

  Gaining electrons
• No new charge is created in the process of rubbing, only a small fraction of the total number of electrons is transferred
• Conductors


  Substances that readily allow passage of electricity through them
• Insulators


  Substances that do not allow electricity to pass through them easily
• Charges on a conductor get distributed over the entire surface, while charges on an insulator stay at the same place
• Point charge


  A charged body whose size is very small compared to the distances between them
• Additivity of charges


  Charges add up like real numbers or scalars
• Charge is conserved within an isolated system, it can be redistributed but not created or destroyed
• Quantisation of charge


  Electric charge is always an integral multiple of a basic unit of charge e
• The basic unit of charge e is the charge on an electron or proton
• Coulomb


  The SI unit of electric charge
• The value of the basic unit of charge e is 1.602192 × 10^-19 C
• All observable charges are integral multiples of the basic unit e
• At the macroscopic level, the grainy nature of charge is lost and it appears to be continuous