Electrostatics

• Production of electric charges:

  • Rubbing a plastic comb through hair and bringing it near small pieces of paper causes attraction
  • Amber rubbed with silk attracts small pieces of paper
  • Attraction or repulsion between substances is due to electric charges acquired during rubbing
  • Electric charge can be produced by rubbing a neutral body with another neutral body
  • Friction produces two different types of charges on different materials
  • Opposite charges are conventionally called positive and negative charge
  • Negative charge is transferred from one object to another during rubbing
• Electrostatic induction:

  • Charged plastic rod near suspended neutral rod causes attraction
  • Attraction between charged and uncharged rods is due to displacement of positive and negative charges on the neutral rod
  • Total charge on the neutral rod remains zero
  • Electrostatic induction: in the presence of a charged body, an insulated conductor develops positive charge at one end and negative charge at the other end
• • Capacitor consists of two thin metal plates parallel to each other separated by a small distance
  • Charge stored on plates is directly proportional to the potential difference across the plates
  • Capacitance is the ability of the capacitor to store charge and is given by the ratio of charge and electric potential
• Electroscope:

  • Gold leaf electroscope is a sensitive instrument for detecting charges
  • Consists of a brass rod with a brass disk at the top and two thin leaves of gold foil hanging at the bottom
  • Charges can move freely from the disk to the leaves through the rod
  • Electric field lines can be used to represent the direction of electric field intensity
  • Electric potential at a point in an electric field is equal to the amount of work done in bringing a unit positive charge from infinity to that point
  • Capacitors and capacitance:
• Coulomb's Law:

  • Force of attraction or repulsion between two point charges is directly proportional to the product of the magnitude of charges and inversely proportional to the square of the distance between them
  • Electric field intensity is the strength of an electric field at any point in space
  • Electric field lines represent the direction of electric field intensity
  • Electric potential is the electric potential energy of a unit positive charge placed at a point in an electric field
• Capacitors can be combined in series or parallel to achieve any desired value of capacitance
• Capacitors in Parallel:

  • Left plate of each capacitor connected to positive terminal of battery
  • Right plate of each capacitor connected to negative terminal of battery
  • Each capacitor connected to a battery of voltage V has the same potential difference V across it
  • Charge developed across the plates of each capacitor will be different due to different capacitances
  • Total charge Q supplied by the battery is divided among the various capacitors
  • Equivalent capacitance of parallel combination of capacitors is the sum of individual capacitances
• Capacitors in Series:

  • Capacitors connected side by side
  • Each capacitor has the same charge across it
  • Potential difference across each capacitor is different due to different capacitances
  • Voltage of the battery is divided among the various capacitors
  • Equivalent capacitance of series combination of capacitors is the reciprocal of the sum of the reciprocals of individual capacitances
• Electrolytic capacitors use chemical reactions to store charge
• Uses of Capacitors:

  • Widely used in electrical and electronic circuits
  • Used for tuning transmitters, receivers, radios, fans, motors, air conditioners, computers, etc.
  • Can differentiate between high and low frequency signals
  • Ceramic capacitors are generally superior and used in various applications
• Hazards of Static Electricity:

  • Lightning can occur due to buildup of electric charge in thunderclouds
  • Lightning conductors are used to minimize damage from lightning strikes
  • Static electricity can cause fires or explosions, especially in presence of petrol vapour
  • Care must be taken to avoid sparks in situations where static charges can discharge
• • Paper capacitors have oiled or greased paper or thin plastic sheet as dielectric
• • Parallel plate capacitors have dielectric between plates and can be rolled into a cylinder shape
• Mica capacitors use mica as dielectric between metal plates
• • Coulomb's law equation: F = k(q1q2)/r^2
• Use of electrostatic charging

  1. Spraying of paint

  2. Dust extraction
• There are positive and negative charges
• Construction and working principle of electroscope

  Not specified
• Coulomb’s law states and explains the relationship between charges
• Volt


  Unit of electric potential
• Potential difference

  Energy transfer per unit charge
• Static electricity can be dangerous, precautions are taken to discharge it safely
• Capacitor


  Charge storing device
• Study of charges at rest is called electrostatics or static electricity
• Production of electric charges can be demonstrated through rubbing neutral bodies
• Opposite charges are conventionally called positive and negative charges
• Opposite charges

  • Positive charge
  • Negative charge
• Process of rubbing


  Negative charge is transferred from one object to another object
• Charge is a basic property of a material body due to which it attracts or repels another object
• Friction produces two different types of charge on different materials (such as glass and plastic)
• Like charges always repel each other
• Unlike charges always attract each other
• Repulsion is the sure test of charge on a body
• Amount of positive charge on the glass rod after rubbing it with silk cloth will not be equal to the amount of negative charge on the silk
• Attraction between charged and uncharged rods is not a sure test of charge on a body
• Electrostatic induction

  In the presence of a charged body, an insulated conductor develops positive charge at one end and negative charge at the other end
• Charging by induction results in equal and opposite charges appearing on each metal sphere
• Electroscope is a sensitive instrument for detecting charges
• Principles of electrostatics

  • Like charges repel
  • Unlike charges attract
• Charges can move freely from the disk to the leaves through the rod