Electrostatics

• The presence of charge on a body causes electricity in the body.
• electricity is of two types : static and current
• Electricity developed by the charges which are at rest is called static electricity
• Electricity developed by the charges which are in motion is called current electricity
• study of static behaviour is electrostatics
• Properties

  1. charge is the property of a body which shows either attraction or repulsion with other bodies
  2. it is scalar
  3. charges are either positive or negative
  4. law of charges
  5. charge is additive
  6. unit is coulomb
  7. charge on a body is quantised
  8. charge is conserved
• Law of charges states that like charges repel and unlike charges attract each other
• Quantisation of charge means the charge on a body can be represented as an integral multiple of the smallest unit of charge
• Smallest unit of charge is electron
• Quantisation principle $Q=$$ne$
• Number of electrons in one coulomb of charge is 6.25 x 10^18
• In Q = ne, n must be an integer not a fraction of decimal
• Conductors cause free flow of electricity because of excess number of free electrons
• Insulators are also called dielectrics
• Examples of semiconductors are germanium and silicon (4th grp elements)
• Methods of electrifying a body are rubbing method, conduction method and induction method
• When glass rod rubbed with silk cloth, glass acquires positive charge and silk acquires negative charge because work function of glass is less than that of silk
• When ebonite is rubbed with woolen cloth wool acquires positive charge and ebonite acquires negative charge
• Contact method is preferred for both.
• Induction method is preferred for conductors
• flow of electrons is from negative to positively charged body (earthing)
• The third hole in electric socket is connected to earth to suppress the excess charge created in the circuit
• When charge is considered in macroscopic quantistation can be ignored
• Two bodies of identical charges can attract each other if one body carries a huge charge compared to other
• In electricity and magnetism the surety test is decided by repulsion
• The force between two static charges separated by a certain distance is directly proportional to product of charges inversely proportional to square of distance between the charges and acts along the line joining the 2 charges
• 1/4πε0 = 9 x 10^9 Nm^2/C^2
• ε0 = 8.85 x 10^-12
• Epsilon is permittivity
• Relative permittivity is also known as dielectric constant
• K of air = 1
• K of any insulator > 1
• K of metallic conductor is infinity
• Coulombs force is a central force
• Coulombs force obeys Newton’s third law or action-reaction pair
• When asked to find least force btw 2 charges separated by a certain distance the charges (q1 and q2) must be taken as electrons and their charge must be taken as 1.6 x 10^-19 C

  q1q2 = 2.56 x 10^-38
• The ratio of electrostatic force to gravitational force btw 2 elementary charges electrons separated by a certain distance is 4.2 x 10^42
• When a charge is surrounded by multiple charges the net force on one of the charges is the vectorial sum of individual forces acting on the charge
• Due to presence of multiple charges the net force on a given charge may change but the individual forces will not change