Charge and its properties

1. Charge:    A property associated with matter responsible to produce electric and magnetic effects.
   

    Unit:    SI Ampers-sec or Coulomb    1 A.s=1C

            CGS    stat Coul or esu    or Franklin        1 Stat Coul=1 esu        1C=3´10⁹ esu

            Practical Unit    Amp.hr        1 A.hr=3600 C

            Faraday    1 faraday=96500 C

    Properties:

• Two kinds of charges – positive and negative. Similar charges repeal and opposite charges attract.
  
• Scalar:    Charge is scalar
  
• Additive
  
• Transferable:    Charge is transferable from one body to another.
  
• Quantized:    Charge is quantized. Smallest charge is a charge on an electron (-) and charge on a proton (+). It is known as fundamental charge and equals to 1.6´10^-19 C.
  
• Conserved:    Cannot be created or destroyed.
  
• Invariant:    It is invariant to frame of reference.
  

Effects:

• Stationary charge: Produces electric field known as electrostatic field.
  
• Charge in uniform motion:    Produces magnetic field in addition to electric field.
  
• Accelerated charge:    Produces electric as well as magnetic field and radiates energy in form of electromagnetic waves (EM Waves).
  

 

1. Materials:
   

   Conductors:
   

• Conduct electricity due to presence of free charge carries. In metals, which are conductors free charges carriers are electrons known as free electrons.
  
• Charge given to a conductor is always distributed on its surface. Its distribution on the surface (Surface charge density ) depends on the local radius of curvature. Smaller is the radius of curvature greater is the local surface charge density.
  
• A good conductor of heat is also a good conductor of electricity.
  
• In external electric field due to movement of free charges some charges appear on the surface. They are called induced charge and the phenomenon is called induction.
  
• Net electric field (resultant of the applied electric field and electric field of induced charges) is always zero inside a conductor.
  
• Amount of induced charge £ Amount of inducing charge.
  

Insulators:

• Cannot conduct electricity as no free charge carriers are present.
  
• In external electric field surface charges appear due to polarization. These surface charges are bound charge.
  
  • Net electric field (resultant of the applied electric field and electric field due to polarization) is always smaller than the applied electric field.
    
• Amount of surface charge due to polarization < Amount of charge inducing polarization.
  

 

1. Methods of charging a body:
   

   Charging by friction:    Electron transfer takes place when two different materials are rubbed on each other.
   

        Tribo-electric Series: (Enclosed in the end of class notes)

Charging By Conduction:    Transfer of charge from one conducting body to another by physical contact.

 

 

Charge distribution depends on the shape and size of both the bodies.