What is a magnetic field


  • We all ready know that a stationery charges gets up a electric field E in the space surrounding it and this electric field exerts a force F=q0E on the test charge q0 placed in magnetic field.
  • Similarly we can describe the intraction of moving charges that, a moving charge excert a magnetic field in the space surrounding it and this magnetic field exert a force on the moving charge.
  • Like electric field, magntic field is also a vector quantity and is represented by symbol B
  • Like electric field force which depend on the magnitude of charge and electric field, magnetic force is propotional to the magnitude of charge and the strength of magnetic field.
  • Apart from its dependence on magnitude of charge and magnetic field strength magnetic force also depends on velocity of the particle.
  • The magnitude magnetic force increase with increase in speed of charged particle.
  • Direction of magnetic force depends on direction of magnetc field B and velocity v of the chared particle.
  • The direction of magnetic force is not alonge the direction of magnetic field but direction of force is always perpendicular to direction of both magnetic field B and velocity v
  • Test charge of magnitude q0 is moving with velocity v through a point P in magnetic field B experience a deflecting force F defined by a equation
    F=qv X B 
  • As mentioned earlier this force on charged particle is perpendicular to the plane formed by v and B and its direction is determined right hand thumb rule.


  • When moving charge is positive the direction of force F is the direction of advance of hand screw whose axis is perpendicular to the plane formed by v and B.


  • Direction of force would be opposit to the direction of advance screw for negative charge moving in same direction.
  • Magnitude of force on charged particle is
    F=q0vBsinθ
    where θ is the angle between v and B.
  • If v and B are at right angle to each other i.e. θ=90 then force acting on the particle would be maximum and is given by
    Fmax=q0vB                   ----(3)
  • When θ=180 or θ=0 i.e. v is parallel or antiparallel to B then froce acting on the particle would be zero.
  • Again from equation 2 if the velocity of the palticle in the magnetic field is zero i.e., particle is stationery in magnetic field then it does not experience any force.
  • SI unit of strength of magnetic field is tesla (T). It can be defined as follows
    B=F/qvsinθ
    for F=1N,q=1C and v=1m/s and θ=90
    1T=1NA-1m-1
    Thus if a charge of 1C when moving with velocity of 1m/s along the direction perpendicular to the magnetic field experiences a force of 1N then magnitude of field at that point is equal to 1 tesla (1T).
  • Another SI unit of magnetic field is weber/m2 Thus
    1 Wb-m-2=1T=1NA-1m-1
    In CGS system, the magnetic field is expressed in 'gauss'. And 1T= 104 gauss. Dimention formula of magnetic field (B) is [MT-2A-1]
For full chapter visit physicscatalyst.com

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