We have allready studied about thermal effects of current and now in the present chapter we are studied about magnetic effect of current.
Earlier it was believe that there is no connection between electric and magnetic force and both of them are completely different.
But in 1820 Oersted showed that the electric current through a wire deflect the magnetic needle placed near the wire and the direction of deflection of needle is reversed if we reverse the direction of current in the wire.
So, Oersted's experiments establishes that a magnetic field is assoiated with current carrying wire.
Again if we a magnetic needlle near a bar magnet it gets deflectid and rests in some other direction.
This needle experiences the tourque which turn the needle to a definite direction.
Thus, the reagion near the bar magnet or current carrying where magnetic needle experience and suffer deflection is called magnetic field.
(2) The 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
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
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
for F=1N,q=1C and v=1m/s and θ=90
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]
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