S.N | Electric field | Magnetic field |
1 | Force = Eq, the force experienced by the charge particle is independent of the speed of the charge | Force = Bqv, the force experienced by the moving charge in speed of the particle |
2 | The moving charge particle experiences a force parallel to the field when it enters the electric field and so it travels parabolic path | The moving charge experiences a force at right angles to the magnetic field and it travels circular path inside the field |
In 1820, Orested discovered that current gives rise to magnetic fields (magnetic effect of current)
1. Near a straight conductor carrying current
Anticlockwise Clockwise
(Conventional direction)
Right Handed Rule:
If we imagine right hand grasping a wire in such a way that thumb is pointing up (in direction of conventional current; + to -) then the direction of encircling of the rest of the fingers gives the direction of magnetic field lines.
Alternative way of remembering the Right Handed Rule is Cork Screw Rule:
It states that if a right handed screw moves forward in the direction of the current (conventional) than the direction of rotation of the screw gives the direction of the magnetic field lines.
2. Near a circular coil
3. Near a solenoid
Solenoid behaves as a bar magnet after passing current through it generating north and South Pole.
Motor effect:
Whenever a current carrying conductor is placed inside the magnetic field it experiences a force due to which the conductor moves. This effect is called motor effect of current. Electric motors are based on the same principle.
The force experienced by a conductor of length ‘l’ carrying ‘I’ when placed in magnetic field of strength ‘B’ is given by:
F= B*I*L*sin α
Where, α is the angle between the conductor and the direction of magnetic field.
B is the magnetic field strength or flux density
Case I
When, α = 0 (i.e. the conductor is placed parallel to the magnetic filed)
F= 0 (i.e. no force experienced by the conductor when it is placed parallel the magnetic filed)
Case II
When, α= 90 (i.e. the conductor is placed perpendicular to the magnetic filed)
F= B*I*L max (i.e. maximum force experienced by the conductor when it is placed perpendicular to the magnetic filed)
So, B= F/ (I*L)
The magnetic flux density is defined as the force acting per unit length on a conductor which carries unit current and is at right angles to the direction of the magnetic fields.
SI Unit: N/Am Or, NA-1m-1 Or, Tesla (T)
Direction of the force
Fleming’s left hand rule (Motor Rule):
It states that if the thumb and first two fingers of the left hand are held each at right angles to the other, with the first finger pointing in the direction of the field and the central finger in the direction of the current then the thumb predicts the direction of the force/ motion of the conductor.
Inwards force is experienced by the conductor.
In CD, F=BIL (inwards)
Electric energy gets converted to mechanical energy.
Force on a moving charge in a magnetic field,
F= BIL sin αCurrent (I) = charge/ time
F= B q (L/T) sin α
= B*q*v*sin α
Where, v is the speed of the charged particle when α=90
F=Bqv
The moving charge experiences a force F=Bqv when enters the magnetic filed. This force is perpendicular to the direction of magnetic field as well as to the direction of motion of charged particle. As a result the moving charge particles travel circular path when it enters the magnetic field.
Magnetic flux (B):
The magnitude flux through a small plane surface is the product of the flux density normal to the surface and the area of surface.
Leny’s law states that any induced current will flow in a direction so as to produce effects which oppose the change that is producing it i.e.
When the charged particle enters a magnetic filed it travels circular path of radius ‘r’,
Magnetic force= centripetal force
Or, Bqv= mv^2/r
Therefore, r=mv/q
If the coil has N turns
Flux linkage, Nф= NBA
We know,
Ф= BA
Or, B= Ф/A = flux/ area
Unit is Whm^-2
Whenever there is relative motion between the conductor and the magnet current (per e.m.f.) is induced in the conductor. The induced current increases and when a soft iron core is used inside the coil. This phenomenon is called electromagnetic induction.
When there is relative motion between the conductor and the magnet, the magnetic flux linking the conductor changes. As a result, current is induced. The induced current lasts only till the flux is changing. The direction of induced current can be predicted by using Fleming’s right hand rule.
It states that if we stretch first three fingers of our right hand, mutually perpendicular to each other such that the thumb is the direction of motion of conductor, first finger in magnetic field then the central finger gives the direction of induced current.
Electric generators and dynamos which convert mechanical energy to electrical energy are based on the principle of electromagnetic induction.
The magnitude of induced e.m.f is given by faraday’s law. It states that the magnitude of induced e.m.f is proportional to the rate of change of magnetic flux or magnetic flux linkage i.e. E (induced e.m.f.) is proportional to dФ/dt (if there is only 1 coil)
If there is ‘n’ number of coils E is proportional n dФ/dt
South Pole: clockwise direction of current
North Pole: anticlockwise direction of current
Leny’s law states that any induced current will flow in a direction so as to produce effects which oppose the change that is producing it i.e.
E is proportional -dФ/dt
Therefore, E= -n dФ/dt
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