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CLASS1 2 BEST NOTES NCERT CLASS 12 MAGNETIC EFFECT OF CURRENT PHYSICS WALLAH
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CLASS 12 NCERT FORMULA SHEET AND SHORT NOTES OF MAGNETIC FIELD AND CHARGES FROM A WELL KNOWN PLATFORM PHYSICS WALLAH
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1Parishram (2024)
FORMULA SHEET_(Magnetic Effect of Current)
➢ Hans Christian Oersted observed that, when a compass is placed near a straight wire carrying a current, the
compass needle aligns so that it is tangent to a circle drawn around the wire (neglecting the influence of
the Earth’s magnetic field on the compass).
➢ Oersted’s discovery provided the first link between electricity and magnetism.
➢ The space or region around the current carrying conductor within which its influence can be felt by the
magnetic needle is called the magnetic field of the current carrying conductor in general way
However, the definition of B can be given as the number of lines of magnetic field per unit area or the force
acting on unit charge which is moving with unit velocity perpendicular to the magnetic field.
➢ S.I. unit of magnetic field is Wm–2 or T (tesla).
➢ The strength of magnetic field is called one tesla, if a charge of one coulomb, when moving with a velocity
of 1 ms–1 along a direction perpendicular to the direction of the magnetic field experiences a force of one
newton.
1 tesla (T) = 1 weber metre–2 (Wb m–2)
= 1 newton ampere–1 metre–1 (N A–1 m–1)
➢ C.G.S. units of magnetic field are called gauss or oersted.
1 gauss = 10–4 tesla.
➢ Right hand thumb rule: If the linear conductor is grasped in the palm of the right hand with thumb pointing
along the direction of the current, then the curl fingers will point in the direction of lines of force.
➢ Maxwell’s cork screw rule: If a right handed cork screw is rotated so that its tip moves in the direction of
flow of current through the conductor, then the rotation of the head of the screw gives the direction of magnetic
lines of force.
➢ The conventional sign for a magnetic field coming out of the plane and normal to it is a dot i.e. ⊙. The magnetic
field perpendicular to the plane in the down ward direction is denoted by .
, 2
➢ Biot savart’s Law: According to this law, the magnetic field (in magnitude) due to a current element of length
dl carrying a current I at a point at distance r from it, is given by
0 I | dl rˆ | 0 I dl sin
| dB |= . = .
4 r2 4 r2
where is the angle between the direction of the current and the line joining the current element to the point and
µ0 is absolute permeability of the free space (µ0 = 4 × 10–7 T A–1 m).
The direction of magnetic field dB is that of I dl r .
➢ Magnetic field due to a current carrying conductor:
The magnetic field at a point at perpendicular distance a from a straight conductor carrying current I is given by
0 I
B= . (sin 1 + sin 2 ),
4 a
where 1 and 2 are angles, which the lines joining the two ends of the conductor to the observation point make
with the perpendicular from the observation point to the conductor.
In case, the straight conductor is of infinite length 1 2 , the magnetic field is given by
2
0 2 I
B= .
4 a
➢ Magnetic field due to a current carrying circular coil: For a coil of radius a, consisting of N turns
and carrying current I,
(i) the magnetic field at a point on axis at distance d from its centre is given by
0 2NIa 2
B= . 2
4 (a + d 2 )3/2
(ii) the magnetic field at its centre is given by
0 2NI
B= .
4 a
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