CBSE Class 12 Physics Magnetic Effect of Current Theory and Examples

CBSE Class 12 Physics Magnetic Effect of Current Theory and Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

1. OERSTED EXPERIMENT (1820)
(a) Oersted found that a magnetic field is established around a current carrying conductor.
(b) Magnetic field exists as long as there is current in the wire.
(c) It is concluded that moving charges produce magnetic field in the surrounding space.
(d) A moving charge produces magnetic as well as electric field, unlike a stationary charge which only produces electric field.
(e) The direction of magnetic field was found to be changed when direction of current was reversed.

2. MAGNETIC FIELD OR MAGNETIC INDUCTION OR INTENSITY OF MAGNETIC FIELD(→B )

(a) The field produced by flow of current or charge in a conductor is called magnetic field.
(b) This is a vector quantity.
(c) Unit : CGS – Gauss or Maxwell / cm²
MKS – Tesla or Weber / m² or N/Amp-meter
1 Tesla = 1 weber / m² = 10⁴ Gauss
= 10⁴ Maxwell / cm2
(d) Magnetic field is shown by magnetic lines of force.

3. MAGNETIC LINES OF FORCE

(a) These are the imaginary closed curves drawn in magnetic field, which represent the direction of the magnetic field.
(b) The tangent drawn at any point on a line of force shown the direction of magnetic field at that point.
3.1 Properties:
(i) The magnetic lines of force always starts from the north pole and following a curved path enter the south pole and reach back the north pole inside the magnet. Thus these are
closed curve. Fig (a) - Magnetic lines of force due to current carrying solenoid.

Note : Electric lines of force are not closed curves.

(ii) Two lines of force never intersect each other.
(iii) Larger the number of lines of force at a given point, stronger is the magnetic field at the point.

Note : At poles there are maximum number of lines of force.

4. BIOT - SAVERT LAW
(i) Biot - Savart - a French scientist in 1820 gave a law to study the magnetic field generated by an infinitesimal element of a current carrying conductor at a point P.
(ii) Let the infinitesimal element be of length 'dl' and magnetic field at P be dB, then

(l) Direction of magnetic field : The direction of magnetic field is determined with the help of the following simple laws :
(i) Maxwell’s cork screw rule : According to this law if a right handed cork screw is rotated in such a way that it moves forward in the direction of current in the conductor, then the direction of the rotation of the screw will show the direction of lines of force. [fig . (a)]

(ii) Right hand palm rule : According to this rule if a current carrying conductor is held in the right hand such that the thumb of the hand represents the direction of current flow, then the direction of folding fingers will represent the direction of magnetic lines of force. [Fig. (b)]

(iii) Right hand palm rule of circular currents : According to this rule if the direction of current in circular conducting coil is in the direction of folding fingers of right hand, then the
direction of magnetic field will be in the direction of stretched thumb. [Fig. (c)]

(m) The magnetic lines of force due to a current carrying element are in the form of concentric circles with their centres at the element.
Note : Biot - Savart law in terms of current density j

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