This chapter is concerned chiefly with magnetic field and the entire syllabus is covered under the following heads.

  • Magnetic field and Magnetic field lines
  • Magnetic field around a straight conductor carrying current
  • Magnetic field due to a current carrying circular coil
  • Magnetic field due to a current in a Solenoid
  • Electromagnet and permanent magnet
  • Force on a current carrying conductor in a magnetic field
  • Electric motor
  • Electromagnetic Induction.
  • Direct and Alternating Current
  • Electric Generator
  • Overloading and Short Circuiting.

Magnetic Compass: It is a compact of magnetic needle which is pivoted at the centre of a small brass box with glass top. It is used to (a) To find the magnetic north-south direction.(b) To find the direction of magnetic field at a place and (c) To test the polarity of a magnet.
Magnetic field: It is the space around a magnet in which the force of attraction or repulsion due to the magnet can be detected. It has both magnitude and direction.
Sources of magnetic fields : (i) Natural and artificial magnets (ii) Electro magnets (iii)A conductor, a coil and a solenoid carrying current. (iv) Earth.
Magnetic field lines: It is the curved paths along which the iron filings arrange
themselves due to the force acting on them in the magnetic field of the bar magnet.
Magnetic Flux: It is the number of magnetic lines of force passing through the given area.

Properties of Magnetic field lines:
(i) They start from the north pole of a magnet and end at its south pole (outside the magnet).
(ii) They are always normal to the surface of the magnet.
(iii) They are closed and continuous curve.
(iv) Two lines of force do not intersect one another. If they intersect at a point, it would mean that compass needle will point towards two directions at that point which are not possible.
(v) They come closer to one another near the poles of a magnet but they are widely separated at other places.

Oersted observed that when a magnetic needle is brought near the current carrying conductor, he observed that it undergoes deflection and also observed that when the direction of current is reversed, direction of deflection is also reversed.
Observation: –
• The North Pole of the needle is deflected towards east when current flows from North to South.(Fig a)
• The North Pole of the needle is deflected towards the west when the current flows from South to North. ( Fig b)
• There is no deflection in the needle if no current is passed.

The direction of deflection is given by Ampere’s swimming rule.
Ampere’s swimming rule: –
Imagine a man swimming along the conductor, the direction of current is from feet to head, looking at the needle, and then the north pole of the needle is deflected towards his left hand.
Magnetic field around a straight conductor carrying current:-
The magnetic field around a current carrying straight conductor consists of concentric circles of magnetic lines of force lying in a plane, which is right angle to the current carrying conductor. The conductor acts as the centre of magnetic lines of force. These lines are crowded near the conductor and become farther apart as the distance from the conductor increases. This indicates magnetic field near the conductor is stronger and becomes weaker as the distance from the conductor increases.
The magnitude of magnetic field produced by a straight current carrying wire at a given point is:
(i) Directly proportional to the current passing in the wire, and (ii) Inversely proportional to the distance of that point from the wire.
Magnetic field (µo I) / (2 π r)
Where o = Permeability of free space (constant)
I = Current flowing through the wire
r = radius of the circular wire
The direction of magnetic field is given by right -Hand Thumb Rule.
* Diagram – Refer NCERT Text Book

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