Important points

  • A piece of loadstone can attract iron nails and when it is suspended freely it aligns in the north-south direction.
  • A compass needle is a light bar magnet, whose north pole points towards geographic north approximately. Like poles of magnets repeal whereas unlike poles attract.
  • The region or space around a magnet where magnetic force can be experienced by other magnetic substances is called magnetic field.
  • If we face a coil such that current in the coil is in anticlockwise direction, then magnetic field will be towards us i.e. there will be a N-pole.
  • If we face a coil such that current in the coil is in clockwise direction, then magnetic field will be away from us, i.e. there will be a S-pole
  • A solenoid consists of many circular turns of an insulated wire in the shape of cylinder.
  • Magnetic field due to a solenoid is directly proportional to (i) current through it, (ii) number of turns, (iii) magnetic permeability of material of core and (iv) inversely proportional to the length of solenoid.
  • When soft iron and steel are placed inside strong magnetic field, these get magnetized. In case of soft iron magnetization is temporary whereas in case of steel it is permanent.
  • A solenoid with soft iron is called an electromagnet. An electromagnet is a temporary magnet, its magnetism lasts as long as current flows through it.
  • In the year 1831 Michael Faraday found that moving magnet with respect to a coil produce emf and current, which is called electromagnetic induction.
  • Number of magnetic lines of force passing through a given area is called a magnetic flux.
  • Whenever there is a relative motion between a coil and a magnet, magnetic flux passing through coil changes which results in induced emf and current.
  • To avoid short-circuiting and overloading, wires of better quality insulation must be used. Another important prevention against overloading and short-circuiting is use of ‘Fuse’. Its a metallic wire of alloy of tin and lead (35% lead and 65% tin) with low melting point, whenever there is overloading and short-circuiting and current exceeds the tolerance limit ‘fuse’ melts and break the circuit and saves the other appliances.

Important Formulae