Home » Handbook of O Level Physics » UNIT 21: SIMPLE PHENOMENON OF MAGNETISM


  • Four magnetic materials are nickel, iron, cobalt and steel.

  • Poles are ends of a magnet where the magnetic field is strongest.

  • Unlike poles attract each other.

  • Like poles repel each other.

  • Repulsion is the only sure way to find out if an object is a magnet or not.

  • Magnetic Induction: When an unmagnetized material is attracted to a magnet, the material itself becomes a magnet. This phenomenon is called magnetic induction.

  • Magnetic materials have tiny regions called domains which act as “mini-magnets”. When these domains are lined up, the material becomes a magnet. If the domains point in random directions then the material does not behave as a magnet.

  • Methods of making magnets are:

    • stroking a steel bar repeatedly with a permanent magnet

    • putting the steel bar in a solenoid (coil) having direct current (D.C.)

  • Methods of demagnetization of magnets are:

    • heat until red-hot

    • hammer repeatedly (or subject to some other impact)

    • Alternating current (A.C.) in a solenoid (coil) with the magnet inside of it

      • Remove magnet slowly from inside of the coil OR

      • Gradually decrease the current to zero

  • Two variations of Right-hand Grip Rule:

  • Magnetic field: It is the region of space where a magnetic object experiences a magnetic force.

  • Experiment to plot field lines of a magnetic field.

    • Mark at one end of compass needle.

    • Move compass so that the other end of compass needle is on the mark AND re-mark.

    • Repeat along one line and join the dots.

    • Repeat with different start points to draw more lines.

    • Direction of field lines is from N-pole to S-pole.

  • Iron is easier to magnetize and demagnetize than steel. This means that iron makes a stronger magnet when placed inside a current carrying solenoid. It also loses magnetism when the current is switched off.

  • An electromagnet consists of insulated copper wire wound on a core of soft iron.

  • The strength of magnetic field of a solenoid can be increased by:

    • increasing number of turns

    • increasing current

    • using iron core

  • Iron is used to shield sensitive equipment from magnetic fields. It carries those fields along the metal.

  • Reed switch consists of two pieces of soft iron called reeds. When these reeds are magnetized they are attracted to each other. Current flows when they make contact.

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