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

# 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.