Magnetism , [ electricity and magnetism]

1. Molecular theory of magnet and magnetic effect of
electric current

2. What is magnetism?
 Force of attraction or repulsion due to electron
arrangement
 Magnetic forces are the strongest at the poles
 Magnets have two poles: North and South
 When free to rotate, to come to rest pointing in a
north-south direction
 Like magnetic poles repel
 Unlike magnetic poles attract

3. Types of magnet
 Natural magnet: known as lodestones which is a type of iron
ore with magnetic properties
 Artificial magnet : materials in which magnetic properties are
produced artificially such as iron, cobalt, nickel, steel etc
 Temporary magnet : that loses their magnetic properties
easily and quickly e.g. soft iron
 Permanent magnet : which retain their properties more longer
and difficult to magnetise e.g. steel
 Electromagnet : an electric current produces magnetic effects
and a coil of wire carrying a current act as an electromagnet

4. Molecular theory of magnetism
 If we break a magnet into two parts, each part become a
complete magnet
 Individual molecule of the magnet act as a tiny molecular
magnet
 Their magnetic properties may be due to electrons rotation
in their orbits, which constitute minute electric current and
thus produce magnetic properties
 When material is unmagnetized, molecular magnets lie in
haphazard manner and their magnetic properties neutralize
each other

5.  When material is magnetized, the molecular magnets are
arranged in an order so that their properties augmented
and become apparent
 Heating or hammering a magnet accelerate loss of
magnetic properties why?
 Point of saturation: is a point where further magnetizing
a magnet cannot increase the strength of a magnet and all
molecular magnets are fully alligned

7. Properties of magnet
 Setting in north-south direction: utilized in compass
 Behavior of like and unlike poles
 Magnetic field
 Attraction for objects of magnetisable material
 Transmission of properties
 Magnetization by contact
 Magnetic induction

8. Magnetic field
 A magnetic field is the area around magnet in which
magnetic forces are apparent
 Travel away from north and towards south pole
 Magnetic field lines are close continuous curves
 No two lines intersect each other
 Tend to repel each other helps distribution of
magnetic field
 Travel more easily through magnetisable material
than others

9. Magnets have two ends or poles,
called north and south poles. At
the poles of a magnet, the
magnetic field lines are closer
together.
Unlike poles of magnets attract
each other and like poles of
magnets repel.

10. Transmission of properties
Magnetization By contact
 Stroke a piece of iron or steel with one pole
of bar magnet
 Same pole is used throughout and stroke is
carried out in same direction
 same polarity where stroke commence and
opposite polarity where stroke finishes
 If north pole of magnet is used it will
attract the south poles of molecular
magnets of steel or iron and draws them
towards the point where it leaves the bar

11. Magnetic induction
 Production of magnetic properties in an object
by magnet without contact
 demonstrated by using a magnet to pick up
some paperclips or iron tacks.
 The paperclip in contact with the magnet
attracts another paperclip due to the fact that it
has become magnetised.
 This second paperclip is also magnetised and so
on
 Opposite polarity develop at adjacent end and
same polarity at far end
 As unlike pole attract each other and repel like
pole

12. Magnetic effect of electric current
Magnetic field around a straight wire
 An electric current sets up magnetic field around a
conductor through which it pass
 The shape of the magnetic field lines for a straight
conductor is concentric circles.
 These concentric circles become larger as we move
away from the wire.

13. Demonstration
 Take a thick copper wire and pass it
through a horizontal cardboard as
shown.
 Pass a strong current through the wire.
 Sprinkle iron filings on the cardboard
around the wire.
 Tap the cardboard gently. You would
see a pattern as shown here.
 You may plot the field lines with a
compass needle also.

14. Right-hand Thumb Rule
 When you wrap your right hand
around the straight conductor such
that the thumb points in the direction
of the current, the fingers will wrap
around the conductor in the direction
of the field lines of the magnetic field.
 Anticlockwise if current flow from
negative to positive
 Clockwise if flow from positive to
negative

15. Magnetic field around coil of wire
 The magnetic field lines are circular
at the points where the current enters
or leaves the wire
 Within the space enclosed by the
coil, the field lines are in same
direction and parallel to each other
and uniform
 Mag. lines are clockwise in upper
turns, positive to negative
 Mag. lines are anticlockwise in lower
turns, so direction is towards positive
 Emerging from north pole and
returning to south pole

16. Magnetic polarity
Magnetic poles lie at end of coil and polarity of each pole
depends upon:
1. Direction of current flow: reversal of current reverses
direction of mag. lines of forces
2. Direction in which coil wound
Many rules for determining magnetic polarity assume the
current flow to be from positive to negative

17. Electromagnet
 Consist of coil of wire wound on a soft
iron bar
 When current pass through the coil,
magnetic field set up and soft iron core is
magnetised by induction, so that its field
is added to that produced by current
 A strong magnetic field is produced,
which can turn on and off as required by
starting or stopping current
 Soft iron is chosen for core because it can
be easily magnetised and demagnetized

18. Moving-coil Milliammeter
 Used to measure the intensity of electric current
 Based on principle of magnetic effect of electric current
and interactions of magnetic fields
 Suitable for measurement of D.C
Principle of working
 Coil of fine insulated wire, the solenoid, situated on pivot
between the poles of a permanent magnet
 The current which has to be measured is passed through
coil which set up mag. field around it
 Like poles of magnetic field repel and unlike poles attract
each other

20.  Solenoid is free to rotate on pivot as it does so, and pointer
attached to it will move over the scale of meter
 Movement is controlled by hairspring which resists the
rotation of solenoid
 Deflection of needle depends on intensity of current and
strength of mag. field
 Solenoid and needle return to resting, when current stop
 Shunt circuit: larger current would damage solenoid as it
is very light and delicate
 Parallel shunt circuit is included in circuit which allow a
portion of current to pass through solenoid
 Resistance one-ninth times less than that of solenoid

21.  Wiring into circuit: Milliammeter is placed in series with
other components in which current intensity has to be
measured
 Because in series, current intensity remain same
 Resistance of meter should kept low
 Care of meter:
 Current of great intensity should not pass as it causes
deflection of needle beyond scale
 Interrupted d.c should be avoid as it will strain the
hairspring and needle would not come to zero position

22. Voltmeter
Construction and working
 Used to measure the p.d. between two points
 Constructed same as that of moving coil ammeter except
large resistance is placed in series with solenoid
 If total resistance is 1000 ohm, p.d. of 1 volt will produce
a current of 1mA
 As I= E/R, so if resistance is 8000 ohm, p.d. of 1 volt will
produce 1/8 mA
 8 volt will be required to produce 1 mA
 16 volts for 2mA

23. Wiring into circuit
 Voltmeter is connected in parallel to any circuit, p.d. of
which has to be measured
 As p.d. in parallel circuits remains same
 Resistance of voltmeter should be high
Meters for measuring A.C
 Convert a.c. into d.c then pass it through the moving
coil ammeter