ELECTROMAGNETISM

1. ELECTROMAGNETI
SM

2. FORCE ON A CURRENT CARRYING
CONDUCTOR
The current carrying wire has magnetic field around
it. If we place the current carrying wire into the
magnetic field, the two magnetic fields may interact,
and produce a force on the wire. This can be shown by
the experiment set up below.

4.  When a current is passed through the wire, the wire
moves upward. A force is acting on the wire segment
inside the magnetic field.
 When the direction of current reversed, the wire
move downward.
 When there is changing of direction of the magnetic
field, the force acting on a wire also change.
 When the current and magnetic field strength are
increase, the wire experience a large force. The force
acting on the wire is therefore proportional to
current and magnetic field strength
 The direction of force can be determine by Fleming’s
Left Hand Rule.

5. Fleming’s left hand rule
Place the forefinger, second finger and thumb of the left
hand mutually at right angles. Then , if the forefinger
points in the direction of the field and second finger in
the direction of current, the thumb will point in the
direction of the force or motion.

6. FORCE ON A BEAM OF CHARGE PARTICLE
When a beam of moving charged particles enters a
magnetic field, there is a force acting on the charged
particles. They are deflected inside the magnetic field.
Fleming’s Left Hand Rule can be applied to determine the
direction of deflection of the beam of charged particles.
A beam of positive charged
particles
 Direction of current is same as direction of movement of
the charged particles
 If beam of positive charged particles enters magnetic
field into the paper, the charged particles move towards
top of the paper as shown below.

7.  When the direction of magnetic field changes the
force acting on a charge particle also changes.
 If the positive charge particle enters magnetic field
out of the page the charge particles move towards
bottom of the page. So by using Fleming’s Left Hand
rule we can find the direction of force acts on a
charged particle.

8. Current is in an opposite direction to that of the flow
of negative charges.
If beam of negative charged particles enters magnetic
field into the paper, the charged particles move
towards bottom of the paper as shown below.
A beam of negative charge
particles (electrons)

9.  When the direction of magnetic field changes the
force acting on a charge particle also changes.
 If the negative charge particle enters magnetic field
out of the page the charge particles move towards
top of the page. So by using Fleming’s Left Hand
rule we can find the direction of force acts on a
charged particle.

10. When two parallel wires carrying current at same
direction, both the wires move towards each other
and magnetic field pattern is shown below.
MAGNETIC FIELDS BETWEEN
PARRALEL CURRENT CARRYING
CONDUCTORS

11. When two parallel wires carrying current in an
opposite direction, both the wires move away from
each other and magnetic field pattern is shown below.

12. D.C MOTOR
A simple direct current electric motor consists of a coil (ABCD)
connected to two split-ring commutators (X and Y), two
permanent magnets and two carbon brushes (P and Q)
connected to an external battery. The commutators rotate with
the coil. Two carbon brushes are made to press lightly against
the commutators so that current can pass through when they
are in contact.

13. (a) When current flows from A to D through the coil,
the side CD experiences an upward force and the side
AB experiences a downward force. These forces
produce a turning effect and cause the coil to rotate in
a clockwise direction.

14. (b)When the coil rotates 90° and reaches the vertical
position, the contact between the carbon brushes (P
and Q) and the commutators (X and Y) are broken.
No current flows through the coil. Because of its
inertia, the coil keeps rotating until the commutators
are in contact with the carbon brushes again.

15. (c) The current along the sides AB and CD is reversed.
The side AB experiences an upward force and the side
CD experiences a downward force. These two forces
produce a clockwise moment. Hence the coil
continues to rotate in a clockwise direction.

16.  The purpose of the split-ring commutators is to
reverse the direction of current in the coil whenever
the commutators change contact from one carbon
brush to another. This ensures that the coil will rotate
in a fixed direction.
Four ways to increase the rotating speed of a motor:
(i) increasing the current,
(ii) increasing the number of turns of the coil,
(iii) increasing the strength of the magnetic field,
(iv) increasing the area of the coil

17. Electromagnetic induction is the production of an e.m.f
(voltage) in a conductor when there is a change in magnetic
flux linked with the conductor.
When a wire is moved across a magnetic field, as shown below,
a small e.m.f. (voltage) is generated in the wire. If the wire
forms part of a complete circuit, the e.m.f. makes a current
flow. This can be detect by using a sensitive meter called
galvanometer.
ELECTROMAGNETIC INDUCTION

18. When the direction of movement of wire changes the direction
of induced current also changes.
If the wire moves downward, the direction of current carried
is shown below and the deflection of galvanometer need is
also same direction that is right side.
If the wire moves upward, the direction of current carried is
shown below and the deflection of galvanometer need is also
same direction.

19. The direction of induced current in the straight wire can be determined by
using Flemings Right Hand Rule.

If the magnetic field direction changes the current direction also changes.

If the wire is in rest in the magnetic field, no e.m.f is induced.

20. The factors effecting magnitude of induced e.m.f
The induced e.m.f. (and current) can be increased by:
moving the wire faster
using a stronger magnet
increasing the length of wire in magnet in the magnetic field
– for example, by looping the wire through the field several
times as shown below.

21. The direction of induced current can be determined by Lenz’s
law.
An induced current always flows in a direction such away
that its magnetic field opposes the change which produce it.
LENZ’S LAW
Example 1
The N-pole of magnet is moving towards the solenoid as shown
in the diagram below.
The change that induces current is the N-pole moving towards the solenoid.
According to Lenz’s Law, the direction of induced current opposes the
change producing it. To oppose the N-pole moving to the coil, the induced
current must produced a N-pole at the end X. Hence the direction of
induced current is as shown.

22. Example 2
The N-pole of magnet is moving away from the solenoid as shown in
the diagram below.
The change that induces current is the N-pole moving out the solenoid.
According to Lenz’s Law, the direction of induced current opposes the
change producing it. To oppose the N-pole moving out of the coil, the
induced current must produced a
S-pole at the end X. Hence the direction of induced current is as shown
above.

23. A.C. GENERATOR
A simple a.c. generator consists of a coil rotating about an axis
between the poles of a permanent magnet as shown below.
When the coil rotates, it cuts magnetic field lines, so an e.m.f. is
induced. This makes a current flow through the coil.
As the coil rotates, each side travels, upwards, down wards, upwards
and downwards… and so on, trough the magnetic field. So the
current flows backwards, forwards and an a.c. current is produced.

24. The direction of induced current changes every half rotation
of the coil and this can be determined by using Fleming’s
Right Hand Rule.
The end of coil are connected to a pair of slip rings. The slip
rigs rotate with the coil and are in close contact with two
carbon brushes which rub against the slip rings and keep the
coil connected to the out side part of the circuit.
The induced current is maximum when the plane of the coil
is parallel to the magnetic field. There is no induced current
when the plane of the coil is perpendicular to the magnetic
field.
The induced current can be increased;
• using the coil with more turns
• using stronger magnet
• rotating the coil faster

25. Graph of voltage output against time for a
simple a.c. generator

26. TRANSFORMER
Transformer is a device used to increase or decrease the
voltage of a.c. supply.
The transformers only worked with alternative current (a.c.).
The diagram below shows how transformer works.
It make use of electromagnetic induction.
When the primary coil has alternative current flowing
through it. It is thus an electromagnet, and produces an
alternating magnetic field.

27. The core transports this alternating field around the
secondary coil.
Now secondary coil is a conductor in a changing magnetic
field. A current is induced in the coil.
There are two types of transformers:
1. Step-up transformer
A step-up transformer is used to increase the out put voltage,
so there are more turns on the secondary coil than primary coil
as shown below.

28. 2. Step-down transformer
A step-down transformer is used to decrease the out put voltage,
so there are more turns on the primary coil than secondary coil
as shown below.
The ratio of number of turns tells us the factors by which the
voltage will be changed. Hence we can write an equation, known
as transformer equation, relating two voltages Vp and Vs, to the
number of turns on each coil, Np and Ns.

29. Advantages of high voltage transmission
From the power houses the electricity is transmitting by high
voltage, using step up transformer as shown below. This is because
using higher voltage for power transmission reduces power loss in
the transmission cables.

30. Environmental and cost implications of underground
power transmission compared to overhead lines.
To prevent sparking, the only effective way of insulating the cable
is to keep huge air spaces around them. That’s why we have to be
suspended from pylons. Underground cables are more difficult
insulate and must be used at lower voltages, to transmit same
power they have to carry higher current. This means that we have
to use thicker cables and it will be very expensive to lay. Despite
the extra cost.
Underground cables are used in areas of outstanding natural
beauty so the destruction of ground.