Magnetic field of magnets Interaction between magnetic poles: like ...

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18 Οκτ 2013 (πριν από 4 χρόνια και 26 μέρες)

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Electromagnetism


1

Electromagnetism

(A)
Magnetic field of magnets

Interaction between magnetic poles: like poles repel; unlike poles attract.

A magnet sets up a magnetic field in surrounding space. It is represented by field lines
which go from a north pole (N
-
pole) round t
o a south pole (S
-
pole).
The direction of the field
line gives the direction of the magnetic force acting on a north pole
.
The point at which the
magnetic fields cancel each other out is called a neutral point.


Where the field lines are closely
-
spaced,

the field is strong. Where the field lines are
widely
-
spaced, the field is weak.


The N
-
pole of a compass needle points north. The S
-
pole of the `earth magnet' is actually in
the north.


Electromagnetism


2


(B)
Magnetic field of electric currents

(1) Magnetic field arou
nd a current
-
carrying conductor

Current sets up a magnetic field.



The field lines around a wire are circular. Their direction can be worked out with the
right
-
hand grip rule for straight wire.




The field lines at the centre of a flat coil is straight and
at right angles to the plane of the
coil. Outside the coil, they run in loops.




The magnetic field outside a solenoid is similar to that of a bar magnet. The poles of the
solenoid can be worked out using the right
-
hand grip rule for solenoid.



The

magne
tic field of the solenoid can be increased by



increasing

the current,



increasing

the number of turns in the solenoid (for the same length of solenoid),



inserting a soft
-
iron core through the solenoid.
(Note that the solenoid retains its
magnetism even a
fter the current has been switched off if the core is made of steel
instead.)

Electromagnetism


3


(2) Electromagnets

An electromagnet consists of a coil of many turns of insulated copper wires.

The coil is
wound round a core made of a magnetic material, like soft
-
iron, to ma
ke the magnetic field
stronger.

An electromagnet can be made stronger by the following:



increasing

the current,



increasing

the number of turns in the solenoid,



inserting a soft
-
iron core through the solenoid.

Electromagnets are used in the following:

(i)

Elec
tric bells and buzzers


(ii)

Telephones


(iii)

Maglev train

Electromagnetism


4



(C)
Force on electric currents in a magnetic field

(1) Force on a current
-
carrying conductor

A current
-
carrying conductor experiences a force when placed in a magnetic field. The
directions of t
he force, the magnetic field and the current

can be worked out using Flemin
g
's
left
-
hand rule.


(2) Turning effect on a coil

Current through a coil produces a turning effect on
a

coil.



Greatest clockwise turning effect (for the same
current and magne
tic field strength)

C
lockwise
(not the greatest)
turning effect (for the
same current and magnetic field strength)



Antic
lockwise
(not the greatest)
turning effect (for the
same current and magnetic field strength)

No

turning effect

Electromagnetism


5


(3) Applications

(i)
Moving
-
coil loudspeakers

The

following figure shows the structure of a moving
-
coil
loudspeaker
.


As the alternating current flows
backwards

and forwards through the coil, the coil is
pushed

in and out. This makes the cone vibrate and give out sound
waves


(
ii)
Electric motors

An electric motor contains a rectangular coil which is free to rotate between the pales of a
magnet. Current flows into the coil via carbon brushes and a commutator (or split ring).


The commutator is a device that reverse
s the current direction every time the coil makes a
half
-
turn and thus enables the coil to rotate continuously.

The carbon brushes are used to
enhance contact.

In a
practical

motor,
the

coil is made up of a large number of turns wound on a soft
-
iron core
(
armature) to increase the magnetic field strength. The coils are set at different angles and
Electromagnetism


6

the pole pieces of the magnet are made curved to give a smoother running and a greater
turning effect.