magnetism

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16 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

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Magnetism

Forces between bar magnets

S

N

N

S

S

N

S

N

Repulsion


(a)

Attraction


(b)

Like magnetic poles repel; unlike poles attract


Magnetic field surrounding a bar magnet

N

S

Magnetic Flux & Flux Density

N

S

Area A

magnetic flux density = magnetic flux/area


B =


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Molecular Theory of Magnetism

(Breaking A Permanent Magnet)

N

S

N

N

S

N

S

N

S

S

S

S

N

N

Ferromagnetic Materials (Hard & Soft)


Materials such as iron, steel, cobalt, nickel and a number of alloys
which are attracted by magnets are called
ferromagnetic
materials
.



Any of these materials can become magnetised when ‘stroked’
with another magnet. This is called
induced magnetism
.




Ferromagnetic materials can be subdivided into
hard

and
soft

magnetic materials.



Hard materials retain their magnetism once they have become
magnetised, so forming permanent magnets.



The magnetism induced in soft magnetic materials is lost as soon
as the source of the magnetism is removed.

Domain Theory of Magnetism

(a) Magnetic material in
demagnetised condition

Atomic magnets in
alignment inside

domains but domain
magnetic axes in

random directions

(b) Magnetised state


Atomic magnets turn
to bring domain
magnetic axes in
direction of
magnetising field

Hysteresis

W

X

Y

O

I

-

B

Saturation

Saturation

Magnetising Current

Z

Coercitivity

Flux density B (T)

Remanence

-
I

Electromagnetic Fields


A magnetic field produced by an electric
current is described as an
electromagnetic field.



The direction of this field is determined
by the direction of current flow, and is
always at right angles to the conductor
through which the current is moving.



Electromagnetic Fields


The strength of an electromagnetic field depends on
two factors:



the size of the current,


the arrangement of the conductor.



the electromagnetic effect can be greatly strengthened
by using coils of wire rather than straight lengths,




and also by placing a core of ferromagnetic material
such as iron within the coil of wire.


(Electro) Magnetic field surrounding a long
straight conductor carrying a current

-

+

Direction of
Current
flow

Direction
of field

+

-

Direction of
Current
flow

Direction
of field

Illustration of the right
-
hand grip rule

Current

Field

Thumb


Direction of Current

Fingers


Direction of Field

Simplified schematic of a magnetic
field around a long straight conductor

X

Current
flowing

into page

Conductor

Current
flowing

out of page

Magnetic field surrounding a solenoid

X

X

X

X

X

X

(a) Actual Solenoid

(b) Cross
-
section through Solenoid and
surrounding Magnetic Field Pattern



B = magnetic flux density at centre of coil



L = the length of the coil;



N = the number of turns in the coil;



I = the current through the coil;




o

= permeability of free space



permeability defines the degree of ease with


which magnetism (magnetic flux) flows


through it





r

= t
he relative permeability of the material


inside the coil (air in this case, with

r



1.



The units of permeability are henries per


metre (H/m). (The henry is the unit of


inductance
-

see later).

L
NI
r
o
L
NI
B





Concentration of flux density in a ferromagnetic material
inside a coil of wire (solenoid)


I

Flux concentrated

mainly in core
-


not in surrounding air