Transcript Document

Magnetism
Forces between bar magnets
S
S
N
N
Repulsion
(a)
Attraction
(b)
N
S
S
N
Like magnetic poles repel; unlike poles attract
Magnetic field surrounding a bar magnet
N
S
Magnetic Flux & Flux Density
Magnetic Flux ( Ø) is measured in webers (Wb)
N
Area A
S
magnetic flux density = magnetic flux/area
B =  / A (measured in tesla (T) )
Molecular Theory of Magnetism
(Breaking A Permanent Magnet)
S
N
N
N
N
S
S
N
S
N
S
S
N
S
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
Flux density B (T)
Saturation
Remanence
X
W
Coercitivity
-I
Y
O
Z
Saturation
-B
I
Magnetising Current
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
Field
Current
Thumb – Direction of Current
Fingers – Direction of Field
Simplified schematic of a magnetic
field around a long straight conductor
Current
flowing
into page
Conductor
X
Current
flowing
out of page
Magnetic field surrounding a solenoid
B
(a) Actual Solenoid
X
X
X
X
X
NI
NI
 o  r
L
L
X
(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 = the 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).
Concentration of flux density in a ferromagnetic material
inside a coil of wire (solenoid)
Flux concentrated
mainly in corenot in surrounding air
I