1. EEE107
Magnetism

2. Forces between bar magnets
Repulsion
(a)
Attraction
(b)
Like magnetic poles repel; unlike poles attract

3. Magnetic field surrounding a bar magnet

4. Magnetic Flux & Flux Density
Magnetic Flux ( Ø) is measured in webers (Wb) N S
Area A
magnetic flux density = magnetic flux/area
B =  / A (measured in tesla (T) )

5. Molecular Theory of Magnetism (Breaking A Permanent Magnet)

6. 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.

7. 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

8. Hysteresis Saturation Flux density B (T) Remanence X W Coercitivity -I
Magnetising Current Z
Saturation
- B

9. 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.

10. 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.

11. (Electro) Magnetic field surrounding a long straight conductor carrying a current- +
Direction of Current flow
Direction of field

12. Illustration of the right-hand grip rule
Field
Current
Thumb – Direction of Current
Fingers – Direction of Field

13. Simplified schematic of a magnetic field around a long straight conductor
Current flowing
out of page
Current flowing
into page
Conductor X

14. Magnetic field surrounding a solenoid
(a) Actual Solenoid X
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).
(b) Cross-section through Solenoid and surrounding Magnetic Field Pattern

15. Concentration of flux density in a ferromagnetic material inside a coil of wire (solenoid)
Flux concentrated
mainly in core-
not in surrounding air