EEE107 Magnetism

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

Magnetic field surrounding a bar magnet

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) )

Molecular Theory of Magnetism (Breaking A Permanent Magnet)

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

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

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 out of page Current flowing into page Conductor X

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

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