MAGNETIC CIRCUITS
4.1 INTRODUCTION TO ELECTROMAGNETISM
•A conductor carrying a Current is always surrounded all along its length by a magnetic field. This important
relationship between magnetism and current electricity which was discovered by Oersted in 1820 forms the basis
of electromagnetism.
Electromagnetism is the branch of engineering dealing with the magnetic effects of an electric current
In this chapter, we shall study certain fundamentals 'of electromagnetism with special emphasis on magnetic
circuits principles.
summarized below, as it is assumed that students are already familiar with them.
4.2 MAGNETISM AND ITS EFFECTS
•Before we begin the study of electromagnetism, some of the common terms used in magnetism are briefly
• Important Properties of a Magnet:
0
() When freely suspended by a piece of silk fibre, it sets itself in a definite direction so that its North pole
It attracts small pieces of iron.
points towards the North direction and the South pole points towards the South direction.
(ii) Like magnetic poles repel and unlike poles attract each other.
(say iron) without actual physical contact.
• Magnetic Induction:
t is the phenomenon due to which a magnet can induce magnetism in a neighbouring piece of magnetic material
• Pole Strength: The magnitude of the force exerted by one magnet on the another magnet gives rise to the idea
of pole strength. If under exactly identical conditions, two magnetic poles are exerting equal forces on another
pole, then they are said to have equal pole strengths.
Unit of pole strength is weber (also called unit pole).
t is defined as the strenath of that pole which when placed from an identical pole at a distance of 1 metre in free
space
force of 1 ne tons
10
Laws of Magnetism: The two fundamental laws of magnetism are as follows:
0) As already mentioned previously, the first law states that like magnetic poles repel and unlike poles attract
(i) According to the second law which is proved experimentally by Coulomb, the force (F) exerted by one
pole on another pole is directiy proportional to the product of the pole strengths and inversely
roportional to the square of the distance (d) between them. It also depends on the nature of the
medium surrounding the poles, ie.
m m2
d
F
F= Kn
The value of the constant K depends on the nature of the surrounding
where m
edium
and m are the pole strengths. The value of the constant K
Magnetic Field:
The reqion in the neighbourhood of a magnet within which the influence of the magnet is felt is termed as magnetia
Existence of the magnetic field can be very well tested with the help of a small magnetized compass needle.
• Magnetic Lines of Force or Lines of Magnetic Flux: These are the imaginary lines (having no physical
existence) introduced by Faraday for the pictorial representation of the distribution of a magnetic field.
A line of force may be defined as o line along which an isolated N-pole would travel if free to moe in a magnetic
feld and it is such that the tongent at any point gives the direction of the resultant force at that point.
• Magnetic Field:
The reaion in the neiahbourhood of a magnet within which the influence of the magnet is felt is termed as magnet
field
Existence of the magnetic field can be very well tested with the help of a small magnetized compass needle.
• Magnetic Lines of Force or Lines of Magnetic Flux: These are the imaginary lines (having no physica
existence) introduced by Faraday for the pictorial representation of the distribution of a magnetic field.
A line of force may be defined as a line along which an isolated N-pole would travel if free to move in a magnet
field and it is such that the tongent at any point gives the direction of the resultant force at that point.
The mapping of the magnetíc field into a series of magnetic lines of force can be done with the help of a smal
compass needle (Fig. 4.1).
•Properties of Lines of Force:
(0)
Lines of force are always in the form of closed curves originating on a N-pole and terminating
Lines of force never cross one another.
Parallel lines of force acting in the same direction repel one another.
Lines of force always try to contract in length and thus behave like stretched elastic threads.
Magnetic lines of force always take the path of least reluctance (opposition). Materials which do not
readily allow passage of flux lines are said to have a comparatively high reluctance to magnetic fields
Since air, for example, has a greater reluctance to magnetic fields than iron has, the concentration of the
magnetic field becomes greater in iron than it does in air.
(iv)
()
•Magnetic Flux (0):
The force experienced by a unit North pole (Le. N-pole with l weber pole strength) placed at any point in a mog
field is known as magnetic field strength at that point
nt of mnagnetic field strength is newtons per weber (N / Wb) or amperes per metre (A I m).
Higher the value of this force, stronger is the field.
•
• As already mentioned previously, a straight conductor carrying an electric
• The lines of force are in the form of concentric circles
CTOR
its length by a magnetic field.
Current is always surrounded all alony
point considered.
4.3 MAGNETIC FIELD DUE TO A STRAIGHT CONDUCT
When iron filings are sprinkled on the cardboard, they arrange themselves in concentric circles around t
current carrying conductor. The direction of the field can be found by replacing the iron filings by compa
needles.
.Fig. 4.2 (b) illustrates the conventional representation of current carrying conductor alongwith the direction
Current flowing through it and the direction of the magnetic field around it.
• Here, the small circle represents the conductor in section. The cross on it represents the rear view of t
feathered end of an arrow and indicates a current flowing into the plane of the paper (e. away from th
observer) in a conventional manner. On the other hand, the dot represents the tip of an arrow and indicates
current flowing out of the plane of the paper (ie. towards the observer).
. The direction of the field may be quickly determined by means of the following two rules:
(0) The Right Hand Gripping Rule: Grip the current carrying conductor in the right hand with the
outstretched parallel to the conductor and pointing in the direction of the current (Fig. 4.3).
The curled fingers then point in the direction of the magnetic field around the conductor.
(ii) The Corkscrew Rule: Imagine a right handed corkscrew or wood screw (Fig. 44) placed alongside of th
Current carrving conductor with its axis parallel to the conductor and tip pointing in the direction of the curren
the direction of the magnetic field is grven by the direction in which the screw must be turned e
ddvance it in the direction of the
Straight
Then the direction of the
current.
tabt Conductor:
agnetic Field Strength of a Long
Oue to maanetic field in the neighbourhood of a current carrying conductor, effect
those in the vicinity of a permanent magnet ie. the forces are exerted on magnets iron o
Conductors which ar
carrying conductor, effects are produced very much like
he forces are exerted on magnets, iron or other current carrying
atic field in the neighbou
eh are introduced in this
ontally verified that th
ie effect or in other words, the magnetic field strength (H)
To be continued.............
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