Download What is a Magnet?

PERMANENT MAGNETS,
ELECTROMAGNETS AND ASSOCIATED
TERMS
Group # 8
Hira Azhar CS-125
Sara Naz Haider CS-131
Marwan Qaiser CS-75
CONTENTS:
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What is a Magnet?
Types of Magnet
Magnetic Field
Magnetic Flux
Magnetic Field due to an electric current
Ampere’s Law
Application of Ampere’s Law
Electromagnetic Induction
Faraday’s Laws of Electromagnetic Induction
Direction of induced EMF
Mutual induction
Self induction
What is a Magnet?
Magnet is a body having the property of attracting other magnetic materials such as iron. It
produces a magnetic field external to itself.
Types of Magnet
There are various types of magnets depending on their properties. Some of the most well
known are listed below.
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Permanent magnets
These are the most common type of magnets that we know and interact with in our
daily lives. E.g.: The magnets on our refrigerators. These magnets are permanent in the
sense that once they have been magnetized, they retain a certain degree of magnetism.
Permanent magnets are generally made of ferromagnetic material. Such material
consists of atoms and molecules that each have a magnetic field and are positioned to
reinforce each other. They do not lose their property of magnetism that’s why they are
called permanent magnets.
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Electromagnets
Electromagnets are extremely strong magnets. They are produced by placing a metal
core (usually an iron alloy) inside a coil of wire carrying an electric current. The
electricity in the current produces a magnetic field. The strength of the magnet is
directly proportional to the strength of the current and the number of coils of wire. Its
polarity depends on the direction of flow of current. While the current flows, the core
behaves like a magnet. However, as soon as the current stops, the core is demagnetized.
They are used in large cranes to lift cables, etc.
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Temporary Magnets
Temporary magnets are those that simply act like permanent magnets when they are
within a strong magnetic field. Unlike permanent magnets however, they lose their
magnetism when the field disappears. Paperclips, iron nails and other similar items are
examples of temporary magnets. Temporary magnets are used in telephones and
electric motors amongst other things
Permanent Magnets
Electromagnets
A permanent magnet’s never loses its An electromagnetic magnet only displays
magnetic property.
magnetic properties when an electric
current is applied to it
Permanent magnet strength depends upon The strength of an electromagnet can be
the material used in its creation.
adjusted by the amount of electric current
allowed to flow into it
Used in fridge doors,screwdrivers,hybrid Used in CD players, heads on Hard drives,
engines, VHS tapes, hard drives, etc.
automatic doors, electric windows,
motors, cranes, etc.
Magnetic Field
Magnetic field is the space or region around a magnet or moving charge within which the
effects of magnetism such as deflection of a compass needle can be detected. Magnetic
field is a vector quantity and is represented by lines of induction.
Magnetic field lines connect the north and south poles of a magnet. Magnetic field lines are
always given a direction, marked by an arrow; they always go from north to south and they
never cross. The more field lines there are in a particular area, the stronger the magnetic
field. With magnets the simple rule is ‘unlike poles attract, like poles repel’ in other words, a
north pole will stick to a south pole but two north poles will resist being forced together.
Determination of the direction of the magnetic field
The direction of the magnetic field can be determined by using the right hand grip rule
which states that:
“If a current carrying wire is grasped in the right hand with the thumb pointing in the
direction of current, the curled fingers of the hand will circle the wire in the direction of
magnetic field.”
It can also be determined by using Fleming’s left hand rule which states that:
“Stretch the first finger, second finger and the thumb of the left hand so that they are
mutually perpendicular. The thumb will represent the direction of the magnetic field, the
second finger will represent the direction of the motion of the positive charge, and the
thumb will point in the direction of the force.”
Magnetic Flux
The total number of magnetic lines of induction passing through a surface is called magnetic
flux. The magnetic field depends on the following factor:
1) The greater the Magnetic field of induction “β”, the greater will be the magnetic flux.
That is φm ∝ β
2) The greater the Area of the surface “ΔΑ”, the more will be the magnetic flux. That is
φm ∝ ΔΑ.
3) Cosine of the angle θ between vector “β” and vector “ΔΑ”. The greater will be the
value of cos θ,the greater will be the flux. That is Δφm ∝ cos θ
From the above points it can be concluded that magnetic flux is the dot product of
magnetic field of induction and unit vector.
Δφm= β.ΔΑ
The unit of Magnetic flux is Weber.
Magnetic Field due to an electric current
During the motion of a charged particle such as electron, proton or an ion, magnetic
.
lines of force rotate around the particle Since electrical current moving through a wire
consists of electrons in motion, there is a magnetic field around the wire. The direction of
this magnetic field can be determined by the help of right hand grip rule.
Ampere’s Law
Ampere`s law states that:
“The sum of the products of the tangential components of magnetic field of induction and
the length of an element (ΔL) of a closed curve taken in a magnetic field is μο times the
current enclosed.”
Mathematically,
Ʃβ. ΔL= μοI
Application of Ampere’s Law
Ampere’s law can be used to determine the magnetic field “β” due to current carrying
conductors of simple shape such as solenoid and toroid, etc.
1) Magnetic Field of a solenoid
A solenoid is a long tightly wound cylindrical coil of wire. The turns of the winding are
ordinarily closely spaced and may consist of one or more layers. When current is passed
through a solenoid, a magnetic field is produced. The field lines inside the solenoid are
nearly parallel to the axis of the solenoid (in the z-direction in figure below), uniformly
distributed and close together. The field is thus uniform and strong.
On the other hand, Components of the magnetic field in other directions are cancelled by
opposing fields from neighbouring coils. Outside the solenoid the field is very weak due to
this cancellation effect and for a solenoid which is long in comparison to its diameter, the
field is very close to zero.
Mathematically, the magnetic field of toroid can be represented by,
Β= μοNI
Where,
μο is the permeability
N is the number of turns of the coil
I is the current flowing through the solenoid
2) Magnetic Field of a Toroid
Toroid is coil of insulated copper wire wound on a circular core. When a current is passed
through a toroid, circular magnetic field is produced. The field inside the turns is strong but
almost zero outside the toroid.
Mathematically, the magnetic field of toroid can be represented by,
Β= μοNI/2 πr
Where,
μο is the permeability
N is the number of turns of the coil
I is the current flowing through the toroid
2 πr is the circumference of the circular solenoid
Electromagnetic Induction
The phenomenon of producing the EMF in a loop or coil by changing magnetic flux passing
through it by moving a loop or coil across the magnetic field is called Electromagnetic
Induction. This EMF is called Induced EMF. The electric current in the closed circuit is due to
induced EMF is called induced current.
Methods of producing induced EMF:1) By relative motion between a loop or a coil and a magnet:
When a magnet is moved towards a coil or a coil is moved towards the magnet, the
flux through the coil changes and a current is induced in the coil. If the coil and the
magnet are moved away from one another, again an induced EMF is produced but in
opposite direction.
2) By changing area of a loop or wire:
When the area of a loop of wire is changed either by sliding or twisting it in a
magnetic field the magnetic flux changes which induces an EMF in the loop.
3) By changing current in a nearby coil:
When current passing through a loop or coil is changed, the magnetic flux passing
through another loop or coil changes due to which an induced EMF ( or induced
current) is produced in it.
Faraday’s Laws of Electromagnetic Induction
Michael Faraday in 1831 created the following two laws of electromagnetic induction:
1) When magnetic flux changes through a circuit (loop or coil), an EMF is induced in it. This
EMF lasts as long as the change in the flux through the circuit continues.
2) The magnitude of the induced EMF is directionally proportional to the number of turns of
the coil and the rate of change of flux (that is flux linked) through the coil.
If the flux changes through by an amount of “Δθ” in time “Δt” through a coil of “N” turns,
then the average induced EMF, Mathematically is given by:
E=-N Δθ/ Δt
Here the negative sign is due to lens law which indicates the direction of induced
emf.
Direction of induced EMF
(1) By Lenz’s law
The direction of induced EMF can be determined by using Lenz’s law which states
that:
“The direction of induced current (or induced emf) in a conductor is always such that
it opposes the cause which produces it”
(2) By Fleming’s Left hand rule.
Mutual induction
This is the phenomenon in which a change of current in one coil causes an induced emf in
the other coil. The coil in which the current is altered is called primary coil and the other one
is called coil.
Mutual induction occurs because the change of current in primary coil produces a change in
magnetic flux which is linked to the secondary coil and causes an induced emf.
The direction of induced current in secondary coil is opposite to that in primary coil in
accordance to the Lenz’s law.
The induced emf in secondary coil is directly proportional to the rate of change of current in
primary coil. Mathematically it can be written as:
Emf ∝ ΔI/Δt
Emf =constant ΔI/Δt
Emf=-M ΔI/Δt
Where M is a constant called Mutual inductance and its unit is Henry.
Self induction
This is the phenomenon in which a change of current in a coil produces and induced emf in
the same coil due to change of flux passing through it, is known as self induction.
The self induced emf is directly proportional to the rate of change of current in the coil
itself. Mathematically it can be written as:
Emf ∝ ΔI/Δt
Emf =constant ΔI/Δt
Emf=-L ΔI/Δt
Where L is a constant called Self inductance and its unit is Henry