Magnets, magnetism, Laws of Magnetism, Faraday's Laws, Biot-Savart Law, Ampere's Law, Lenz's Law

1. Basics of Electrical Engineering
 Magnets and Magnetism
 Laws of Magnetism
By
Ms. Nishkam Dhiman
Assistant Professor -EEE Deptt.
Chitkara Institute of Engg. & Technology

2. Magnet
 A magnet is a material or
object that produces magnetic
field. This magnetic field is
invisible but is responsible for
the most notable property of a
magnet: a force that pulls on
her ferromagnetic materials,
such as iron, and attracts or
repels other magnets.

3. Magnetic Domains
 Iron and other ferromagnetic substances, though,
are different. Their atomic makeup is such that
smaller groups of atoms band together into areas called domains,
in which all the electrons have
the same magnetic orientation. Inside each of
these domains, the electrons are oriented in the
same direction. But the iron is not magnetic
because these domains themselves are not aligned.
 If you put that piece of iron within a magnetic field, though, the
domains all begin to point in the same direction. The result? A
magnet! A permanent magnet is nothing more than such an
object in which all the domains are aligned in the same
direction.

4. Types of Magnets
 Permanent magnets: are magnets that retain their magnetism
once magnetized.
Ferromagnetic material which
include iron, nickel, cobalt, some alloys of rare earth
metals, and some naturally occurring minerals such
as lodestone.
 Temporary magnets: are materials magnets that perform like
permanent magnets when in the presence of a magnetic field,
but lose magnetism when not in a magnetic field. Example :
Electromagnets.
 Electromagnets: are wound coils of wire that function as
magnets when an electrical current is passed through. By
adjusting the strength and direction of the current, the strength
of the magnet is also altered. Often, the coil is wrapped
around a core of "soft" ferromagnetic material such as steel.

5. Properties of Magnets
1. Magnets attract objects of iron, cobalt and nickel.
2. The force of attraction of a magnet is greater at its poles
than in the middle.
3. Like poles of two magnets repel each other.
4. Opposite poles of two magnets attracts each other.
5. If a bar magnet is suspended by a thread and if it is free
to rotate, its South Pole will move towards the North Pole
of the earth and vice versa.

6. Magnetic Dipoles
 Every magnet is a magnetic dipole.
 If a magnetic piece of steel rod is cut into smaller pieces, each piece is a
magnet with a N or a S pole.
 Therefore a magnet can be said to be made of lots of "tiny" magnets all
lined up with their N poles pointing in the same direction. At the ends,
the "free" poles of the "tiny" magnets repel each other and fan out so
the poles of the magnet are round the ends.
 Magnetic Monopole does not exists

7. Magnetic Field
 A magnetic field is the area around a permanent magnet or
a wire carrying a current in which a force is experienced.
 Also called magnetic flux density
 Units : Weber/meter sq or Tesla
 The direction of the field at any
point should be the direction of
the force on a N pole.
 The direction is shown by
arrows - these point away
from N pole towards S pole.

8. Magnetic Flux
 Magnetic flux can be defined as a measure of magnetic
field in a certain medium. In simple terms, if the
magnetic field had to pass through a certain medium,
it will always travel as "flux" lines (flux lines are
imaginary, but continuous lines traveling from north
pole of a magnet to its south pole). Units are Weber.

9. Permanent Magnet and Electromagnet
 A permanent magnet is an
object made from a material that
is magnetized and creates its own
persistent magnetic field. As the
name suggests, a permanent
magnet is 'permanent'. This
means that it always has a
magnetic field and will display a
magnetic behavior at all times.

An electromagnet is made from
a coil of wire which acts as a
magnet when an electric current
passes through it. Often an
electromagnet is wrapped around
a core of ferromagnetic material
like steel, which enhances the
magnetic field produced by the
coil.

10. Permanent Magnet Vs Electromagnet
 1.Properties: Permanent magnet has persistence magnetic
field. An electromagnetic magnet only displays magnetic
properties when an electric current is applied to it
 2. Magnetic Strength: Permanent magnet magnetic
strength depends upon the material used in its creation.
The strength of an electromagnet can be adjusted by the
amount of electric current allowed to flow into it
 3. Advantages : The main advantage of a permanent
magnet over an electromagnet is that a permanent magnet
does not require a continuous supply of electrical energy to
maintain its magnetic field. However, an electromagnet’s
magnetic field can be rapidly manipulated over a wide range
by controlling the amount of electric current supplied to the
electromagnet.

11. Laws of Electromagnetism:
There are four laws of electromagnetism:
 The law of Biot-Savart: magnetic field generated by
currents in wires
 Ampere's law : the effect of a current on a loop of flux
which it threads
 Force law : the force on an electron moving through a
magnetic field
 Faraday's law: the voltage induced in a circuit by
magnetic flux cutting it

12. Biot Savart Law
 The Biot–Savart law is used for computing the
resultant magnetic field B at position r generated by
a steady current I (for example due to a wire): a
continual flow of charges which is constant in time
and the charge neither accumulates nor depletes at any
point.Units Weber/m-sq

13. Ampere’s Law
 Ampere's Law states that for any closed loop path, the
sum of the length elements times the magnetic field in
the direction of the length element is equal to the
permeability times the electric current enclosed in the
loop.
 Ampere's Law: ΣB|| Δl = μ0 I

14. Force Law: Force Due to Magnetic
Field
F = qv B
 F = force (vector)
 q = charge on the particle (scalar)
 v = velocity of the particle relative
to field (vector)
 B = magnetic field (vector)
 The magnitude of a cross is the product of the magnitudes
of the vectors times the sine of the angle between them. So,
the magnitude of the magnetic force is given by
 F = qvBsin
where
is angle between q v and B vectors

15. . The direction of the force is given by the right hand
rule

16. Faraday's law of
electromagnetic induction
Faraday’s First Law
 Any change in the magnetic field of a coil of wire will cause
an emf to be induced in the coil. This emf induced is called
induced emf and if the conductor circuit is closed, the
current will also circulate through the circuit and this
current is called induced current.
Method to change magnetic field:
1. by moving a magnet toward or away from the coil
2. by moving the coil into or out of the magnetic field.
3. by changing area of a coil placed in the magnetic field
4. by rotating the coil relative to the magnet.

17. Faraday's Second Law
 It states that the magnitude of emf induced in the coil
is equal to the rate of change of flux linkages with the
coil. The flux linkages of the coil is the product of
number of turns in the coil and flux associated with
the coil.
Flux Φ in Wb = B.A
B = magnetic field strength
A = area of the coil
Generator action (RHR)

18. LENZ'S LAW
 Lenz law states that when an emf is generated by a change in magnetic
flux according to Faraday's Law, the polarity of the induced emf is such
that it produces a current whose magnetic field opposes the change
which produces it.
 If the magnetic flux Ф linking a coil increases, the direction of current
in the coil will be such that it oppose the increase in flux and hence the
induced current will produce its flux in a direction as shown below
(using right hand thumb rule).
 If magnetic flux Ф linking a coil is decreasing, the flux produced by the
current in the coil is such that it aid the main flux and hence the
direction of current is as shown below.

19. Applications of Faradays Laws
 Electrical Transformers
 Electrical Generators
 Induction Cookers
 Musical Instruments

20. Solenoids
 Linear solenoids are electromechanical devices which
convert electrical energy into a linear mechanical motion
used to move an external load a specified distance. It
consists of
 Cylindrical coil
 A steel or iron frame
 A steel or iron plunger
 A stationary magnetic
pole/ travel stop

21. Working of Solenoid
 Current flow through the solenoid coil winding creates
a magnetic field which produces an attraction between
a movable plunger and a fixed stop.
 When electrical power is applied, the solenoid’s
plunger and its external load accelerates and moves
toward the solenoid’s stop until an impact occurs. The
plunger rides inside the core of the coil assembly.
 Removal of power from the solenoid eliminates the
current flow through the coil , the plunger with
external

22.  APPLICATIONS OF SOLENOIDS
 Electronically activated door locks, pneumatic or hydraulic
control valves, robotics, automotive engine management,
irrigation valves to water the garden and even the "DingDong" door bell has one