Electromagnetic field (emf)

PRESENTED BY:
• Group No.: 04
• Group Member:
Amarta Sarkar (001)
Asadul Islam (005)
Nasim Ali (006)
Shawan Roy (023)
Sabrina Wazir (039)
Department of Textile, BUBT
• 5th Intake , Section : 1

ELECTROMAGNETIC FIELD
• An electromagnetic field (also EMF or EM field) is a physical field
produced by moving electrically charged objects.
• It affects the behavior of charged objects in the vicinity of the field.
• The electromagnetic field extends indefinitely throughout space and
describes the electromagnetic interaction.
• It is one of the four fundamental forces of nature (the others are
gravitation, the weak interaction, and the strong interaction).
• The field can be viewed as the combination of an electric field and a
magnetic field.
• The electric field is produced by stationary charges, and the magnetic
field by moving charges (currents); these two are often described as the
sources of the field.

ELECTROMAGNETISM
• Electromagnetism is one of the fundamental phenomenon in nature. It
is responsible for almost all the phenomena in our daily life.
• Electromagnetism spans both electric fields and magnetic fields.
• When observed individually, electricity and magnetism behave
differently but when unified, we can observe that both are
interdependent on each other and they cannot be separated from each
other.
• In order to fully understand Electromagnetism, we have to look at the
four laws that govern electricity and magnetism.
• These are Gauss’s laws in Electrostatics, Gauss’s law in
Magnetism, Ampere’s law and Faraday’s law.
• These laws were combined by James Clerk Maxwell in the year 1864 to
give a complete set of relation and connection between both the forces
of electricity and magnetism.

ELECTIC FILEDS AND MAGNETIC FIELDS
MAGNETIC
ELECTRIC FIELDS FIELDS
1. Electric fields arise from 1. Magnetic fields arise from
voltage. current flows.
2. Their strength is measured 2. Their strength is measured in
amperes per meter (A/m).
in Volts per meter (V/m) Commonly, EMF investigators
3. An electric field can be use a related measure, flux
density (in micro tesla (µT) or
present even when a device mille tesla (mT) instead.
is switched off.
3. Magnetic fields exist as soon as
4. Field strength decreases a device is switched on and
with distance from the current flows.
source. 4. Field strength decreases with
5. Most building materials distance from the source.
shield electric fields to 5. Magnetic fields are not
some extent. attenuated by most materials.

USES FOR ELECTROMAGNETS
• An electromagnet does all the things that ordinary magnets can
do, but you can switch them on and off.
• An electric bell – uses an electromagnet to rapidly pull the
hammer over to the gong then release it.
• For sorting scrap – an electromagnet can be used to pick up
and put down magnetic materials, sorting them from non-
magnetic scrap.
• In speakers – an electromagnet is used to move a cone very
rapidly, causing sound waves.
• In switches – a small current can be used to operate an
electromagnet, which in turn can control another circuit in
which a much larger current might be flowing. This isolates the
large current from the person operating the switch, making it
safer.


Diagram of
an electric
bell


An
electromagnet
being used to
pick up scrap


Relays are used
in circuit
control.

THE MOTOR EFFECT
“A conductor carrying an electric
current may experience a force when
placed into a magnetic field.”
To increase this force:
 Increase the current
 Increase the number of coils
 Increase the strength of the magnet
 Increase the length of conductor in
the field
To reverse this force:
 Reverse the direction of the current
 Reverse the direction of the
(permanent) magnetic field

NOTE: There is NO FORCE if the
conductor is parallel to the field.

Reverse the Keep the field
field the same
Keep the current Reverse the
the same current

Motion

Current

Field Motion Motion
reverses reverses

ELECTROMAGNETIC INDUCTION
 A potential difference is induced across the ends of a conductor when it
cuts across magnetic field lines. This is called Electromagnetic
Induction.
 The same effect occurs if the conductor is held still and the magnetic
field changes.
 The faster the conductor cuts the field lines (or the faster the magnetic
field changes) the bigger the potential difference induced.

A SIMPLE DYNAMO
 If the conductor forms part of
a circuit, a current will flow.

 In a dynamo, a coil is rotated
inside a magnetic
field, causing an alternating
current to flow.

 You can use the right hand
rule to prove to yourself that a
current will flow all the way
around the coil of wire when
the coil is rotated.

TRANSFORMERS
• A coil of wire is wound on to
one side of a soft iron core.
This coil is called the primary
coil.

• When an alternating current
flows through this wire, an
alternating electromagnetic
field is set up in the core.

TRANSFORMERS

• If a secondary coil is then wound
on to the other side of the
core, this changing magnetic field
will induce an alternating potential
difference across the ends of the
secondary coil.

TRANSFORMERS

Transformers step voltage up or down. The
size of the induced voltage is given by the
ratio:
p.d. acrossprimary number of turns on primary
p.d. acrosssecondary number of turns on secondary
or
Vp Np
Vs Ns

TRANSFORMERS AND MAINS SUPPLY
• Electricity is generated at the power station at about
33,000V.
• A step-up transformer steps this up to about 400,000V for
transmission in overhead cables.
• This is then stepped down for use in homes, to 230V (or
for industrial uses, to 11,000V).
• WHY?

TRANSFORMERS AND MAINS SUPPLY
• When the potential difference is stepped up, the current is
stepped down.
• So there is a lower current flowing through the wires.
• This means that less energy is lost to heat (P=I2R).
• So more of the power supply’s energy gets to the
appliance, rather than being lost in the wires.

WHAT HAPPENS WHEN YOU ARE EXPOSED TO ELECTROMAGNETIC FIELDS?

 Exposure to electromagnetic fields is not a new phenomenon.
 However, during the 20th century, environmental exposure to man-made
electromagnetic fields has been steadily increasing as growing
electricity demand, ever-advancing technologies and changes in social
behavior have created more and more artificial sources.
 Everyone is exposed to a complex mix of weak electric and magnetic
fields, both at home and at work, from the generation and transmission
of electricity, domestic appliances and industrial equipment, to
telecommunications and broadcasting.
 Tiny electrical currents exist in the human body due to the chemical
reactions that occur as part of the normal bodily functions, even in the
absence of external electric fields.

ELECTROMAGNETIC FIELDS AT HOME
 Electricity is transmitted over long distances via high voltage power
lines.
 Transformers reduce these high voltages for local distribution to homes
and businesses.
 Electricity transmission and distribution facilities and residential wiring
and appliances account for the background level of power frequency
electric and magnetic fields in the home.
 In homes not located near power lines this background field may be up
to about 0.2 µT.
 Directly beneath power lines the fields are much stronger.
 House walls substantially reduce the electric field levels from those
found at similar locations outside the house.

TYPICAL ELECTRIC FIELD STRENGTHS MEASURED NEAR HOUSEHOLD APPLIANCES

SUMMARY OF THE ICNIRP EXPOSURE GUIDELINES

European Mobile phone Microwave
power base station oven
frequency frequency frequency

50 Hz -50 Hz 900 MHz -1.8 2.45 GHz
Frequency
GHz

Electric field Power density Power density
(V/m) -Magnetic (W/m2) -Power (W/m2)
field (µT) density (W/m2)

ELECTROMAGNETIC AND GRAVITATIONAL FIELDS

o Sources of electromagnetic fields consist of two types of
charge –> positive and negative.
o This contrasts with the sources of the gravitational
field, which are masses.
o Masses are sometimes described as gravitational
charges, the important feature of them being that there is
only one type (no negative masses), or, in more colloquial
terms, 'gravity is always attractive'.

ELECTROMAGNETIC AND GRAVITATIONAL FIELDS

REFERENCES
 Wikipedia
 WHO
 Google Search
 Some Books:
1. Electromagnetic Fields (2nd Edition), Roald K.
Wangsness, Wiley, 1986. ISBN 0-471-81186-6 (intermediate level
textbook)
2. Schaum's outline of theory and problems of
electromagnetics(2nd Edition), Joseph A. Edminister, McGraw-
Hill, 1995. ISBN 0070212341(Examples and Problem Practice)

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