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

E L E C T R I C F I E L D S

1. Electric fields arise from voltage.

2. Their strength is measured in Volts per meter (V/m)

3. An electric field can be present even when a device is switched off.

4. Field strength decreases with distance from the source.

5. Most building materials shield electric fields to some extent.

M A G N E T I C F I E L D S

1. Magnetic fields arise from current flows.

2. Their strength is measured in amperes per meter (A/m). Commonly, EMF investigators use a related measure, flux density (in micro tesla (µT) or mille tesla (mT) instead.

3. Magnetic fields exist as soon as a device is switched on and current flows.

4. Field strength decreases with distance from the source.

5. Magnetic fields are not 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.

USES FOR ELECTROMAGNETS

Diagram of an

electric bell

USES FOR ELECTROMAGNETS

An electromagnet being used to pick up scrap

USES FOR ELECTROMAGNETS

Relays are used in circuit control.

THE MOTOR EFFECT

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

“A conductor carrying an electric current may experience a force

when placed into a magnetic field.”

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

Keep the field the same

Reverse the field

Field

Current

Motion

Motion reverses

Reverse the current

Motion reverses

Keep the current the same

DIAGRAM OF AN ECLECTIC MOTOR

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

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:

s

p

s

p

N

N

V

V

or

secondaryon turnsofnumber

primaryon turnsofnumber

secondary across p.d.

primary across p.d.

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 power

frequency

Mobile phone base

station frequency

Microwave oven

frequency

Frequency 50 Hz -50 Hz 900 MHz -1.8 GHz

2.45 GHz

Electric field (V/m) -

Magnetic field (µT)

Power density (W/m2) -Power

density (W/m2)

Power 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)