Electric and Magnetic
Fields (2)
Presented by:
Dr. Mona Ibrahim
Lecture 1
Introduction
What is Electric Field?

An electric field is a force field that acts upon material bodies
by virtue of their property of charge, just as a gravitational
field is a force field that acts upon them by virtue of their
property of mass.

While the electric fields are generated around the particles
which obtains electric charge. During this process, positive
charges are drawn, while negative charges are repelled.
What are positive and negative charges?
Electric Field Lines

The lines for a positive charge point away from the charge

The lines for a negative charge point towards the charge
Electric Field Lines

This diagram shows the electric field lines for two equal and opposite point charges

Notice that the lines begin on the positive charge and end on the negative charge
Electric Field Lines

This diagram shows the electric field lines for two positive point charges

Notice that the same number of lines emerges from each charge because they are equal in magnitude
Electric Field Lines

If the charges are unequal, then the number of lines emerging from them will be different

Notice that the positive charge has twice as many lines
Link:
http://nd-rc.org/Physics30/2Electricity/day_05_objectives.htm
Capacitors:

Stare into the sky most days and you'll see some huge
capacitors floating over your head.

Capacitors (sometimes known as condensers) are
energy-storing devices that are widely used
in
televisions,
radios,
and
other
kinds
of electronic equipment.

The capacitors that drift through the sky are better
known as clouds and, though they're absolutely
gigantic compared to the capacitors we use in
electronics, they store energy in exactly the same
way.
Capacitors:
What is magnetic field?

An electric field as previously mentioned surrounds any stationary electric
charge

A magnetic field surrounds:

Moving Electric Charge (Current)

Magnetic Material
Magnetic field due to moving electric charge

A stationary electric charge is not affected by a
magnetic field, but a moving charge usually is.
Note that Anything that produces a magnetic field
will itself be affected by other magnetic fields.

Oersted found that when a compass needle is
placed near an electric wire, the needle deflects as
soon as the wire is connected to a battery and the
current flows
Electric current produces a magnetic field -The first
indication that electricity and magnetism are linked

What about a stationary electric charge and
magnet?
They don’t affect each other
Magnetic Materials
Natural Magnets

The naturally occurring materials which have the property of attracting iron.

These are weak magnets.
Magnetic Materials
•
•
•
•
•
•


Because electrons in atoms are charged particles in motion about the
nucleus, they produce magnetic fields.
The electrons have their own magnetic fields associated with their spin.
In any unmagnetized material, the individual magnetic fields of the
electrons are randomly oriented and cancel each other out.
In ferromagnetic materials, these fields can be aligned with one another
by an external magnetic field;
The material then produces a net magnetic field.
So we can conclude that moving electric charges are the causes of
magnetic fields even in ordinary bar and horseshoe magnets.
When the external field is removed, whether or not the orientation will
remain, depends on the material. If the orientation remains the material
becomes a permanent induced magnet, whereas if it returns to random
order, the material was only a temporary magnet.
Many metals display such properties and are attracted to magnetic
material. If a paperclip is on a magnet, it itself can act as attract others.
Unmagnetized piece of
aluminum
After magnetization under the
effect of an external magnetic
field
Magnetic Materials:
Electromagnets:






An electromagnet is a solenoid with a soft iron core.
When using iron, the magnet is temporary.
When using steel, the magnet is permanent
An iron core inside a solenoid
the magnetic field of the electromagnet is the magnetic
field of the iron core + the magnetic field of the solenoid.
The combined magnetic fields are much stronger than the
magnetic field of one or the other individually
Electromagnets are very useful and easy to use because:
 They can be turned on and off.
 The magnetic field only exists when electricity is
running through the wire making up the solenoid.
Comparison between a Bar Magnet and an
Electromagnet
Bar (permanent)
Magnet
Electromagnet
• Made of steel
•
Made of soft iron
• Poles cannot be reversed
• Poles can be reversed
• It produces a permanent • It produces a temporary
magnetic field
magnetic field
• Strength of magnetic field • Strength of magnetic field
cannot be changed
can be changed
• Does not need electric
• Requires electric current
current to act as a
to act as an
magnet
electromagnet
Applications for Magnetism

In terms of applications, magnetism is one
of the most important fields in physics.

Large electromagnets are used to pick up
heavy loads.

Magnets are used in such devices as meters,
motors, and loudspeakers.

Intense magnetic fields are used in magnetic
resonance imaging (MRI) devices to explore
the human body with better resolution and
greater safety than x-rays can provide.

Maglev Trains
Magnetic Fields

Magnetic Field lines point from the north pole to the south pole of the magnet
“LIKE POLES REPEL, UNLIKE POLES ATTRACT “
Why does a magnet suspended freely comes to rest
only in North-South direction?
• This is because the earth behaves like a
huge bar magnet.
• A piece of magnetite, when made to
hang and swing freely, would align
itself with the magnetic field of the
earth following a north-south direction
How a Compass Works?
“LIKE POLES REPEL, UNLIKE POLES
ATTRACT “
• That's all there is to a compass: the red pointer in a compass is a magnet
and it's being attracted by Earth's own magnetism (sometimes called the
geomagnetic field—"geo" simply means Earth).
Now if the needle in your compass is pointing north, that means it's being
attracted (pulled toward) something near Earth's north pole.
Since unlike poles attract, the thing your compass is being attracted to must
be a magnetic south pole. In other words, the thing we call Earth's
magnetic north pole is actually the south pole of the magnet inside
Earth.
That's quite a confusing idea, but
it'll make sense if you always
remember that unlike poles attract.
Types of magnetic fields
The magnetic field could be either:
 A steady magnetic field
Produced by the flow of a DC current in a
coil. The magnetic field of such a coil has
the same shape as the field around a bar
magnet.
 A time varying magnetic field
•An alternating current in a coil will produce
a magnetic field that oscillates:
It increases, decreases, and switches polarity
with the same frequency as the current.
•The
oscillating magnetic field of a coil with
AC in it is used in many common devices
such as AC Motors and Generators.
What are electromagnetic fields?

Although steady current can produce a steady magnetic
field, a steady magnetic field cannot directly induce a
current in a conductor. Electric current in conductors is
actually induced by the change in a magnetic field, known
as the law of electromagnetic induction.
•
Currents can be produced in loop of wire rotated in
uniform magnetic field .
•
Electromagnetic induction is used in the most important
device for the production of electricity: the generator.
Electromagnetic Induction

If a permanent magnet is passed through
loops of wire, as the wires experience the
change in the field around them, they will
have current.
Electromagnetic Induction

All that is required is that the magnet and coil move relative to each other.

If the coil moves and the magnet remains stationary, a current is induced and if the magnet moves and the coil
remains stationary also a current is induced.

If the motion is steady in either case, the induced current is in one direction.

If either the coil or the magnet oscillates back and forth, the current alternates with the same frequency—it is AC.
Course Content:

Week Number 1: The steady magnetic field

Week Number 2: Force on a current carrying wire in a magnetic field

Week Number 3: Biot Savart Law
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Week Number 4: Force between two parallel conductors carrying steady current

Week Number 5: Magnetic flux and flux density

Week Number 6: Magnetic flux over a closed surface

Week Number 7: Torque on a loop and magnetic moment +7Th week exam
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Week Number 8: Solenoid and the definition of inductor and its inductance

Week Number 9: Inductance of simple geometrics

Week Number 10: Ampere’s law and its application for thin and thick co-axial cables

Week Number 11: Magnetization, Magneto-static potential and Magneto-motive force
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Week Number 12: Energy and Co-energy in magnetic devices+ 12Th week exam
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Week Number 13: Magnetic circuits

Week Number 14: Time varying fields
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Week Number 15: Applications
Evaluation

7TH week exam
30 Marks

12th week exam
20 Marks

Reports
10 Marks

Bonus Report
5 Marks