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Electromagnetism
chapter 21
Magnets
• Magnetic forces are known since 6th century
B.C.
• Permanent magnets made from: Co, Ni,Fe
• each magnet has two poles: (south and north)
• Like poles repel; unlike poles attract
• Difference with electric phenomena: the
poles of a magnet cannot be separated (no
magnetic monopole)
• Similarity with electric phenomena: magnets
are surrounded by magnetic fields
- the magnetic field can be detected using a
small compass
- the compass points in the direction of the
field lines
- iron filings line up also in the direction of
the magnetic field
- strength of the magnetic field ~ density of
field lines
•
Electric
magnetism
Connection between electric and magnetic phenomena (1820 Hans Christian
Oersted)
- current-carrying wire produces a magnetic field
- stationary charges do not produce magnetic field
• Direction of the magnetic field produced by
currents: right-hand rule
• Different field patterns can be produced by bending the wire,
making loopes
• All magnetic fields originate from current loops (Andre Ampere)
- the currents can be at microscopic level, originating from the
rotation of elementary charges
- the macroscopic magnetic properties of an object are determined
by the combined effects of these atomic level magnets
- if the orientation of the atomic magnets is random --> the object has no net
magnetization (the same if no atomic magnet exist)
- if the atomic magnets are aligned --> the object is magnetized
Electromagnetic interactions
• Interaction between a current-carrying wire and permanent magnets
• Interaction between two current carrying wires
- the source of the force on each wire is the magnetic field produced by the other
- parallel wires attract if current directions are the same, and repel if current
directions are opposite
- this effect is used to define the unit of current ( Ampere) in the metric system:
consider two long parallel wires separated by 1m distance and carrying the same
current. If the force between these wires is 2 x 10-7 N on each meter of wire, the
current is defined as 1 Ampere.
- strength of the magnetic field, units in T (Tesla) or G (gauss)
F q
the strength of the magnetic field at a distance of 1m from
a long straight wire carrying a current of 1A is 2 x 10-7 T.
• Interaction between magnetic field and moving charged particles
magnetic field interacts only with moving charged particles
force: depends on the velocity of the particle (v), intensity of`
the magnetic field field (B), charge of the particle (q), and
angle () between field and velocity:
F=q v B sin()
direction: perpendicular on the plane formed by v and B
Magnetic field of the Earth
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Earth is a giant magnet with its south magnetic pole in the Northern Hemisphere
one pole (south) located north of Hudson Bay (Canada)
the other pole (north) located directly on the other side of Earth
strength of the magnetic field at the surface of the Earth ~ 0.5 G
magnetic axes tilted with 12 0 relative to the rotational axis
origin of the Earth’s magnetic field --> still a mystery (modern theories presume
that is the result of the convection currents in melted metals in the core, but no
accepted theories)
Earth’s magnetic field is a shield against high energy and charged cosmic
particles
the strength of the magnetic field is varying--> momentary: decreasing
The Earth switched the poles 171 times during last 17 million years
During switching: long periods without any magnetic field--> danger to living
organism.
Next disappearance of the magnetic field (prognoses) after 4000 years.
Magnetic electricity
• Electric current produces magnetic field
• QUESTION: Does magnetic field produce electric current?
• ANSWER: YES, but not by putting simply the wires in magnetic field!
(Michael Faraday, in 1983)
• In order to produce electric current:
- the wire must move relative to the magnetic field
- OR the intensity of the magnetic field has to change
- OR the or the amount of magnetic field lines surrounded by the wire has to
change
• Direction of the current given by Lenz’s law:
The induced current always produces a magnetic field to oppose the
change!
Transformers
• Used to change voltage of alternating currents
• The same amount of electric power can be delivered through wires at low voltage
and high current as at high voltage and low current
• Using low electrical current for electric energy transportation is more efficient: loss
of electric energy in the wires smaller
• Transformer: changes the voltage!
• Transformers made up by two coils on one iron core
• first coil: primary coil (N1 turns), connected to a source of alternating current (V1)
• second coil: secondary coil (N2 turns), produces an alternating voltage (V2)
• principle (the alternating magnetic field produced by the primary coil, induces an
alternating voltage in the secondary coil)
• basic equation:
N N

V V
1
1
2
2
Generators and motors
• Principle of a generator: rotating loop of wire in
magnetic field
• While the loop is rotating the number of field lines
within the loop changes --> induces voltage
between the two ends of the wire
• The voltage is an oscillating one
• A simple change in the way the voltage is carried to
the external circuits (with a commutator) converts
the generator to produce pulsating direct current
• Electric motor: basically a generator run backward
(we apply voltage at the end of the wires, causing
the loop to rotate!)
A question of symmetry….
• Changing magnetic field produces electric current.
• Changing electric current can produce magnetic field? The answer is yes!!!
• Maxwell equations: describe mathematically the electromagnetic field as a
whole, with connections between them. (James Clerk Maxwell, 1860)
• Changing magnetic field can produce electric field, and changing electric field
can produce magnetic field EVEN in empty space!
Electromagnetic waves
• If the magnetic field changes at a constant rate, the electric field is constant
• If the rate of change in the magnetic field varies, this produce a changing
electric field, which can produce again a changing magnetic field and so
on……--> possibility to propagate the electromagnetic field!!!!
• Propagating electromagnetic field --> electromagnetic waves (Heinrich Hertz,
1887)
• In order to generate electromagnetic waves, we need to change constantly the
electric or magnetic field at one point of space with non-constant rate!
• Electromagnetic waves: transverse waves (electric and magnetic field oscillates
perpendicularly to each other, and to the direction of the propagation)
• speed in vacuum: c=3 x 108 m/s;
• light, together with many other radiation are electromagnetic waves!
• As a function of the frequency many different waves!
Radio and TV
• Radio: coding electromagnetic waves with the useful information (sound), so
they can be transmitted through space, intercepted and converted back into
sound.
• Sound waves are transformed in electrical signals (oscillating electric current)
• This electrical signal is combined with a broadcast signal or carrying signal
(much higher frequency), which is than is amplified and radiated through
antennas as electromagnetic waves
• Electromagnetic waves travel in vacuum and reaches the radio sets.
• the frequency of this electromagnetic waves (carrier frequency), depends on the
broadcasting station.
• Combination of the useful signal with the carrying signal called: modulation!
• Two types of modulation: amplitude modulation (AM) and frequency
modulation (FM)
• Radio set in order to receive the
modulated signal, is tuned so it only
resonates with on carrying frequency
at a given time.
• The radio filters out the carrying
frequency and retain the electrical
version of the sound information
• amplified and reconverted to sound
Summary
• magnets have two poles; like poles repel and unlike poles attract; poles cannot exist
without each-other. Magnets attract some objects and have no effects on others
• magnets have no effect on stationary charges
• field lines can be used to represent magnetic field at all points in space
• current-carrying wire produces a magnetic field and interacts with other magnets
• all magnetic fields originate from current loops.
• the Earth’s magnetism is still a mystery, its strength at the surface is ~ 5 x 10-5 T
• a charged particle moving in magnetic field experiences force at right angles to its
velocity and to the magnetic field.
• If the number of magnetic lines passing through a loop of wire changes for any
reason, a current is produced in the loop
• Connections between electricity and magnetism are best expressed in terms of the
fields
• Changing magnetic field can generate changing electric field, and changing electric
field can generate a changing magnetic field, creating electromagnetic waves
• Electromagnetic waves are transverse waves, and travel with the speed of light
Home-work: Part I. 542/1-5; 543/6-11,13-17,20; 544/21,22; 546/1-10
Part II. 544/27-30,33-38; 545/39-43,45,46,49,50,52,55-57;
546/58-62,11-15,19,20; 547/21,22