Electromagnetism chapter 21

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Electromagnetismchapter 21
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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

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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 --gt the object has no net magnetization
  (the same if no atomic magnet exist)
• - if the atomic magnets are aligned --gt the
  object is magnetized

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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)
• 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
• Fq v B sin(?)
• direction perpendicular on the plane formed
  by v and B

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Magnetic field of the Earth

• 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 Earths magnetic field --gt still a
  mystery (modern theories presume that is the
  result of the convection currents in melted
  metals in the core, but no accepted theories)
• Earths magnetic field is a shield against high
  energy and charged cosmic particles
• the strength of the magnetic field is varying--gt
  momentary decreasing
• The Earth switched the poles 171 times during
  last 17 million years
• During switching long periods without any
  magnetic field--gt danger to living organism.
• Next disappearance of the magnetic field
  (prognoses) after 4000 years.

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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 Lenzs law
• The induced current always produces a
  magnetic field to oppose the change!

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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

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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 --gt 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!)

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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!

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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--gt possibility to propagate the
  electromagnetic field!!!!
• Propagating electromagnetic field --gt
  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 c3 x 108 m/s
• light, together with many other radiation are
  electromagnetic waves!
• As a function of the frequency many different
  waves!

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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

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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 Earths 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