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Bell Work: Magnetism


Magnetic fields are strongest near the ( poles / center ) of the magnet. Bell Work: Magnetism ... It converts mechanical energy to electrical energy. – PowerPoint PPT presentation

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Title: Bell Work: Magnetism

Bell Work Magnetism
  1. When regions of iron atoms are aligned, a
    magnetic ( block / domain / pole ) is created.
  2. When a magnet attracts a paperclip, the clip is (
    permanently / temporarily ) magnetized.
  3. Name two metals that are not ferromagnetic.
  4. A compass lines up ( parallel / perpendicular )
    to the magnetic fields.
  5. Magnetic fields are strongest near the ( poles /
    center ) of the magnet.

Earth as a Magnet
  • When there is current through a wire, the filings
    will form a pattern of concentric circles, around
    the wire.
  • A wire with current will create a magnetic field
    perpendicular to the current flow

Which way?
  • To determine the current and the magnetic field,
    we use right hand rules.
  • Is the current flowing into or out of the screen?
  • Is the magnetic field rotating
    clockwise or counter- clockwise?

1st Right Hand Rule
Imagine holding a length of insulated wire
with your right hand. Keep your thumb pointed
in the direction of the conventional (positive)
current. The fingers of your hand circle
the wire and point in the direction of the
magnetic field. Determines Direction of the
Magnetic Field relative to the current
Determine the direction of the current flow.
2nd Right Hand Rule
  • Imagine holding an insulated coil with your right
    hand. If you then curl your fingers around the
    loops in the direction of the conventional
    (positive) current, your thumb will point toward
    the north pole of the electromagnet.
  • Determines Polarity

Which end is North?
Which end is North?
Which way is the current flowing?
  • When a wire is looped several times to form a
    coil and a current is allowed to flow through the
    coil, the field around all the loops is in the
    same direction.

  • A long coil of wire consisting of many loops is
    called a solenoid. The field from each loop in a
    solenoid adds to the fields of the other loops
    and creates a greater total field strength.

  • When there is an electric current in a coil of
    wire, the coil has a field similar to a permanent
  • One end of the electromagnet will attract one end
    and repel the other end of a magnet.
  • Thus, the current-carrying coil has a north and a
    south pole and is itself a magnet.
  • This type of magnet, which is created when
    current flows through a wire coil, is called an

  • The strength of the field is directly related to
    the current in the coil.
  • The strength of the field is directly related to
    the number of coils.
  • The magnetic field produced by each loop is the
    same. Because these fields are in the same
    direction, increasing the number of loops
    increases the strength of the magnetic field.
  • The strength of an electromagnet also can be
    increased by placing an iron rod or core inside
    the coil.
  • It increases the magnetic field because the field
    of the solenoid creates a temporary magnetic
    field in the core, just as a nearby permanent
    magnet does when brought near a metal object.

Electromagnetic Induction
  • In 1822 Micheal Faraday wronte a goal in his
    notebook, it was Convert magnetism into
  • After nearly 10 years he found that he could
    induce electric current by moving a wire through
    a magnetic field.
  • The same year, Joseph Henry an American
    high-school teacher, also showed that a changing
    magnetic field could produce electric current.

Electromagnetic Induction
  • When the wire is moved parallel to the magnetic
    field or held stationary, there is no current,
    but when the wire moves up through the field, the
    current is in one direction.
  • When the wire is moves down through the field,
    the current is in the opposite direction.

Electromagnetic Induction
  • An electric current is generated in a wire only
    when the wire cuts magnetic field lines.
  • It is relative motion between the wire and the
    magnetic field that produces a current.
  • Electromagnetic Induction

Electric Generator
  • The Electric Generator was invented by Michael
  • It converts mechanical energy to electrical

Electric Generator
  • An electric generator consists of a number of
    wire loops placed in a strong magnetic field. The
    wire is wound around an iron core to increase the
    strength of the magnetic field.

Electric Generator
  • The iron and wires are called the armature, which
    is similar to that of an electric motor.

Electric Generator
  • The armature is mounted so that it can rotate
    freely in the magnetic field. As the armature
    turns, the wire loops cut through the magnetic
    field lines and induce a potential difference.
    Commonly called the voltage.

Electric Generator
The current is greatest when the motion of the
loop is perpendicular to the magnetic field (the
loop is in the horizontal position).
When the loop is in the vertical position, the
wire segments move parallel to the field and the
current is zero.
Electric Generator
The current changes smoothly from zero to some
maximum value and back to zero during each
half-turn of the loop. Then it reverses direction.
Electric Generators
  • The entire loop does not contribute to the
    induced EMF?
  • If the fourth right-hand rule is applied to
    segment ab, the direction of the induced current
    is toward the side of the wire. The same applies
    to segment cd.
  • No current is induced along the length of the
    wire in ab or cd.

Generators and Motors
  • Generators and motors are almost identical in
    construction, but they convert energy in opposite
    directions. Generators convert mechanical energy
    to electrical energy, motors converts electrical
    energy to mechanical energy.

  • Transformers are used o increase or decrease AC
  • Transformers change voltages with relatively
    little loss of energy.

  • There are two types of transformers
  • Step-Up Transformers - the secondary voltage is
    larger than the primary voltage.
  • Step-Down Transformers - the voltage coming out
    of the transformer is smaller than the voltage
    put in the transformer.