Bell Work: Magnetism

1
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.

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Earth as a Magnet
3
Electromagnetism

• 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

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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?

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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
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Determine the direction of the current flow.
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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

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Which end is North?
N
9
Which end is North?
N
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Which way is the current flowing?
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Solenoid

• 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.

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Solenoid

• 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.

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Electromagnets

• When there is an electric current in a coil of
  wire, the coil has a field similar to a permanent
  magnet.
• 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
  electromagnet.

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• 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.

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Electromagnetic Induction

• In 1822 Micheal Faraday wronte a goal in his
  notebook, it was Convert magnetism into
  electricity.
• 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.

http//www.youtube.com/watch?vWX1fkfJPWpY
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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.

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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

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Electric Generator

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

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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.

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Electric Generator

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

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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.

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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.
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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.
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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.

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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.

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Transformers

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

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Transformers

• 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.