Electromagnetism 1

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Electromagnetism 1

• SPH4U Grade 12 Physics
• Unit 1

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Magnetism Review

• Every magnet has a north and south pole
• ? Like Poles Repel ? Opposite Poles
  Attract

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Magnetism Review

• The magnetic field around a magnet is strongest
  at the poles.
• The north pole and the south pole of the same bar
  magnet are in general, equally strong.
• If you cut a magnet in two pieces, each piece
  will have a north and south pole. You can keep
  cutting to make smaller and smaller magnets but
  each one will be weaker in strength.

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Magnetism Review

• A magnetic field is the space around a magnet
  where a magnetic force can be felt. It is very
  similar in theory to an electric field. The
  symbol for magnetic field is
• Lines of magnetic force show how the magnetic
  force acts around the magnet. The force is
  strongest at the poles (where the lines are close
  together) and weaker the further out you go.

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Magnetism Review

• When drawing Magnetic Field lines, remember
• Lines go from North to South outside the magnet
• Lines go from South to North inside the magnet
• Lines never cross

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Magnetism Review
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Magnetism Review

• The direction of a line of force is defined as
  the direction in which the north pole of a
  compass points when placed along that line.
  Therefore, lines of magnetic force point to the
  South pole.

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Magnetism Review

• A horseshoe magnet

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Predicting Magnetic Forces

• Parallel fields from two different magnets show
  us that there is a repulsion.

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Predicting Magnetic Forces

• Opposite fields from two different magnets show
  us that there is an attraction.

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The Earth
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Electricity Magnetism

• Electricity and Magnetism are very closely
  related, since both are based on the properties
  of electrically charged particles.
• The Electromagnetic force is one of the four
  fundamental forces in nature (including the
  strong nuclear, the weak nuclear, and gravity).

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Principal of Electromagnetism

• Moving electric charges will produce a magnetic
  field. (this was discovered by Oersted).
• This means that when an
• electric current moves through
• a wire, a magnetic field
• is produced.

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Magnetic Fields and Current

• The magnetic field exists in circular rings
  around a straight conductor.
• The direction of the magnetic field depends on
  the direction of the current.

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Magnetic Fields and Current

• The Right-hand Rule when holding a straight
  conductor with your right hand and thumb pointing
  in the direction of conventional current, your
  curled fingers will point in the direction of the
  magnetic field lines.

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Example 1

• Draw the magnetic field lines around each wire.

Current going into the page
Current going out of the page
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Example 1

• Draw the magnetic field lines around each wire.

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Magnetic Field around a Loop

• If you make a circular loop from a straight wire
  and run an electric current through the wire, the
  magnetic field will circle around each segment of
  the loop.

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Magnetic Field around a Loop

• You can still use the right hand rule to find the
  direction of the magnetic field for a single
  loop.
• The field will be stronger inside the loop than
  on the outside.

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Magnetic Field around a Solenoid

• A solenoid is a conducting wire that is wound up
  into many loops forming a coil.

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Magnetic Field around a Solenoid

• If you run an electric current through a
  solenoid, the magnetic field is the sum of all
  the magnetic fields of each loop.
• The field will be strongest inside the loop
  because the field lines are closer together.

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Magnetic Field around a Solenoid

• The more tightly you wind the coil, the stronger
  the magnetic field will be.

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Magnetic Field around a Solenoid

• When we run a current through a solenoid like
  this, the magnetic field that is created looks
  just like a bar magnet. So it is as if a
  temporary bar magnet is created. One end is North
  and one is South.

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Magnetic Field around a Solenoid

• To determine the direction of the magnetic field
  around a solenoid, use the right-hand rule for a
  solenoid

Right-hand rule for a solenoid If you curl your
fingers in the direction of the conventional
current, your thumb will point in the direction
of the magnetic field lines in the core. This
means your thumb points towards North.
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The Motor Principal

• Since a current carrying wire has a magnetic
  field around it, an external magnetic field
  around the wire can cause the wire to move.
• This is because of the attraction or repulsion of
  the two magnetic fields (the one around the wire
  and the external one).
• http//www.youtube.com/watch?vtUCtCYty-ns

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The Motor Principal

• This property is called the Motor Principal
• A current-carrying conductor that cuts across
  external magnetic field lines experiences a force
  that is perpendicular to both the magnetic field
  and the direction of the electric current.

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The Motor Principal
The force is caused because the magnetic field
lines on the magnet, and on the wire are going in
the same direction. Therefore there is a
repulsion.
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The Motor Principal

• Right-hand Rule for the Motor Principal
• If the fingers of your open right hand point
  in the direction of the external magnetic field,
  and your thumb points in the direction of the
  conventional current, then your palm faces in the
  direction of the force on the conductor.

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Example 2

• Determine the direction of the force on the wire.

X
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Example 2

• Determine the direction of the force on the wire.

Solution Wire will go into the magnet by the
right hand rule.
X
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Electromagnetic Induction

• Because of the relationship between electricity
  and magnetism, when a magnetic field changes near
  a conductor, an electric current can be produced.
• This is called Electromagnetic Induction. The Law
  of Electromagnetic induction states An
  electric current is induced in a conductor
  whenever the magnetic field in the region of the
  conductor changes with time.

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Electromagnetic Induction

• This is how electricity is produced in a
  power-plant
• When water (or steam) pushes the turbine, it will
  rotate a magnet.
• When the magnet is rotated, the magnetic field
  changes.
• When the magnetic field changes, a current is
  produced.

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Video

• Overview of the electricity
• http//www.youtube.com/watch?vXiHVe8U5PhU

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Homework

• Read Sections 8.1, 8.2, 8.3,
• Make additional notes to supplement the lesson
  notes. (For now, you do not need to know the
  formulas for these sections)
• Complete the following questions
• Pg. 385 2, 3, 4,
• Pg. 391 2