Magnetic Forces, Fields, and Oersted

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Magnetic Forces, Fields, and Oersteds Principle

• Lesson 10

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• A magnet is a material or object that produces a
  magnetic field.
• The distribution of a magnetic force in the
  region of a magnet.
• Magnets are labelled North and South and have the
  same field laws as electric charges.
• Similar magnetic poles repel and dissimilar poles
  attract.

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• To map a magnetic field a test compass can be
  used. Much like a test charge, a test compass
  will point north in line with the south field.

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• Magnetic forces can act between some metals that
  are not magnetic. These metals are called
  Ferromagnetic metals and include metals such as
  cobalt, iron, nickel, or mixtures of the three.

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• The atomic structure of these metals seems to
  make them strongly magnetic. Think of magnetic
  materials, being made up of a lot of smaller
  magnets.

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Domain Theory of Magnets

• All large magnets are made up of many smaller and
  rotatable magnets, called dipoles, which can
  interact with other dipoles close by. If dipoles
  line up, then a small magnetic domain is
  produced.

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Electromagnets

• Force at a distance is the common element between
  electrostatics and magnetism. Hans Oersted
  studied this and came up with the following
  principle.
• Charge moving through a conductor produces a
  circular magnetic field around the conductor.

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• Mapping the magnetic field of a conductor enables
  one to be able to predict the direction of the
  electromagnetic force from the current.
• There are several hand signs developed to predict
  how magnetic forces will act.

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Left-hand rule 1 for Conductors (LHR 1)

• Grasp the conductor with your left hand such that
  the thumb points in the direction of the electron
  (-) current flow. The curved fingers point in the
  direction of the circular magnetic field around
  the conductor.

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• Using this rule allows us to produce a weak
  magnet that we can turn on and off.
• To make the electromagnet stronger and straighten
  out the field so that it is more like a bar
  magnet, the wire conductor is made into a coil.
  The individual field lines fall on top of each
  other strengthening the entire field. Coiling the
  wires also straightens out the field.

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Left hand rule 2 (LHR2)

• Grasp the coiled conductor with the left hand
  such that the curled fingers point in the
  direction of the electron (-) current flow
  through the conductor. The thumb points in the
  direction of the magnetic field within the coil.
  Outside the coil, the thumb represents the north
  (N) end of the electromagnet produced by the
  coil.

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• This allows us to create a magnet that acts like
  a bar magnet but that can be tuned off when it
  needs to be. The other advantage is that strength
  of the electromagnet can be controlled by the
  following factors.

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1. Current in the coil

• The greater the current flow, the greater the
  field strength. Strength varies directly as the
  current in the coil.

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2. Number of turns in the coil

• The greater the number of coils, the greater the
  field strength. Strength varies directly as the
  number of turns in the coil of the current is
  constant.

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3. Type of material in the coils centre

• The more ferromagnetic the material within the
  coil, the greater the magnets strength. Iron is
  one of the better materials to use.

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4. Size of the coil

• the smaller the diameter of the coil, the
  stronger the magnetic field.

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Questions

• Given the direction of current flow in the
  conductor seen below, find the direction of the
  magnetic field.  
• Given the direction of the magnetic field, find
  the direction of the current in the conductor
  seen to the right.
• What happens to the strength of the magnetic
  field around a coil if the current through the
  conductor is increased from 1.0 A to 2.5 A?
  (Hint Look at the 4 factors that affect
  electromagnetic strength.)

Current
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• What would happen to the field strength if the
  number of turns in the coil of the electromagnet
  were reduced by half and the current remained the
  same? (Hint Look at the 4 factors that affect
  electromagnetic strength.)
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• What would happen to the strength of an
  electromagnet if over time the conductor started
  to corrode and increased the resistance of the
  conductor?
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• What would happen to the strength of the
  electromagnet if the coiled conductor started to
  unwind causing the diameter of the coil to
  increase?