The Nature of Magnetism

1
The Nature of Magnetism

• Chapter 18 Section 1

2

• Properties of Magnets
• The magnetic property was discovered over 2,000
  years ago when a rock containing magnetite was
  found to attract iron.
• Magnetism is the attraction of a magnet for
  another object.
• Any material that attracts iron or things made of
  iron are called magnets.
• Magnetic Poles
• Any magnet, no matter its shape, has two ends,
  each one called a magnetic pole.
• A pole is the area of a magnet where the magnetic
  effect is the strongest.

3

• North and South
• One pole of a magnet (when allowed to swing
  freely) always points to magnetic north and is
  called the north pole.
• One pole of a magnet always points to magnetic
  south and is called the south pole.
• Magnetic Forces
• When you bring two magnets close together, the
  magnets exert a magnetic force on the other,
  which results from spinning electric charges in
  the magnets.
• The magnetic force between magnets depends on how
  the poles of the magnets line up.
• Magnetic poles that are alike repel each other
  and magnetic poles that are unlike attract each
  other.
• If a magnet is broken in two, each piece will
  have its own north and south poles.

4
(No Transcript)
5

• Magnetic Fields
• The region of magnetic force around a magnet is
  known as its magnetic field.
• Magnetic field lines map out the magnetic field
  around a bar magnet. They spread out from the
  north pole, curve around a magnet, and return to
  the south pole.
• A magnetic field is invisible, but you can
  demonstrate the effects using iron filings.
• Fields from two magnets can overlap and create a
  combined field.

6

• The Cause of Magnetism
• Atoms and Domains
• In most materials, the magnetic fields of the
  atoms point in random directions and cancel each
  other out and are not magnetized.
• In some materials, the magnetic fields of the
  atoms are aligned -- a cluster of billions of
  atoms that have magnetic fields lined up is known
  as a magnetic domain.
• In a magnetized material, all or most of the
  domains are arranged in the same direction.
• The arrangement of domains in an object
  determines whether the object is magnetic.

7
(No Transcript)
8

• Losing Alignment
• When domains move, magnets are demagnetized, or
  lose its magnetic properties.
• You can lose alignment by
• Dropping a magnet
• Putting the magnet in a strong magnetic field
  that is opposite to its own domains
• Increase the temperature
• Making Magnets
• Magnets can be made from ferromagnetic materials
  by placing the material in a strong magnetic
  field or by rubbing it with one pole of a strong
  magnet.
• Domains that already point in the direction of
  the magnetic field become larger.
• Domains that do not already point in the
  direction of the magnetic field rotate until they
  do.

9

• Making Magnets cont.
• Some materials can be magnetized easily, but lose
  their magnetism quickly, such as paper clips.
• Some materials, like some types of steel, are
  more difficult to magnetize, but retain their
  magnetism longer.
• Iron, Cobalt, or Nickel can make magnets
• Cutting a Magnet
• When a magnet is broken, the domains are still
  lined up, which is why each piece still has a
  north and south pole.

10

• Kinds of Magnets
• A material that shows strong magnetic effects is
  said to be a ferromagnetic material.
• Iron, nickel, and cobalt are the common
  ferromagnetic materials. Magnets can also be
  made from alloys.
• Electromagnets are magnets made by electric
  current and they usually have an iron core.
• Temporary and Permanent Magnets
• Temporary magnets are made form materials that
  are easy to magnetize, but they tend to lose
  their magnetization easily.
• A magnet made of a material that keeps its
  magnetism is called a permanent magnet.
• Permanent magnets are hard to magnetize.

11

• Earth as a Magnet
• A compass uses a small bar magnet (needle) that
  aligns with Earths magnetic field in order to
  help us navigate.
• As Earth rotates, the liquid in the core flows.
  Electric charges move, which means a magnetic
  field.

12
Magnetism from Electricity

• 18-2

13

• Using Electromagnetism
• Electromagnetism is the interaction between
  electricity and magnetism.
• Solenoids
• A current carrying wire has a magnetic field that
  forms a cylinder around the wire. If the wire is
  twisted into a loop, the magnetic field lines
  bunch up inside the loop, getting stronger with a
  larger number of loops.
• A current carrying wire with many loops is called
  a solenoid. A solenoid acts as a magnet that can
  be turned on and off by switching the current on
  and off.

14

• Electromagnets
• A solenoid with a ferromagnetic core is called an
  electromagnet. Adding the core makes the
  strength of the magnetic field many times that of
  a solenoid alone.
• When there is no current, then the electromagnet
  is off.
• Applications of Electromagnetism
• Uses of electromagnets
• Lift heavy objects that contain iron
• Doorbell
• Electric motors
• Galvanometers