MAGNETISM

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MAGNETISM
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• The Earth itself has a huge magnetic field - as
  if it had a huge bar magnet embedded at its
  centre. The Earths magnetic field lines emerge
  from near the geographical north pole and
  re-enter it at the south pole. The nature of the
  field around the Earth varies in both strength
  and direction. The Earths magnetic field is
  strongest at the magnetic poles and weakest near
  the Equator. The field also varies with time, for
  example, the poles are known to wander as shown
  in the bottom picture and sometimes the polarity
  reverses completely!

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• The metals affected by magnetism consist of tiny
  regions called 'Domains' which behave like tiny
  magnets. Normally they are arranged in the
  magnetic material all pointing in different
  directions in a completely random fashion and so
  their magnetic effects cancel each other out. If
  an object is magnetised it is because the domains
  are all made to point in the same direction. This
  can be done by stroking the magnetic material
  with a magnet (or magnets) as shown in the
  diagram. When aligned the domains reinforce one
  another and create north and south poles at
  either end.

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Field lines run from North to South. The magnetic
field is strongest at the poles.
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• Unlike poles attract.
• North and South.
• Like poles repel.
• North and north.
• South and south

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MAGNETIC FIELD PATTERNS

• This diagram shows the magnetic field patterns
  around the unlike and like poles of a pair of
  magnets. Note that magnetic field lines never
  cross. The strength of the magnetic field is
  proportional to the space between the field
  lines. The poles, where the field is strongest,
  is also where the field lines are closest
  together.

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Like poles are repelling so the magnet on the
right is suspended in the air.
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Investigation You are given two magnets find out
which one is strongest
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SOME MAGNET FACTS
The poles of Earth can change. Magnetic lines of
rocks laid down long ago show that once the North
pole was the South pole and the South pole the
North pole
Your favourite music and television programmes
can be stored magnetically. Sound and video tapes
are plastic ribbons with a magnetic coat.
Information is recorded on the tapes as magnetic
fields. Stroke them with a magnet and all this
information which includes sound and pictures
will be lost forever
A letter written in 1269 by the Scientist calling
himself Peter the Pilgrim has the first mention
of the poles of a magnet
An Alnico magnet is a combination of aluminium,
nickel and cobalt, and can be very powerful.
The strongest magnets are ceramic. They are made
from metal powders, heated together under very
high pressure
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Things that will make a magnet stronger

• The bigger horseshoe magnet will be strongest and
  the larger bar magnet will also be stronger. (The
  bigger the magnet the stronger it will be.) There
  are more domains which can become magnetised and
  they all contribute to the total magnetic
  effect.

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The horseshoe magnet will be stronger because it
is folded so it is bigger, also both poles are
closer together and the poles are the strongest
part of a magnet, having them closer will
increase the magnetic field. They will be the
same because even when you straighten the
horseshoe magnet the bar magnet is thicker.
More domains I.e. it has more metal. Short and
fat will be stronger than long and thin more
domains near the poles creating a strong magnetic
effect at the poles. The plastic cover will
weaken the magnetic field
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Plan of the Investigation

• Suspend two magnets using sellotape
• Add paperclips in a long chain and see which
  chain is longest.

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Fair Testing

• The same size paper clip
• Same place not near another magnetic field , or
  near another metal object.
• Same temperature
• Same pole of magnet.
• Paper clips of the same metal must not be
  magnetised before, not joined together.

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What I will measure.
The number of paper clips in a row.
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Results and conclusions
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What My graph shows

• The larger the magnet the greater the magnetic
  strength.
• The magnetic field is increased because if there
  are more domains exerting a magnetic field the
  combined strength will be greater.
• The large magnet held 23 more pins than the
  smallest magnet and 11 more than the medium
  magnet.

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Summary of Magnetism

• Only iron, steel,nickel and cobalt are magnetic.
• Bar magnets have to be made using one of these
  substances.
• All bar magnets have invisible magnetic fields.
• One end of a magnet is called the north pole
  this is the side which will swing to face the
  Earths magnetic north.
• The field lines always point from the North pole
  to the South pole.

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Magnetic fields

• Magnetic fields are regions where they experience
  a magnetic force.
• You can investigate these using a plotting
  compass or iron filings.
• Like poles repel and unlike poles attract.

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Electromagnets

• A wire with a current in it has a magnetic field
  around it.
• The strength of a magnetic field can be increased
  in three ways
• Increase the amount of current flowing
• Increase the number of coils
• A larger soft iron core.
• A reed relay is an electromagnetic switch.

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An electromagnet

• A soft iron rod has no magnetic field
• When current flows in the wire the soft iron
  becomes magnetised so a magnetic field is
  detected by the plotting compasses.

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The first electromagnet was made by an English
scientist called William Sturgeon in 1825. It
could lift a few pins. Modern electromagnets can
lift whole cars. Some are so powerful they affect
metals, like copper, which are not usually
magnetic.
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Reed relay

• A small current in one circuit is used to switch
  on a much larger current in another circuit.
• When the small current is switched on the
  electromagnet A activates and the iron lever is
  attracted to it. This causes the arm to move
  which closes the contact B in the other circuit.
• The circuit is complete so the current then flows.

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An electric bell also uses an electromagnet. See
if you can describe in your own words how the
bell works.
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Your Answer should be like this

• A The bell push is closed which allows a current
  to flow.
• B The electromagnet becomes magnetized and the
  armature is pulled to the magnet this makes the
  hammer hits the gong.
• As this happens the contact is broken so it not a
  complete circuit so the electromagnet is
  de-magnetized.
• C The armature is no longer attracted so it jumps
  back.
• This closes the contact, so electricity flows
  again and the armature is once more attracted to
  the electromagnet.The whole cycle is repeated
  until you stop pressing the bell push.