Download magnetism2

• The Earth itself has a huge
magnetic field - as if it had a
huge bar magnet embedded at
its centre. The Earth’s magnetic
field lines emerge from near the
geographical north pole and reenter it at the south pole. The
nature of the field around the
Earth varies in both strength and
direction. The Earth’s 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!
• 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.
Field lines run
from North to
South.
The magnetic
field is
strongest at the
poles.
Unlike poles
attract.
North and South.
Like poles repel.
North and north.
South and south
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.
Like poles are repelling so the magnet on the right
is suspended in the air.
Investigation:
You are given
two magnets
find out which
one is
strongest
SOME MAGNET FACTS
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
The strongest magnets
are ceramic. They are
made from metal
powders, heated
together under very
high pressure
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
An Alnico magnet
is a combination
of aluminium,
nickel and cobalt,
and can be very
powerful.
Things that will make a magnet
stronger
o 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.
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
Plan of the Investigation
•Suspend two magnets using sellotape
•Add paperclips in a long chain and see which
chain is longest.
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.
What I will measure.
The number of paper clips in a row.
Results and conclusions
50
45
40
35
30
25
20
15
10
5
0
magnet 1 small
magnet 2
medium
magnet 3 large
1st
2nd
3rd
average
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.
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.
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.
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.
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.
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.
• 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.
Reed relay
An electric bell also uses an electromagnet. See if you can
describe in your own words how the bell works.
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 demagnetized.
• 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.