magnetic surveys
Magnetism
Magnetised needles have been used to aid navigation for
more than a thousand years. The use of magnetic compasses
seems to have originated in China in the eighth century and
by the twelfth century both floating and pivoted compass
needles were used in the West.
magnet is attracted to the south pole of another magnet. The fact that the
north pole of a suspended magnet swings towards the North implies that
the ‘North’ geomagnetic pole must be the south pole of the Earth’s
magnetic field.
and
and
Magnetism
magnet
The permeability of a vacuum, µ0, h
the value for a silicon steel it is about 67
An air-cored coil of radius 5 mm, with 2
of 1 A would have a maximum field s
(microtesla). With a steel core it migh
The direction of a magnetic field is the direction that a compass needle
(i.e. 0.0025 × 6700); however steel b
points, that is, from a north magnetic pole to a south magnetic pole.
magnet is attracted to the south pole of anotheratmagnet.
The fact that the
navigation for
about 1 T.
Magnetised needles have been used to aid
north pole of a suspended magnet swings towards the North implies that
What
a magnet?
moreis
than
a thousand years. The use of magnetic compasses
The Earth’s magnetic
eld
the ‘North’ fi
geomagnetic
pole must be the south
pole of the
Earth’s
Generating
electricity
u
A magnet is usually made of iron
Fig.1
Bar magnet
seems to
have
originated
in
China
in
the
eighth
century
and
The Earth
has its own magnetic field. Its position and strength change
magnetic
fi
eld.
or steel. Individual atoms of iron
and a ‘horseshoe
If a wire is moved across a magnetic fi
over time and it can even reverse direction; the last reversal occurred
magnet’
by the twelfth
century
bothmagnetic
floating
and pivoted
compass
contain
properties.
In an
field lines – an electric potential, or vo
780,000 years ago. The location of the poles changes by between 10 and
The direction of a magnetic field is the direction
thatThe
a faster
compass
piece of iron these tiny
ends.
it cuts needle
the field the gre
40 km a year. At present the ‘North’ geomagnetic pole is located about
needles were used in unmagnetised
the
West.
magnets are not all aligned in the
points, that is, from a north magnetic pole to a south magnetic pole.
1 Wb per second then 1 volt is generate
83°N, 115°W and the ‘South’ pole is about 64°S and 138°E.
same direction and therefore there is
magnetic field continues. This phenom
no net magnetic effect. However, when the iron is placed in a strong
Michael Faraday in 1831, is called elect
Magnetic flux
magnetic field the atoms tend to align in one direction. In a sufficiently
strong magnetic field all the atoms point the same
way.
When
this
occurs
If iron filings are sprinkled
A magnet is usually made of iron
Measuring
variations
in
Bar magnet
The Earth has its near
owna magnet,
magnetic
field. Its position
and strength
change
the material cannotFig.1
be magnetised
further and is said to be magnetically
a pattern like
or steel. Individual atoms of iron
Local magnetic fields result from the
and a ‘horseshoe
saturated.
over time and it can
reverse
direction; the last reversal occurred
that even
in Fig.2
is produced.
magnetic field and the magnetic field
magnet’
contain magnetic properties. In an
Although
nothingofactually
780,000 years ago.
The location
the poles changes
by between
10 and
When the external magnetic field is removed some of the atoms may
smaller deposits
of magnetic
material (u
unmagnetised piece of iron these tiny
fl
ows
from
the
magnet,
the
return to their previous orientation and the piece of iron loses some of
40 km a year. At present the ‘North’ geomagnetic
pole is located
about
of magnetometer
have been
developed
concept of magnetic flux is
magnets
noteasily
all aligned in the
its magnetism. Hard steel (e.g. a sewing
needle) are
is not
monitored
83°N, 115°W and the
‘South’ pole is about 64°Sbe
and
138°E.from fixed or mobile stat
used to describe these lines.
direction
andiron
therefore there is
magnetised but it retains its magnetisation same
much better
than soft
magnetic field strength at a location bu
The unit of magnetic flux is
(e.g.
nails
or
pins).
the field; these are caller scalar magneto
no net magnetic effect. However, when the iron is placed in a strong
the weber (Wb, pronounced
strength and direction of the field along th
field (notably
the atoms
to align
in one
direction.
A magnetic
few other metals
nickeltend
and cobalt)
exhibit
the same
kind of In a sufficiently
vay-ber). Nearer the poles of
Fig. 2 Pattern produced by iron filings
magnetism
as iron; they
areall
said
to atoms
be ferromagnetic.
a
magnet
the
lines
are
close
strong magnetic
field
the
point theAlnico
samemagnets,
way. When this
occurs
If
iron
filings are sprinkled
near a bar magnet.
which
were first produced
in the
1930s, are alloys
of aluminium,
nickel,
The Geological Survey o
together and we can say that
the material
cannot be
magnetised
further
and is said
to be magnetically
near
a
magnet,
a pattern like
cobalt and iron mainly.
the magnetic flux density is
The Geological Survey of Ireland carr
saturated.
thatcaesium
in Fig.2
ismagnetometer
produced. – a sca
larger. The strength of a magnetic field (or the magnetic flux density)
marine
Ceramic magnets are made of compressed mixtures of iron oxide (Fe2O3)
is measured in webers per square metre. One Wb/m2 is also called a
Although
nothing
ship
this
is
known
as
aactually
‘fish’. Some of th
, SrCOof
). the atoms may
and
barium
or strontium
or carbonate
SrO, BaCO
When
the
externaloxide
magnetic
field(BaO,
is removed
some
3
3
tesla (T).
striping
themagnet,
west (left the
hand side
flows
fromonthe
They
can to
be their
manufactured
in aorientation
variety of shapes
and piece
magnetic
eld loses some of
return
previous
and the
of fiiron
field reversals.
orientations and are commonly used, for example, in DC electric motors
concept
of
magnetic
fl
ux
is
its magnetism. Hard steel (e.g. a sewing needle) is not easily
and as ‘fridge’ magnets.
Comparative values
Airborne magnetometers are used over
What is a magnet?
The Earth’s magnetic field
Magnetic flux
magnetised but it retains its magnetisation much better than soft iron
In recent decades stronger and more expensive magnets have
(e.g. nails or pins).
been made from alloys of samarium or neodymium with cobalt and
other elements.
used to describe these lines.
can be trailed
the
Theland.
unitThese
of magnetic
fluxbehind
is
plane (‘a bird’) or fixed to the nose (‘a
thestinger’)
weber (Wb,
pronounced
as shown in Fig. 3. Some of
vay-ber).
Nearer
the poles
0.000030 to 0.000060 T
the results
are shown
in Fig.of
5. When
Fig. 2 Pattern produced by iron filings
(= 30 to 60 µT)
interpreted
with are
otherclose
geological
a magnet
the lines
near a bar magnet.
data such
as can
drill say
holes,
together
and we
thatgeology,
variable, less than 0.1T
geochemical, geophysical, the data
the
magnetic
fl
ux
density
is
0.1 T
can be used when exploring for mineral
larger. The strength of a magnetic0.15
field
(or the
magnetic
flux density)
or other
deposits.
T
is measured in webers per square metre. One Wb/m2 is also called a
0.15 T
tesla (T).
The list below gives a sense of the magnetic field strength of various
magnets
A few other metals (notably nickel and cobalt) exhibit the sameEarth
kind of
magnetism as iron; they are said to be ferromagnetic. Alnico magnets,
Most
not ferromagnetic
whichmetals
were firstare
produced
in the 1930s, are alloys of aluminium,
nickel,
Lodestone
Some
people
all metals are ferromagnetic; however this is not
cobalt
andthink
ironthat
mainly.
Steel magnet
the case. Familiar examples of non-magnetic metals are aluminium, zinc,
copper,
brass
(an alloyare
of made
copper of
and
zinc), lead, silver
and gold.
magnet
)
Ceramic
magnets
compressed
mixtures
of iron oxideAlnico
(Fe2O
3
Cupro-nickel coins are attracted by a magnet while brass or copper
and
barium
or
strontium
oxide
or
carbonate
(BaO,
SrO,
BaCO
,
SrCO
).
Ceramic
3
3magnet
coins are not.
They can be manufactured in a variety of shapes and magnetic
field
Samarium-cobalt
orientations and are commonly used, for example, in DC electric
motors
Neodymium-iron-boron
Properties
of bar magnets
and as ‘fridge’ magnets.
Comparative values
1T
1.25 T
A bar magnet is one that is magnetised from end to end, and it therefore
‘Superconducting electromagnets’
20 T
has two magnetic poles. If the magnet is free to rotate (on a thread, a
The list below gives a sense of the magnetic field strength of various
In recent decades stronger and more expensive magnets have
pivot or a float) it acts as a compass – pointing approximately in a northmagnets
Electromagnetism
been
madeTofrom
alloys
of itsamarium
neodymium
south
direction.
be more
precise,
points in the or
direction
of the localwith cobalt and
When
there
is
an
electric
current in a coil of wire, the coil becomes a
other
elements.
magnetic field.
magnet. At the centreEarth
of the coil the magnetic field strength (B) is
0.000030 to 0.000060 T
The end of the magnet that points roughly north is called the north pole
proportional to the electric current (I) and to the number of turns in the
(= 30 to 60 µT)
of the magnet.
coil (N), and is inversely proportional to the radius (r): B = µNI/2r.
The
symbol
µ
represents
the
magnetic
permeability
of
the
material
within
Lodestone
variable,
less than 0.1T
If a second magnet is brought near a suspended magnet it is found that
Some people think that all metals are ferromagnetic; however of
this
not
theiscoil.
like poles repel and unlike poles attract one another. A north pole of one
Most metals are not ferromagnetic
the case. Familiar examples of non-magnetic metals are aluminium, zinc,
copper, brass (an alloy of copper and zinc), lead, silver and gold.
Cupro-nickel coins are attracted by a magnet while brass or copper
coins are not.
Steel magnet
Alnico magnet
Ceramic magnet
Samarium-cobalt
Properties of bar magnets
Neodymium-iron-boron
A bar magnet is one that is magnetised from end to end, and it therefore
has two magnetic poles. If the magnet is free to rotate (on a thread, a
pivot or a float) it acts as a compass – pointing approximately in a northsouth direction. To be more precise, it points in the direction of the local
magnetic field.
‘Superconducting electromagnets’
The end of the magnet that points roughly north is called the north pole
of the magnet.
If a second magnet is brought near a suspended magnet it is found that
like poles repel and unlike poles attract one another. A north pole of one
0.1 T
Fig. 4 Marine magnetometry results
0.15 T
0.15 T
1T
1.25 T
20 T
Electromagnetism
When there is an electric current in a coil of wire, the coil becomes a
magnet. At the centre of the coil the magnetic field strength (B) is
proportional to the electric current (I) and to the number of turns in the
coil (N), and is inversely proportional to the radius (r): B = µNI/2r.
The symbol µ represents the magnetic permeability of the material within
of the coil.
magnetic surveys
IN
AND
SCIENCE TECHNOLOGY
SECOND
EDITION
EDITION
ACTION THIRD
netic surveys
Magnetised needles have been used to aid navigation for
more than a thousand years. The use of magnetic compasses
seems to have originated in China in the eighth century and
by the twelfth century both floating and pivoted compass
needles were used in the West.
and
Magnetism
magnet is attracted to the south pole of another magnet. The fact that the
north pole of a suspended magnet swings towards the North implies that
the ‘North’ geomagnetic pole must be the south pole of the Earth’s
magnetic field.
The direction of a magnetic field is the direction that a compass needle
points, that is, from a north magnetic pole to a south magnetic pole.
The permeability of a vacuum, µ0, has a value of 4π×10−7 N A−2;
the
value
silicon steel it is about 6700 larger.
What
isfor
a amagnet?
The permeability of a vacuum, µ0, h
the value for a silicon steel it is about 67
An air-cored coil of radius 5 mm, with 2
of 1 A would have a maximum field s
(microtesla). With a steel core it migh
(i.e. 0.0025 × 6700); however steel b
at about 1 T.
The Geological Survey of
The Earth’s magnetic field
Generating electricity u
magnet
usually
iron
Fig.1
Barof
magnet
An air-cored
coil
radius 5Amm,
withis20
turnsmade
and aofcarrying
a The
current
Earth has its own magnetic field. Its position and strengthIreland
change (GSI), founded in 1845, is
or steel. Individual atoms of iron
and a ‘horseshoe
If a wire
is moved
across a magnetic fi
Ireland’s
National
Earth
Science
over
time
and
it
can
even
reverse
direction;
the
last
reversal
occurred
of 1 A would
have a maximum
field strength of 0.0025 T or 2500 µT
magnet’
contain magnetic properties. In an
field
lines
– an electricfor
potential, or vo
780,000
years ago. The location of the poles changes by between
10 and
Organisation.
It
is
responsible
(microtesla). With a steel core
it
might
be
expected
to
be
about
16
T
unmagnetised piece of iron these tiny
ends. The faster it cuts the field the gre
40 km a year. At present the ‘North’ geomagnetic pole is located
about
providing
geological
advice
and
magnets
are becomes
not all aligned
in the
(i.e. 0.0025 × 6700); however
steel
magnetically
saturated
1 Wb per second then 1 volt is generate
83°N, 115°W and the ‘South’ pole is about 64°S and 138°E.
same direction and therefore there is
information, and
for thefield
acquisition
magnetic
continues. This phenom
at about 1 T.
no net magnetic effect. However, when the iron is placed in a strong
Michael
Faraday
in 1831,
of data for this
purpose.
The
GSIis called elect
Magnetic flproduces
ux
magnetic field the atoms tend to align in one direction. In a sufficiently
a range of products
strong
magnetic
fi
eld
all
the
atoms
point
the
same
way.
When
this
occurs
Generating electricity using magnetism.
If iron filings are sprinkled
Measuring
in
including maps,
reports variations
and
the material cannot be magnetised further and is said to be magnetically
near a magnet, a pattern like
Local
magnetic
fields result from the
Ifsaturated.
a wire is moved across a magnetic field – ‘cutting’ through magnetic
databases
and
acts
as
a
knowledge
that in Fig.2 is produced.
magnetic field and the magnetic field
field lines – an electric potential, or voltage, is generated between its
Although nothingcentre
actuallyand project partner in all
When the external magnetic field is removed some of the atoms may
smaller deposits of magnetic material (u
ends.
The
faster
it
cuts
the
fi
eld
the
greater
the
voltage.
If
it
cuts
through
fl
ows
from
the
magnet,
the
aspects
of
Irish
geology.
It
functions
as
a
division have
of the
return to their previous orientation and the piece of iron loses some of
ofline
magnetometer
been developed
concept of magnetic flux is
1itsWb
per second
1 volt
while the
movement
across the
magnetism.
Hardthen
steel
(e.g.is agenerated
sewing needle)
is not
easily
be monitored
from
fixed or mobile stat
Department of
Communications,
Energy
and
Natural
used to describe these lines.
magnetised
but
it
retains
its
magnetisation
much
better
than
soft
iron
magneticinfield
strength of
at a location bu
magnetic field continues. This phenomenon, which was discovered by
Resources (DCENR)
an flux
annual
budget
excess
The unit of with
magnetic
is
(e.g. nails or pins).
the field; these are caller scalar magneto
Michael
Faraday in 1831, is called electromagnetic induction.
the weber approximately
(Wb, pronounced 70 staff.
€7 million, and employs
strength and direction of the field along th
A few other metals (notably nickel and cobalt) exhibit the same kind of
Fig. 2 Pattern produced by iron filings
vay-ber). Nearer the poles of
To meet GSI’s goals
to the
deliver
information
and advice to today’s
a magnet
lines are
close
The
Geological
together and operates
we can say that
society the organisation
with small
teams
in specificSurvey o
cobalt
and
iron
mainly.
the magnetic flux density is
Local magnetic fields result from the combined effect of the Earth’s
Theoffshore
Geologicalmapping
Survey of Ireland carr
programme areas.
Bedrock, quaternary and
larger. The strength of a magnetic field (or the magnetic flux density)
caesium marine magnetometer – a sca
magnetic
field are
and
theofmagnetic
eld of underlying
rock
O ) strata and of
Ceramic magnets
made
compressedfimixtures
of iron oxide (Fe
2 3
produce
geological
maps
which
are
used
by
a
range
of
2
is measured in webers per square metre. One Wb/m is also called a
ship this is known as a ‘fish’. Some of th
, SrCO3). Various types
and barium
or strontium
oxide or carbonate
(BaO,
SrO, BaCO
smaller
deposits
of magnetic
material
(usually
magnetite).
3
professionals
such
as
geologists,
engineers
and
researchers.
tesla (T).
striping
on
the
west
(left
hand side
They can be manufactured in a variety of shapes and magnetic field
of
magnetometer have been developed so that magnetic variations can
In addition these products are further enhanced
by the GSI
field reversals.
orientations and are commonly used, for example, in DC electric motors
be
from fixed or mobile stations. Some measure theComparative
overall
and monitored
as ‘fridge’ magnets.
to produce, Groundwater Protection Schemes,
Aggregateare used over
values
Airborne magnetometers
magnetic field strength at a location but cannot indicate the direction
of
andfield
Geological
Sites.
works
are behind the
land.These
These can
be trailed
The
list below gives a Potential
sense of theMaps
magnetic
strength of Heritage
various
In recent decades stronger and more expensive magnets have
the
caller
scalar magnetometers.
both
the
bird’)
or fixed to the nose (‘a
magnets
commonly carried out with Local Authoritiesplane
and(‘aare
important
beenfield;
madethese
from are
alloys
of samarium
or neodymium withOthers
cobalt measure
and
as shown
in Fig.
strength
and direction of the field along three axes (vector magnetometers).
other elements.
planning tools aimed to protect our resourcesstinger’)
and develop
them
in 3. Some of
Earth
0.000030 to 0.000060 T
the results are shown in Fig. 5. When
a sustainable manner.
(= 30 to 60 µT)
interpreted with other geological
magnetism as iron; they are said to be ferromagnetic. Alnico magnets,
near a bar magnet.
Measuring
variations
which were first produced
in the 1930s, are in
alloysgeomagnetism
of aluminium, nickel,
Most Geological
metals are notSurvey
ferromagnetic
The
of Ireland (GSI)
Lodestone
Some people think that all metals are ferromagnetic; however this is not
Steel magnet
The
Geological
Surveyof non-magnetic
of Ireland carry
offshorezinc,
surveys using
a
the case.
Familiar examples
metals out
are aluminium,
copper, brass
(an alloy
of copper and
lead,
silver and gold.
caesium
marine
magnetometer
– azinc),
scalar
instrument;
when towed
by magnet
a
Alnico
Cupro-nickel
coins areasattracted
a magnet
while
brassare
or copper
ship
this is known
a ‘fish’.by
Some
of the
results
shown in Fig.Ceramic
4; the magnet
coins are not.
striping on the west (left hand side) is characteristic of magnetic
Samarium-cobalt
field reversals.
Properties of bar magnets
data such as drill holes, geology,
less than
0.1T of projects, these include
The GSI is involvedvariable,
in a large
range
geochemical, geophysical, the data
T workings,
investigating and cataloguing old 0.1
mine
change
can beclimate
used when
exploring for mineral
or mapping,
other deposits.
and carbon sequestration, geological
airborne
0.15 T hazards
geophysical surveys and geotourism.
0.15 T A special programme of
events is planned for 2008 which is 1designated
by the UN General
T
Assembly as IYPE (International Year of Planet Earth).
Neodymium-iron-boron
1.25 T
Airborne
magnetometers
are used
over
In addition the GSI is responsible for
th Geoscience Research
A bar magnet
is one that is magnetised
from
end to end, and it therefore
‘Superconducting electromagnets’
20 Griffi
T
has twoThese
magnetic
If the magnet
is free
land.
canpoles.
be trailed
behind
theto rotate (on a thread, a
Awards aimed at building research capacity in the area of
pivot or a float) it acts as a compass – pointing approximately in a northplane
(‘a bird’) or fixed to the nose (‘a
Geosciences in Ireland.
Electromagnetism
south direction. To be more precise, it points in the direction of the local
stinger’)
as
shown
in
Fig.
3.
Some
of
When
there
is
an
electric
current in a coil of wire, the coil becomes a
magnetic field.
magnet. At the centre of the coil the magnetic field strength (B) is
the results are shown in Fig. 5. When
The end of the magnet that points roughly north is called the north pole
proportional to the electric current (I) and to the number of turns in the
interpreted
other geological
of the magnet. with
coil (N), and is inversely
to the radiuson
(r):the
B =Geological
µNI/2r.
For proportional
more information
Survey of Ireland
data such as drill holes, geology,
The symbol µ represents the magnetic permeability of the material within
If a second magnet is brought near a suspended magnet it is found that
geochemical,
geophysical,
the
data
please
visit
our
site
at
www.gsi.ie
or
www.sta.ie
of
the
coil.
Fig.
3
Magnetometer
attached
like poles repel and unlike poles attract one another. A north pole of one
can be used when exploring for mineral to the front of a plane
Fig. 4 Marine magnetometry results
or other deposits.
Fig. 4 Marine magnetometry results
Fig. 5 Airborne magnetometry results
magnetic surveys
and
and
Magnetism
Magnetism
Magnetised needles have been used to aid navigation for
more than a thousand years. The use of magnetic compasses
seems to have originated in China in the eighth century and
by the twelfth century both floating and pivoted compass
needles were used in the West.
Syllabus
Reference
What
is a magnet?
magnet is attracted to the south pole of another magnet. The fact that the
north pole of a suspended magnet swings towards the North implies that
the ‘North’ geomagnetic pole must be the south pole of the Earth’s
magnetic field.
The direction of a magnetic field is the direction that a compass needle
points, that is, from a north magnetic pole to a south magnetic pole.
magnet
The permeability of a vacuum, µ0, h
the value for a silicon steel it is about 67
An air-cored coil of radius 5 mm, with 2
of 1 A would have a maximum field s
(microtesla). With a steel core it migh
(i.e. 0.0025 × 6700); however steel b
at about 1 T.
point; then move the compass tail to this dot and repeat until another
The Earth’s magnetic
field Join the dots to form a line. This process should be
edge is reached.
Generating electricity u
The Earth has its own magnetic field. Its position and strength change
A magnet is usually made of iron
repeated for each of the starting dots. Although
the whole
procedure
of iron
If a wire
is moved
across a magnetic fi
over time and it can even reverse direction; the last reversal occurred
contain magnetic properties. In an
is slowof itthereveals
subtlebyvariations
the magnetic
field.
maypotential,
be
field lines
– anYou
electric
or vo
780,000 years ago. The location
poles changes
between 10in
and
Magnetism; Magnetic fields;unmagnetised
Current in apiece
magnetic
fi
eld;
Electromagnetic
of iron these tiny
ends.
The
faster it cuts the field the gre
to detect
the presence
of pipes
etc. under
the
floor.
40 km a year. At present able
the ‘North’
geomagnetic
pole is located
about
magnets are not all aligned in the
induction
1 Wb per second then 1 volt is generate
83°N, 115°W and the ‘South’ pole is about 64°S and 138°E.
same direction and therefore there is
field continues.
This phenom
2. More interesting patterns can be obtained magnetic
by placing
a weak bar
no net magnetic effect. However, when the iron is placed in a strong
Michael Faraday in 1831, is called elect
magnet
in
the
middle
of
the
plotting
area;
this
should
be
held
in
place
Magnetic
fl
ux
magnetic
fi
eld
the
atoms
tend
to
align
in
one
direction.
In
a
suffi
ciently
Junior Certificate Science
with adhesive. IfDifferent
orientations of the magnet will give rise to
strong magnetic field all the atoms point the same way. When this occurs
iron filings are sprinkled
Measuring variations in
the material cannot be magnetised further and is said to be magnetically
different patterns.
Otherwise
followlike
the procedure outlined above.
near
a
magnet,
a pattern
Unit 3C1 Magnetism; 3C2 Static electricity; 3C3 Current
Local magnetic fields result from the
saturated.
that in Fig.2 is produced.
magnetic
and the magnetic
field
3. Nickel alloy coins
(copper-nickel
or silver-nickel)
arefield
attracted
by a
electricity
and
voltage;
3C4
Electric
circuits;
3C5
Electricity
Although
nothing
actually
When the external magnetic field is removed some of the atoms may
smaller deposits of magnetic material (u
fl
ows
from
the
magnet,
the
magnet
but
silver,
copper
or
brass
coins
are
not.
Examine
a
collection
return
to their supply;
previous orientation
and the piece of iron loses some of
in mains
3C6 Electronics
of magnetometer have been developed
of magnetic
flux is and classify them by their
of coins (newconcept
and old
if possible)
its magnetism. Hard steel (e.g. a sewing needle) is not easily
be monitored from fixed or mobile stat
used to describe these lines.
magnetised but it retains its magnetisation much better than soft iron
magnetic field strength at a location bu
appearance and
magnetic properties.
The unit of magnetic flux is
(e.g. nails or pins).
the field; these are caller scalar magneto
the weber (Wb, pronounced
strength
and direction ofathe
field along th
4. Make a model of the Earth’s magnetic field
by embedding
bar
A few other metals (notably nickel and cobalt) exhibit the same kind of
vay-ber). Nearer the poles of
Fig. 2 Pattern produced bymagnet
iron filingsin a ball of plasticene (or within a globe). Indicate the position
magnetism as iron; they are said to be ferromagnetic. Alnico magnets,
a
magnet
the
lines
are
close
near a bar magnet.
which were first produced in the 1930s, are alloys of aluminium, nickel,
of the equator together
and some
meridians.
compasses
investigate
Geological
Survey o
and we
can say that Using The
On completing
cobalt
and iron mainly. this section, the student will be able to:
flux density
the direction ofthe
themagnetic
magnetic
field atisvariousThe
points
on the
surface.
Geological
Survey
of Ireland carr
larger. The strength of a magnetic field (or the magnetic flux density)
• Interpret
of ferromagnets
andoxide
compasses
caesium
marine magnetometer
Ceramic
magnetsthe
arebehaviour
made of compressed
mixtures of iron
(Fe2O3)
Where is the field horizontal (parallel to the
surface)?
Where is – a sca
is measured in webers per square metre. One Wb/m2 is also called a
ship this is known as a ‘fish’. Some of th
and barium or strontium oxide or carbonate (BaO, SrO, BaCO3, SrCO3).
it
vertical?
tesla (T).
• Distinguish
between
and gravitation
striping on the west (left hand side
They
can be manufactured
in magnetism
a variety of shapes
and magnetic field
field reversals.
orientations and are commonly used, for example, in DC electric motors
5.
Place
a
neodymium
magnet
(e.g.
5
mm
diameter)
in the middle of a
• as
Describe
what is meant by electromagnetic induction
and
‘fridge’ magnets.
Comparative values
Airborne magnetometers are used over
Fig.1 Bar magnet
or steel. Individual atoms
and a ‘horseshoe
Leaving Certifi
cate – Electromagnetism
magnet’
Learning Outcomes
shallow dish such as a Petri dish. Add water until the magnet is just
land.
These
be regular
trailed behind the
The from
list below gives a sense
of the Observe
magnetic fithe
eld strength
of various
and
covered.
reflections
of a square
grid
or can
other
plane (‘a bird’) or fixed to the nose (‘a
magnets
pattern from the surface of the water. How is
the
water
affected
by
stinger’) as shown in Fig. 3. Some of
the magnet?
Earth
0.000030 to 0.000060 T
the results are shown in Fig. 5. When
• Outline some methods used to measure the Earth’s magnetism
(= 30 to 60 µT)
interpreted with other geological
Most metals are not ferromagnetic
Refinements: Add a little detergent to thedata
water.
Blacken
top geology,
such
as drillthe
holes,
Lodestone
variable, less than 0.1T
geochemical,
geophysical,
Some people think that all metals are ferromagnetic; however this is not
surface of the magnet. Cover the bottom of the
dish with black
paper the data
Steel magnet
can be used when exploring for mineral
the case. Familiar examples of non-magnetic metals are aluminium, zinc,
to eliminate unwanted reflections.0.1 T
In• recent
decadesthat
stronger
and more
expensive
have time
Appreciate
the Earth’s
magnetic
fieldmagnets
varies over
been place
made to
from
alloys of samarium or neodymium with cobalt and
place
other elements.
General Learning Points
copper, brass (an alloy of copper and zinc), lead, silver and gold.
Cupro-nickel coins are attracted by a magnet while brass or copper
coins
are not.
• Magnetic
compasses can be used for navigation because
has a magnetic field.
Properties of bar magnets
Alnico magnet
0.15 T
Ceramic magnet
0.15 T
the Earth
Samarium-cobalt
1T
Neodymium-iron-boron
• Ordinary bar magnets have poles at each end; unlike poles attract
or other deposits.
True/False Questions
1.25 T
A bar magnet is one that is magnetised from end to end, and it therefore
‘Superconducting electromagnets’
20 T
andmagnetic
like poles
repel.
has two
poles.
If the magnet is free to rotate (on a thread, a
(a)
Compasses
were
invented
by
Michael
Faraday
pivot or a float) it acts as a compass – pointing approximately in a northElectromagnetism
• Today
magnets
canprecise,
be manufactured
with a great
south
direction.
To be more
it points in the direction
of thevariety
local of magnetic
(b) Lodestone
a of
naturally
magnetic
mineral
When there is an electric
current in a is
coil
wire, the occurring
coil becomes
a
fieldfield.
orientations, including numerous (20+) poles
magnetic
magnet. At the centre of the coil the magnetic field strength (B) is
The
of the
magnet
thatcobalt
points roughly
north
is called
the north
pole
(c) Atoms
of(I)iron
magnetic
proportional
current
andare
to the
number of turns in the
• end
Iron,
nickel
and
are the
three
main
ferromagnetic
metals. to the electric
of the magnet.
coil (N), and is inversely proportional to the radius (r): B = µNI/2r.
Most metals are not ferromagnetic
(d) In
steel the
atoms
arewithin
randomly oriented
The symbol µ represents
themagnetised
magnetic permeability
of the
material
If a second magnet is brought near a suspended magnet it is found that
of the coil.
• poles
There
are
other
magnetism
like
repel
and
unlikekinds
poles of
attract
one another. A north pole of one
• The variations in the Earth’s magnetic field are measured using a
magnetometer.
T F
T F
T F
T F
(e) The end of a magnet that points North is called
Fig. 4 Marine magnetometry
a magnetic field
T Fresults
(f) Copper and brass are ferromagnetic
T F
• An electric potential (voltage) can be generated between the ends
of a conductor when it is moved across magnetic field lines.
(g) Electricity can be generated without a battery
T F
(h) The Earth’s magnetic field strength is about 1 Wb/m2
T F
Student Activity
(i) The unit of magnetic flux density is the tesla
T F
(j) There is a limit to the magnetisation of a piece of steel
T F
(k) An instrument that measures magnetic fields is called
an ammeter
T F
(l) Ceramic magnets are made of compressed mixtures
of iron oxide with some other materials.
T F
1. Plotting the magnetic field variations of the classroom. This will
require some large sheets of paper – a few square metres if possible
and plotting compasses. Spread the paper on a clear area of the
ground (classroom floor, hall, school yard); temporarily stick the
sheets to the surface. (It is important to keep magnetic material
away from the area being surveyed.) Mark points at regular intervals
along one edge of the paper (e.g. the South edge). Place the tail of
a compass over one of the dots and mark the position of the compass
Check your answers to these questions on www.sta.ie
What
is a magnet?
Examination
Questions
magnetic surveys
IN
AND
SCIENCE TECHNOLOGY
SECOND
EDITION
EDITION
ACTION THIRD
netic surveys
Magnetised needles have been used to aid navigation for
more than a thousand years. The use of magnetic compasses
seems to have originated in China in the eighth century and
by the twelfth century both floating and pivoted compass
needles were used in the West.
and
Magnetism
magnet is attracted to the south pole of another magnet. The fact that the
north pole of a suspended magnet swings towards the North implies that
the ‘North’ geomagnetic pole must be the south pole of the Earth’s
magnetic field.
The direction of a magnetic field is the direction that a compass needle
points, that is, from a north magnetic pole to a south magnetic pole.
Aurorae
The permeability of a vacuum, µ0, h
the value for a silicon steel it is about 67
An air-cored coil of radius 5 mm, with 2
of 1 A would have a maximum field s
(microtesla). With a steel core it migh
(i.e. 0.0025 × 6700); however steel b
at about 1 T.
The Earth’s magnetic field
Generating
electricity
u
A magnet is usually made of iron
Themagnetic
Sun has
anIts
eleven
year
is characterised
by a cyclical
rise
The Earth has its own
field.
position
andcycle;
strengththis
change
or Ordinary
steel. Individual
atoms of iron
If aus
wire
moved
across on
a magnetic fi
2002 Leaving Certificate
Level
fall
in thedirection;
numberthe
of last
sunspots.
What appear to
asisdark
specks
over time and it canand
even
reverse
reversal occurred
contain magnetic properties. In an
field lines – an electric potential, or vo
780,000 years ago. The
of of
thethe
poles
changes
byfact
between
10 anderuptions
the location
surface
Sun
are in
gigantic
in which thousands
Describe an experiment to demonstrate
a of
current-carrying
conductor
unmagnetisedthat
piece
iron these tiny
ends. The faster it cuts the field the gre
40 km a year. At present
the ‘North’
geomagnetic
pole is(in
located
aboutof
of
millions
of
tonnes
of
matter
the
form
magnets
are not all aligned in the
1 Wb per second then 1 volt is generate
in a magnetic field experiences
a force.
83°N, 115°W and the ‘South’ pole is about 64°S and 138°E.
same direction and therefore there is
charged particles) are ejected from the Sun
magnetic field continues. This phenom
no
magnetic
However, when
the iron is in
placed
in a strong
Thenetforce
on a effect.
current-carrying
conductor
a magnetic
field is given by
Michael Faraday in 1831, is called elect
at typically about 500 km/s and sometimes
Magnetic
fl
ux
magnetic
fi
eld
the
atoms
tend
to
align
in
one
direction.
In
a
suffi
ciently
F = IlB.
more than 2000 km/s. If an ejection happens
strong magnetic field all the atoms point the same way. When this occurs
If iron filings are sprinkled
Measuring variations in
to be in our directionnear
then
when the charged
the
be magnetised
further What
and is said
to beletters
magnetically
a magnet, a pattern like
Thematerial
letter cannot
I stands
for the current.
do the
l and B stand for?
Local magnetic fields result from the
saturated.
particles arrive at that
the inEarth
few days
Fig.2 (a
is produced.
magnetic field and the magnetic field
Although
nothing
actually
The
diagram
shows
a
motor.
later)
they
are
largely
defl
ected
by
the
When the external magnetic field is removed some of the atoms may
A
smaller deposits of magnetic material (u
fl
ows
from
the
magnet,
the
Earth’s
magnetic
fi
eld.
However
some
may
be
‘funnelled’
in along
thedeveloped
return to their previous orientation and the piece of iron loses some of
of magnetometer
have been
Name
the parts
labelled
A, B and
C. needle) is not easily N
concept of magnetic flux is
S
its
magnetism.
Hard
steel (e.g.
a sewing
be monitored
fromcharged
fixed or mobile stat
field lines towards used
the magnetic
poles.
The
interaction
of
the
to describe these lines.
magnetised but it retains its magnetisation much better than soft iron
magnetic field strength at a location bu
particles and the molecules
in the upper
Explain
The unit of magnetic
flux is atmosphere often results in
(e.g.
nails orwhy
pins). A turns when a current flows
the field; these are caller scalar magneto
the(aurora
weber (Wb,
pronounced
spectacular
displays
borealis
and
aurora
australis).
through it.
strength
and direction of the field along th
A few other metals (notably nickel and cobalt) exhibit the same Ckind of
vay-ber). Nearer the poles of
Fig.
2
Pattern
produced
by
iron
fi
lings
magnetism
as iron; they
are said
to labelled
be ferromagnetic.
Alnico magnets,
Intense
bursts
of
solar
charged
particles
can
occasionally
induce
a
magnet
the
lines
are
close
Give the function
of the
parts
B.
near a bar magnet.
which were first produced in the 1930s, are alloys of aluminium, nickel,
Survey o
and we can say that
damaging voltagestogether
in communications
satellitesThe
andGeological
even in national
cobalt
and
iron
mainly.
Give an everyday use of a motor.
The Geological Survey of Ireland carr
B
electricity grids. the magnetic flux density is
larger. The strength of a magnetic field (or the magnetic flux density)
caesium marine magnetometer – a sca
Ceramic magnets are made of compressed mixtures of iron oxide (Fe2O3)
is measured in webers per square metre. One Wb/m2 is also called a
Name
another
device
thatoriscarbonate
based on
theSrO,
same
principle
ship this is known as a ‘fish’. Some of th
, SrCO3). as the motor.
and
barium
or strontium
oxide
(BaO,
BaCO
3
tesla (T).
striping on the west (left hand side
They can be manufactured in a variety of shapes and magnetic field
field reversals.
orientations and are commonly used, for example, in DC electric motors
For further examples of past paper questions
and as ‘fridge’ magnets.
Comparative values
Airborne magnetometers are used over
Fig.1 Bar magnet
and a ‘horseshoe
magnet’
Biographical Notes
check www.sta.ie
In recent decades stronger and more expensive magnets have
been made from alloys of samarium or neodymium with cobalt and
other elements.
Most metals are not ferromagnetic
Some people think that all metals are ferromagnetic; however this is not
the case. Familiar examples of non-magnetic metals are aluminium, zinc,
copper, brass (an alloy of copper and zinc), lead, silver and gold.
Cupro-nickel coins are attracted by a magnet while brass or copper
coins are not.
Did You Know?
The list below givesWilhelm
a sense of Eduard
the magnetic
field strength
of various
Weber
(1804–1891)
magnets
Earth
Lodestone
Steel magnet
Alnico magnet
Ceramic magnet
land. These can be trailed behind the
plane (‘a bird’) or fixed to the nose (‘a
Weber, along with his two brothers, showed an
aptitude
for science
stinger’)
as shown
in Fig. 3. Some of
from an early age. On0.000030
completing
his schooling
he attended
University
to 0.000060
T
the
results are shown
in Fig. 5. When
(= 30 to
60 µT)
other
of Halle where he studied ‘natural
philosophy.’
Atinterpreted
the age ofwith
22 he
wasgeological
datataught
such as
drill holes,
awarded PhD for his study
of reed
organ
in Halle
until geology,
variable,
less than
0.1T pipes. He
geochemical, geophysical, the data
1831 when he was appointed professor
of physics
Göttingen.
0.1 T
can be usedinwhen
exploring for mineral
He believed that if students wished
physics it was
or other deposits.
0.15 to
T understand
important that they carry out their own experiments.
0.15 T
1T
Weber carried out much original research
himself and contributed to
the understanding of areas as diverse
Neodymium-iron-boron
1.25 T as music, acoustics, fluidity,
Properties of bar magnets
anatomy and electromagnetism. He 20
and
There
areisother
of magnetism
A
bar magnet
one thatkinds
is magnetised
from end to end, and it therefore
‘Superconducting electromagnets’
T C.F. Gauss constructed the
first electromagnetic telegraph in 1833.
has two magnetic poles. If the magnet is free to rotate (on a thread, a
Paramagnetism and diamagnetism are different from ferromagnetism.
pivot or a float) it acts as a compass – pointing approximately in a northElectromagnetism
Paramagnetic
are
weakly
to local
strong magnetic
He pioneered the study of the geomagnetism and produced an Atlas of
south
direction. To substances
be more precise,
it points
in theattracted
direction of the
When there is an electric
current in a coil
of wire, the
coil becomes
a published Electrodynamic
magnetic
field.include the elements barium, calcium, oxygen and uranium,
fields; they
Geomagnetism.
Together
with
Gauss he
magnet. At the centre of the coil the magnetic field strength (B) is
and
the
compounds
copper
(II)
sulfate,
magnesium
chloride
and
Proportional
Measures
in
1864;
this
laid
the
foundation for the electrical
The end of the magnet that points roughly north is called the north pole
proportional to the electric current (I) and to the number of turns in the
iron
chloride.
of
the(III)
magnet.
measurements
areradius
still in(r):use
today.
coil (N), and is inversely
proportionalthat
to the
B =
µNI/2r.
Samarium-cobalt
The symbol µ represents the magnetic permeability of the material within
If
a second magnet
is brought near
suspended
magnetby
it isstrong
found that
Diamagnetic
substances
are aweakly
repelled
magneticoffithe
elds;
coil.
like poles repel and unlike poles attract one another. A north pole of one
they include: water, most organic compounds, graphite and gold.
Read more about other famous scientists at www.sta.ie
Fig. 4 Marine magnetometry results
Historic developments
The electric battery was invented in by an Alessandro Volta (Italian) in
1800 and it quickly became the focus of much experimentation. In 1820
Hans Christian Ørsted (Danish) discovered that when a conductor
carries an electric current a magnetic field forms around it. With the
benefit of hindsight it seems surprising that it took twenty years for
someone to make this discovery.
Eleven years later (1831) Michael Faraday (English) discovered what is
called electromagnetic induction – the fact that a voltage is generated
between the ends of a conductor that is moving across a magnetic field.
Faraday’s work led to the development of electric motors and generators.
He also contributed significantly to electrochemistry and to the
understanding of the nature of light.
Revise The Terms
Can you recall the meaning of these terms? Reviewing the
terminology is a powerful tool for recall and retention.
aligned; alloys; brass; DC electric motors; ferromagnetic; inversely
proportional; iron filings; lodestone ; magnet; magnetic compasses;
magnetic field; magnetic field lines; magnetic flux; magnetic flux density;
magnetically saturated; magnetometer ; navigation; neodymium;
orientation; samarium; scalar ; steel; superconducting magnets; tesla;
vector; volt; voltage; weber.
Check the Glossary of Terms for this lesson at www.sta.ie