School Science Lessons
Physics - Magnetism, magnetic fields, and forces, earth's magnetic
field
Updated: 2008-02-28 L
Please send comments to: J.Elfick@uq.edu.au
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websites
Table of contents
29.0.0 Magnetism
29.1.0 Magnetic materials
29.1.1 Magnets, temporary and permanent
magnets
29.1.2 Magnet domains and magnetization
29.1.3 Paramagnetism and diamagnetism
29.1.4 Hysteresis
29.1.5 Magnetostriction and magnetores
29.1.6 Magnetism and temperature
4.67
Simple compass needle
4.68
Magnetic dip
4.69
Make a magnetizing coil
4.70
Freely-suspended magnet
4.71
Natural magnets
4.72
Artificial magnets
4.73
Identify magnetic substances
4.74
Magnetic poles and pin
chains
4.75
Cut an iron wire magnet
4.76
Magnetic fields in two
dimensions
4.77
Magnetic fields in three
dimensions
6.40 Hanging magnets (Primary)
2.10 Magnetic pin chain (Primary)
29.2.0 Magnetic fields and forces
29.2.1 Magnetic fields
29.2.2 Magnetic fields and currents
29.2.3 Forces on magnets
29.2.4 Magnet / electromagnet interactions
29.2.5 Force on moving charge in a magnetic
field
29.2.6 Force on current in wires
29.2.7 Torques on coils
29.3.0 Earth's magnetic field, geomagnetism,
terrestrial magnetism principles, earth magnetism
4.74
Magnetic poles and pin
chains
4.75
Cut an iron wire magnet
29.1.0 Magnetic materials
29.1.1 Magnets, temporary and permanent magnets
29.1.1.0 Storing bar magnets
29.1.1.1 Magnet assortment, natural magnets,
artificial magnets
29.1.1.2 Break a magnet
29.1.1.3 Cast magnetic field
29.1.1.4 Identify which bar is a magnet,
identify
magnetic substances, rule of magnets, attraction and repulsion
29.1.1.5 Lowest energy configuration
29.1.1.6 Magnetic poles, isolated pole, freely
suspended magnets
29.1.1.7 Pin chain
29.1.1.8 Magnetic boats
19.2.18 Extract iron, Fe, from
breakfast
cereal with a magnetic stirrer
29.1.2 Magnet domains and magnetization
29.1.2.1 Iron filings domains
29.1.2.2 Induced magnetic poles, magnetic
induction
29.1.2.3 Hammer iron bar, magnetization in
the earth's field
29.1.2.4 Magnetization by electric current
29.1.2.6 Electromagnets
29.1.2.7 Retentivity
29.1.2.8 Permalloy bar
29.1.2.9 Barkhausen effect
29.1.2.10 Magnetize magnetic material, by
single touch, by double touch (divided touch), by electricity
29.1.2.11 Variation of magnetism along a bar
magnet tested by spring balance
29.2.0 Magnetic fields
and forces
29.2.1 Magnetic Fields
29.2.1.1 Dip needle
29.2.1.2 Oersted's effect
29.2.1.3 Current through an electrolyte
29.2.1.4 Magnetic fields in two dimensions,
magnet and iron filings
29.2.1.6 Magnetic fields in three dimensions,
particles in oil, iron filings in glycerine, iron filings on glass
plate
stack
29.2.1.9 Area of contact
29.2.1.10 Gap and field strength
29.2.1.11 Shunting magnetic flux
29.2.1.12 Magnetic shielding, magnetic
screening
29.2.1.13 Compare magnetic moments of two
bar magnets using a deflection magnetometer (null method)
29.2.1.14 Vibrator with a magnet
29.2.1.15 Substances magnetic lines of force
can pass through
29.2.2 Magnetic fields and currents
29.2.2.1 Iron filings around a wire, parallel
wires, anti-parallel wires
29.2.2.2 Magnetic field around a wire
29.2.2.3 Magnetic fields around currents
29.2.2.4 Fields around currents, uniform and
circular fields
29.2.2.5 Show the right-hand rule, force
on charges moving through magnetic field
29.2.2.6 Biot-Savart law, Ampere's law,
Ampere-Laplace
law
29.2.2.7 Iron filings and a solenoid
29.2.2.8 Length of a solenoid
29.2.2.9 Small coils in a solenoid
29.2.2.10 Demountable Helmholtz coils
29.2.2.11 Field of a toroid
29.2.2.12 Iron filings on the overhead
projector
29.2.2.13 Magnetic field round a bar magnet
with the axis in the magnetic meridian
29.2.3 Forces on magnets
29.2.3.1 Magnets on a pivot
29.2.3.2 Levitation magnets, magnetic
suspension
29.2.3.3 Hanging magnets and inverse square
law, pole strength of a bar magnet in the magnetic meridian using
neutral
points
29.2.3.4 Inverse square law, inverse fourth
power, inverse seventh power
29.2.4 Magnet / electromagnet Interactions
29.2.4.1 Interaction of magnet and magnetizing
coil
29.2.4.2 Solenoid and bar magnet
29.2.4.3 Jumping magnet
29.2.4.4 Magnetically suspended globe, unipolar
motor
29.2.4.5 Floating magnetic balls, float
magnetized
needles
29.2.4.6 Ampere's ants
29.2.5 Force on moving charges
29.2.5.1 Cathode ray tube, CRT
29.2.5.2 e / m for electrons, measurement of
e / m
29.2.5.3 Bending of an electron beam
29.2.5.4 Crookes tube
29.2.5.5 CRT and earth's magnetic field
29.2.5.6 Forces on an electron beam, magnetic
deflection of cathode rays
29.2.5.7 Pinching mercury
(The
use of open surface mercury is illegal in some school systems!)
29.2.5.8 Magnetic pump, ion motor force on
conducting field
29.2.6 Force on current in wires, parallel
conductors
29.2.6.1 Parallel conductors
29.2.6.2 Interacting coils
29.2.6.3 Pinch effect simulation
29.2.6.4 Filament and magnet with a.c. / d.c.,
vibrating
lamp filament
29.2.6.5 a.c. / d.c. magnetic contrast
29.2.6.6 Dancing spring. jumping wire (LC)
29.2.6.7 Current balance
29.2.6.8 Maxwell's rule
29.2.6.9 Barlow's wheel
29.2.6.10 Electromagnetic swing
29.2.6.11 Magnetic grapevine
29.2.6.12 Electromagnetic conical pendulum
29.2.6.13 Ampere's motor, Ampere's frame
29.2.7 Torques on Coils
29.2.7.1 Model galvanometers
29.2.7.2 Force on a current loop
29.2.7.3 Interacting coils
29.2.7.4 Interacting solenoids
29.3.0 Earth's
magnetic
field, geomagnetism
29.3.1 Simple compass needles
29.3.2 True north and magnetic north, magnetic
variation (magnetic declination, magnetic deviation)
29.3.3 Magnetic dip, measure magnetic dip angles
29.3.4 North pole of magnet pointing magnetic
north
Magnetism
29.1.1 Magnets, temporary and permanent magnets
(The
use of open surface mercury is illegal in some school systems!)
Magnetic materials, alloy magnets,
ceramic magnets, temporary and permanent magnets
See diagram 29.4.0: Permanent bar magnets
Magnets are masses of a substance that can repel or attract the same
substance. Magnets may be temporary magnets or permanent magnets, e.g.
bar magnets in school laboratories or compass needles. Permanent
magnets
are made of steel. The magnetic length, 2L, is the distance between the
two poles and is always less than the physical length. Permanent
magnets
can be made from alloys, e.g. "Alnico" and "Alcomax" (nickel, iron,
aluminium,
cobalt, copper), or ceramic iron oxide compounds called ferrites, e.g.
barium ferrite, nickel ferrite. Permanent magnets are used in
telephones,
electric motors and bicycle dynamos. Ferrites as powders are used as a
magnetic coating in audi tapes and computer floppy disks. Do not carry
out magnetism experiments near large masses of magnetic material or
near
apparatus or wires through which an electric current is passing. Moving
electric charges cause magnetism. Diamagnetism occurs when a substance
is weakly affected by a strong magnet, e.g. bismuth, mercury.
Paramagnetism
occurs when substances can produce a weak magnetic field in the same
direction
as that of a strong magnet, e.g. tungsten, aluminium. Ferromagnetism
occurs
in the ferromagnetic materials iron, cobalt, nickel, and ferrite
metallic
oxides when groups of atoms, called domains, have the same directions
of
spin. External magnetic forces can induce magnetism, i.e. magnetize
ferromagnetic
materials. Induced magnetism can be temporary magnetism as in the soft
iron used in electromagnets or permanent magnetism as in hard steel.
Temporary
magnetism lasts only if the external source of magnetism lasts.
However,
even permanent magnetism can be lost by hammering or heating. Some
solutions
of salts, e.g. MnCl2, FeCl3, CoSO4 show
some magnetic susceptibility using a Quincke type glass tube.
29.1.1.0 Storing bar magnets
See diagram 29.01: Stored bar magnets
Magnets can lose their magnetism if you treat them roughly or do not
store them in pairs with soft iron keepers, N to S and S to N. A
magnetized
ring of iron keep its magnetism better than a bar of iron with two
magnetic
poles. So the "keepers" keep the magnetic flux in a magnetic circuit
with
no free magnetic poles. Store artificial magnets in pairs in a box,
north
to south and south to north. Keep magnets away from computer diskettes
and colour television screens!
29.1.1.1 Magnet assortment, natural magnets,
artificial magnets
The most common natural magnets are a form of magnetite, iron (II,
III) oxide, called lodestone that acts as a magnet when freely
suspended.
Lodestone was common in Magnesia in the Kingdom of Lydia, an ancient
kingdom now in western Turkey. Previously, a lodestone was supposed to
have magical properties!
Lodestone attracts small nails. Two pieces of magnetite in paper
stirrups
come to rest on the magnetic meridian. Magnetite was probably first
discovered in
China
and was used for the first compasses. Look for low cost artificial
magnets
in discarded loudspeakers, telephone receivers and other equipment.
Artificial magnets have
different shapes, e.g. "Alnico", horseshoe
magnet, pairs of bar magnets with a soft iron keeper, cylindrical
magnets, C-magnets,
U-magnets,
"Alcomax" magnets, and powerful magnets. Store artificial magnets in
pairs in a box, north to south and
south to north. Keep magnets wall
away from computer diskettes and colour television screens!
1. List all
the different kinds of magnets (a) in the laboratory (b) in the home
(c) in a motor car.
2. Suspend a large lodestone in a cradle with the
south pole painted white. Use a bar magnet is used to show attraction
and
repulsion.
29.1.1.2 Break a magnet.
1. A magnet attracts nails. Break it and note that the broken pieces
have formed new magnetic poles.
2. Break a magnetized steel wire in
half.
Test both ends of each broken portion. The magnetism found on each side
of the break has opposite polarity. Break off a very small piece of the
wire magnet and test it with iron filings. The smallest piece of the
wire
is a magnet with opposite poles.
29.1.1.3 Cast magnetic field. Cast iron
filings
in gelatine. Cast iron filings in acrylic over one pole of a magnet.
29.1.1.4 Identify which bar is a magnet,
identify
magnetic substances, rule of magnets, attraction and repulsion
See diagram 29.01: Like and unlike poles
1. Two bars look alike one is a magnet and the other is not a magnet.
With two similar bars of iron one magnetized use the end of one to lift
the middle of the other. 2. Many iron and steel objects are
magnetized
without you knowing it. You can detect this magnetism with a compass.
If
a rod is magnetized, it must, like the compass needle, have a north
pole
and a south pole. The rule of magnets is that two unlike poles attract
and two like poles repel. So one pole of the needle will be attracted
to
the end of the rod and the other repelled. If the rod is not
magnetized,
both poles of the needle are weakly attracted to the end. Collect
objects
made of paper, wax, brass, zinc, iron, steel, glass, cork, rubber,
aluminium,
copper, gold, silver, wood, tin, etc. Test each object with a magnet to
see which ones are attracted that are not. Bring a soft iron wire and
hard
steel or piano wire near a compass needle to see if it is affected by a
magnetic field.
29.1.1.5 Lowest energy configuration
Magnets held vertically in corks are placed in a dish of water. When
a coil around the dish is energized the magnets move to the lowest
energy
configuration.
29.1.1.6 Magnetic poles, isolated pole,
freely
suspended magnets
See diagram 30.2.1: Magnetic field of a bar
magnet
1. Magnetism is the
strongest at the poles of a magnet. Use a bar magnet, a
horseshoe-shaped magnet, a magnetized needle and
other
magnetized objects. Immerse them in fine iron filings then take them
out.
Note that most filings are at the poles. Scatter iron filings or iron
powder
over every part of a magnet and note that some filings slip off the
magnet
and some filings are attracted at its poles.
2. Magnetize a piece of
iron wire or a needle by rubbing with a bar magnet. Find its poles with
iron filings. Cut into two the magnetized iron wire or needle with
pliers
then test it with filings again. Each piece still has two poles. Cut
each
piece into two parts again then test them with filings. Each small
piece
has two poles. No matter how short the remaining wire is, it has two
poles.
3. Use a 6 cm length of steel wire or piano
wire. Draw one end of a steel
magnet along it once only and in one direction from end to end. Lay the
wire on a piece of paper
then test for magnetism by sprinkling iron filings over it. The iron
filings are not attracted equally along its whole length. They call the
areas of strongest attraction the
"magnetic poles". Pick up a pile of pins with the magnet. Leave one pin
attached to the magnet. Take off another pin and bring it close the end
of the first pin. They will
stick together by magnetic force. Connect all the pins to make a
magnetic pin chain.
4. To isolate a magnetic pole pass a long magnetized
knitting
needle through a cork and float it on water.
E. See diagram 2.167
Make a freely suspended magnet. Use loops of cotton to
suspend two magnets freely. Bring each pole of the two magnets close
to,
but not touching, each other. Show that like poles repel and unlike
poles
attract.
29.1.1.7 Pin Chain
See diagram 29.4.1: Pin chain
1. Pick up a pile of pins with the magnet. Leave one pin attached
to the magnet. Take off another pin and bring it close the end of the
first
pin. They will stick together by magnetic force. Connect all the pins
to
make a magnetic pin chain.
2. Estimate the strength of bar
magnets by
using a magnetized object to attract pins or paper clips and estimate
this
object's magnetization effect by the number of attracted pins or paper
clips.
29.1.1.8 Magnetic boats
Stroke three pins many times with the north pole end of a magnet in
the same direction so that their points attract each other. Put each
pin
in a little paper boat made of greaseproof paper. Put the boats in a
dish
of water. The boats will line up end to end in a north south direction.
29.1.2 Magnet domains and magnetization
Making
a magnet, stroking, electrical solenoid, electrically demagnetizing,
induced
magnetism and paper clip chain
29.1.2.1 Iron filings domains, magnetization
Outside magnetic effects can cause the domains in a ferromagnetic
material
to act strongly in one direction to magnetize the substance temporarily
in soft iron and permanently in hard steel. Soft iron is used in
electromagnets
so the magnetism can be "turned on" and "turned off". Permanent
magnetism
is necessary for compasses and permanent magnets but heating or
hammering
can destroy it. The measure of magnetic strength is magnetic moment, m.
It is the torque on a magnet placed at right angles to a magnetic
field.
The magnetic moment is a vector. As the magnetic moments of molecular
current
inside matter are in the same direction, the matter shows a character
that
can attract the things made by iron, cobalt, nickel, and metallic
oxides
(ferrites) called magnetism. The process of obtaining the magnetism
acted
on by a magnetic field is called magnetization.
A tube of compressed iron filings is magnetized and then the iron
filings are agitated. A set of magnetic needles on pivots orients
randomly
until a magnet is brought close.
29.1.2.2 Induced magnetic poles, magnetic
induction
See diagram 29.2.2(a)(b)
A chain of nails is supported by a magnet each becoming a magnet
by induction. A soft iron bar held collinear with a permanent magnet
will
become magnetized by induction. Use a compass needle to show the far
pole
of the bar is the same as the near pole of the magnet.
2. Put an
iron
bar on a block of wood. Hold an iron nail near one of its ends
vertically.
The nail can drop down when you release it showing that the iron bar
has
not magnetized. Hold a strong magnet near the other end of the iron
bar.
The nail does not drop when you release it, showing that the magnetic
induction
from the magnet has magnetized the iron bar. Remove the magnet and
check
if the iron bar is still magnetized by dropping the nail again.
3. Put
a compass on the table. Hold an iron nail 15 cm in length with its
sharp
end near the north pole of the compass. Bring the north pole of a bar
magnet
near the other end of the iron nail but do not let them touch each
other.
Observe if the north pole of the compass moves. Remove the magnet after
the compass points to the direction of north again. Bring the south
pole
of the magnet near the other end of the nail. Observe how the compass
moves.
29.1.2.3 Hammer iron bar, magnetization in
the
earth's field
See diagram 29.4.0: Plotting compass
1. Hammer the end of a soft iron bar in the earth's magnetic field.
Pound a soft iron bar held in the earth's field. A permalloy bar does
not
need to be pounded. Hammer a soft iron bar held parallel to the field
of
the earth. A bar of permalloy is magnetized by simply holding it in the
earth's field.
2. Temporary magnetism lasts only if the external
source
of magnetism lasts. However, even permanent magnetism can be lost by
hammering
or heating.
3. Hold a soft iron bar pointing to the north and sloping
downwards with the lower end against a thick piece of plastic. Hammer
it
down into the plastic. Lay the iron bar on the plastic, put a piece of
paper over it and sprinkle iron filings on the paper. The iron filings
move into a pattern showing that the iron bar has become slightly
magnetic.
4. Hold a plotting compass near the iron bar and notice any movement
of the compass needle. When you hammer the iron, some of its particles
line
up with the earth's magnetic field lines so that they point to the
north.
29.1.2.4 Magnetization by electric current
Place an iron core in a solenoid. Magnetize with direct current and
demagnetize by reducing alternating current to zero.
29.1.2.6 Electromagnets
A magnet powered by a 1.5 V battery lifts a large weight. An
electromagnet
with 25 turns of wire and one dry cell can lift over 100 kg
29.1.2.7 Retentivity
Retentivity means ability to respond after stimulus is removed, e.g.
in ferromagnetic substances measured as residual flux density. In a
magnetic
hysteresis loop, retentivity is the value of B at zero magnetic field
and
is called remanence. A soft iron bar will cling to a U shaped
electromagnet
when the current is turned off but no longer attract after it is pulled
away.
29.1.2.8 Permalloy bar
Permalloy is an alloy with high permeability and low hysteresis loss.
It contains 78.5% Ni and 21.5% Fe + possibly other elements, e.g. Cu,
Cr,
Co. Permalloys are used in magnetic shields and computer memory chips.
Iron filings stick to a permalloy bar held parallel to the earth's
magnetic
field but fall off when it is held perpendicular. A small strip of iron
sticks to a permalloy rod when it is held in the direction of the
Earth's
field.
29.1.2.9 Barkhausen effect
The Barkhausen effect is observed when the steady increase in a
magnetizing
flux produces jumps in the magnetization of ferromagnetic materials.
Magnetic
domains in the core of a small coil can be heard flipping as a magnet
is
moved by using and an audio amplifier. Insert various cores into a coil
connected to an audio amplifier and spin a magnet around it. Stretch an
iron nickel alloy wire through a coil and bring a magnet close to
show
sudden simultaneous magnetization. Soft iron and hard steel cores are
placed
in a small coil attached to an audio amplifier and the assembly is
inserted
into a magnetic field. A soft iron core inserted in a small coil
connected
to the input of an audio amplifier.
29.1.2.10 Magnetize magnetic material, by
single
touch, by double touch (divided touch), by electricity, magnetizing
iron
by contact
See diagram: 29.4.1
1. Demagnetize the specimen, e.g. steel knitting needle, using a
solenoid
carrying an alternating current or by heating the specimen to dull red
heat along its whole length and plunging it into cold water. Fix the
specimen
under a brass drawing pin stuck into the bench. Stroke the specimen ten
times in the same direction with one pole of a permanent bar magnet
with
marked poles. Note the pole used and mark the end of the specimen where
the pole first meets the specimen. Tests for magnetism in the specimen
using
iron filings. Find the polarity of the specimen with a plotting
compass.
2. Demagnetize the specimen. Fix the specimen under a
brass drawing
pin
stuck into the bench. Stroke the specimen by using opposite poles of
two
permanent bar magnets with marked poles. Stroke the specimen ten times.
Note the poles used and mark the end where the pole first meets the
specimen.
Tests for magnetism in the specimen using iron filings. Find the
polarity
of the specimen with a plotting compass.
3. Demagnetize the
specimen.
Adjust the sliding contact of the rheostat to half the resistance. Put
the specimen in the solenoid. Close the switch. Note the direction of
flow
of the current, from positive through the circuit to negative. Note the
direction of winding of the solenoid. Open the key. Remove the specimen
from in the solenoid. Mark it to show its position in the solenoid.
Test
for magnetism in the specimen using iron filings. Find the polarity of
the specimen with a plotting compass. 4. Magnetize iron by contact
and
demagnetization. Stroke a nail on a permanent magnet and it will pick
up
iron filings. Magnetize an iron bar in a solenoid then pound it to
demagnetize.
Stroke a steel needle with a permanent magnet to magnetize and pass it
through an a.c. solenoid to demagnetize.
29.1.2.11 Variation of magnetism along a
bar
magnet tested by spring balance
See diagram 29.2.1
Place a bar magnet on a piece of squared paper. Tie a soft iron nail
to the hook of a spring balance. Let the bar magnet attract the nail
then
try to pull the nail off the magnet. Record the needed pulling force.
Start
the experiment from one pole of the bar magnet and test every 2.5 cm.
Show
the readings of the spring balance on a graph. Let the distance at the
first end be zero, graph the distance on the horizontal axis, the
needed
pulling force on the vertical axis. Draw a graph to show the
distribution
of the magnetism in a bar magnet. Magnetism is strongest at the
magnetic
poles
29.1.3 Paramagnetism and diamagnetism
Place paramagnetic and diamagnetic crystals between the poles
of a large electromagnet. Place small samples of bismuth, aluminium,
glass
between the poles of a strong electromagnet. Suspend samples of bismuth
and
copper (II) sulfate by threads. A large horseshoe magnet
attracts the copper (II) sulfate and repels the bismuth. A dollar bill
is attracted by a
strong magnet. Pull the bubble in a carpenter's level with a strong
magnet. Pull
liquid
air drops around on a sheet of paper. Liquid oxygen sticks to the pole
pieces of a strong electromagnet until it evaporates. Fill a test-tube
with liquid oxygen. Suspend the test-tube by a long
string attached to the ceiling. Bring a powerful magnet to the
side of the test-tube. The position of the test-tube changes due to
paramagnetsim.
29.1.4 Hysteresis
Hysteresis is the lag of an effect behind the cause of the effect.
A hysteresis loop is a closed figure obtained by plotting magnetic flux
density, B, against magnetizing field, H, when H increases and
decreases.
The area of the loop measures the energy lost during magnetization.
Hysteresis
loops for laminated steel and ferrite cores as saturation is reached
can be displayed on an oscilloscope. The hysteresis loop for the iron
core of
a transformer is shown on an oscilloscope. Parallel iron bars suspended
in a coil show hysteresis when slowly magnetized. Water is boiled by
magnetic
hysteresis waste heat.
29.1.5 Magnetostriction and magnetores
29.1.5.1 Nickel constricts and cobalt steel lengthens when magnetized.
Place sample rods in a solenoid and show the effect by optical lever.
29.1.6 Magnetism
and temperature
29.1.6.1 Curie point
The Curie point is the temperature above which a ferromagnetic material
becomes paramagnetic. Iron under magnetic attraction is heated until it
falls away. Upon cooling it is again attracted. A counterweighted iron
wire is attracted to a magnet until heated red with a flame. A long
soft
iron wire held up by a magnet falls off when the wire is heated past
the
Curie point. A length of soft iron wire heated with 110 V d.c. through
a
rheostat shows loss of magnetic properties when it passes through
recalescence
(loss of heat in ferromagnetic material as crystal structure and
magnetic
properties change). Monel metals (nickel based alloys) have curie
points
between 25 degrees C and 100 degrees C depending on the alloy. A rod of
nickel is attracted to a magnet when cool but swings away when heated.
29.1.6.2 Thermomagnetic motor
Local heating of permalloy tape or nickel rings in a magnetic field
will cause rotation. The rim of a wheel of Monel tape is placed in the
gap of a magnet and heat is applied to one side to make the wheel turn.
A thin strip of magnetic alloy around the rim of a well balanced wheel
is placed in the gap of a magnet with a light focussed on a point just
above the magnet. Heating changes the magnetic properties and the wheel
rotates.
29.1.6.3 Meissner effect
The Meissner effect occurs in a diamagnetic material which expels all
its magnetic flux when cooled below its critical temperature and a
magnetic
field is applied. Such a material can show superconducting
levitation.
Cool a superconductor and a magnet floats over it owing to magnetic
repulsion.
Place a small powerful magnet over a disc of superconducting material
cooled
to liquid nitrogen temperature. A magnet / cork in a vial filled with
salt
water so the float just sinks is placed over the superconductor
29.2.0 Magnetic fields and forces, magnetic
lines of force (field lines), magnetic flux, plot magnetic field lines,
field patterns of permanent magnets and current carrying wires, loops
and
solenoid, force field comparisons to gravitational or electric fields,
measurement of magnetic field strength by the simple current balance
technique,
magnetic field, plot magnetic field lines
See diagram 29.01: Lines of force | See
diagram 29.4.0: Deflection magnetometer
The "amount" of magnetism is called magnetic flux. The magnetic field
at a place, also called the magnetic induction or magnetic flux
density,
is represented by the vector B, unit tesla, T = 1 weber per square
metre,
Wb / m2. At any place in the magnetic field B is tangential
to
the magnetic field line drawn through that place. F (newtons) = q
(coulombs)
x v (volts) x B (tesla) sin a (where a = the angle between the magnetic
field lines and the direction of a moving charge). The cgs unit for B
is
the gauss (G). I G = 10-4 T. Magnetic flux through an area
is
the product of the component of B perpendicular to the surface area, A,
and 1. Magnetic flux = B x A Cos a (the angle between the direction of
the magnetic field and the area at right angles), in weber, Wb. The
deflection
magnetometer is designed to compare the strengths of two magnetic
fields
acting at right angles to one another. It consists of a small magnet ns
pivoted at the centre of a large circular scale graduated in degrees
and
a light aluminium pointer. Any deflection of the magnet causes the
pointer
to move through the same angle. The field to be tested is always placed
perpendicular to the earth's horizontal component; i.e. in a magnetic
east
- west direction. If the strength of the field = H oersted and the
earth's
horizontal component = H0 oersted, by the parallelogram of forces law,
the resultant R makes an angle a with Ho. The magnet aligns itself with
this resultant field, i.e. is deflected through an angle a measured on
the circular scale. H / H0 = tan a, so H = H0 tan a. Magnetic fields of
force can be shown as lines of magnetic force, magnetic flux. You can
draw
lines of magnetic flux around a source of magnetism to show the
magnetic
field so that the tangent at any point gives the direction of the
magnetic
field. The lines of magnetic flux are drawn as going from the north
pole
to the south pole. A magnetic compass aligns itself in the direction of
the magnetic field, i.e. a tangent to the line of magnetic force at
that
point. Magnetic field is sometimes called the "flux density". When two
or more magnetic fields interact, the result is equal to the vector sum
of the separate fields.
29.2.1 Magnetic fields, magnetic fields and
forces
A magnetic field is a field of force that appears on magnetic poles
or magnets, i.e. around a magnetic body. Also a magnetic field appears
around a current-carrying conductor and is associated with the motion
of
electrons in atoms. The strength and direction of a magnetic field is
measured
by magnetic flux density, B, SI unit tesla, or magnetic field strength,
magnetizing force, H, SI unit ampere per metre.
Magnetic flux is related to the product of the magnetic permeability
of the medium and the magnetic field intensity normal to the surface,
SI
unit weber, CGS unit maxwells.
See diagram 9.24
Cover a bar magnet with a piece of stiff white paper. Sprinkle
iron firings on the paper and tap it lightly. The iron filings line up
along the lines of force from north pole to south pole. Hold a small
magnetic
compass, plotting compass, above the paper. It aligns itself to the
direction
of the magnetic field. Move the compass around to see the directions of
magnetic field at different places.
2. Put paper over a magnet.
Scatter
iron filings on it. Tap the paper lightly, and a pattern forms. The
curved
lines of the iron filings show the direction of the magnetic force.
Make
the pattern permanent by dipping paper into melted candle wax and let
it
cool. Scatter iron filings on it. Hold a hot iron over the wax after
the
formation of the magnetic lines. The pattern will be fixed.
3. Place
a piece of heavy paper over a bar magnet. Sprinkle iron filings on the
paper and tap gently. The pattern shows the direction of the field of
the
magnet. You can use a sheet of glass instead of the paper. You may also
plot the field with the aid of a small compass, placing it in various
positions
near the bar magnet and noting the direction in which the needle
points.
The iron filings form themselves into lines because each filing, being
in a magnetic field, becomes itself a tiny magnet. The north pole of
each
tiny magnet is attracted by a south pole of a magnet near by and the
filings
arrange themselves into lines.
29.2.1.1 Dip needle
Use a large compass needle or dip needle as an indicator of magnetic
field. Construct a magnetoscope by hanging needles from the edge of a
small
brass disc. Use a dip needle show the inclination and local direction
of
the earth's magnetic field. Explore the magnetic field around a long
wire
with a compass needle or dip needle.
29.2.1.2 Oersted's effect
Show Oersted's effect with a compass needle and a long wire
carrying a heavy current to explore the magnetic field around a long
wire.
A compass deflects above and below a current carrying wire. Hold a
current
carrying wire over a bar magnet on a pivot and the magnet moves
perpendicular
to the wire. Arrange four compass needles around a vertical wire
running
through plexiglass. Pass a current of 50 amps through a heavy vertical
wire and investigate the magnetic field with a compass needle. Pass a
heavy
current from a storage cell through a long wire and use a compass
needle
to investigate the nearby magnetic field. When demonstrating Oersted's
effect
using large currents, use flat braided brass cable instead of
copper
wire.
29.2.1.3 Current through an electrolyte
Use a compass needle detects the magnetic field from 2 amps flowing
in an electrolyte. Detect a magnetic field produced current in copper
electrolyte
and a gas discharge tube with a large compass needle.
29.2.1.4 Magnet and iron filings, magnetic
fields
in two dimensions, field of a magnet
See diagram: 29.3.2
1. Sprinkle iron filings on a glass sheet placed on top of a bar
magnet.
2. Sprinkle iron filings evenly on a thin card. Hold the card
high
over
a bar magnet then carefully lower it until it almost touches the
magnet.
Tap the card gently with the end of a pencil. The iron filings move
into
a pattern showing the magnetic field. Repeat the experiment with
different
types of magnets or with two bar magnets in different positions. The
iron
filings tend to line up in "lines of force" ("field lines"). Hold a
plotting
compass above the lines of force and compare their direction with the
direction
of the compass needle. Make permanent records of the magnetic field. a.
Spray over the iron filings with a paint sprayer b. Replace the card
with
photographic paper in a dark room. Shine a bright light on it and
develop
the print. Dip a white sheet of paper in melted wax. Let it cool then
sprinkle
iron filings on the solid wax. Hold the paper over a strong magnet to
allow
the iron filings to move into lines of force pattern. Hold a hot iron
over
the iron filings to let them sink into the wax.
3. Sprinkle iron
filings
on a sheet of plexiglass over a magnet. Sprinkle iron filings on a
magnet
between two glass plates. Sprinkle iron filings on a glass sheet
covering
a bar magnet.
4. Put two bar magnets, a piece of
soft
iron, and wood blocks thicker than the magnets and iron on the table
and
cover with a piece of white cardboard. Scatter fine iron filings on the
cardboard. Tap the cardboard from the side and note how the fine iron
filings
settle into a pattern.
5. To make magnet circles, push a wire up
through
a piece of thin cardboard. Drop iron filings evenly over the cardboard.
Connect the wire to a dry cell battery. Tap the cardboard and the iron
filings form circles around the wire because of the magnetic field from
the flow of electric current in the wire.
29.2.1.6 Magnetic fields in three
dimensions,
particles in oil, iron filings in glycerine, iron filings on glass
plate
stack
1. Pour a spoon of fine iron filings into a
bottle
then 3 / 4 fill with sticky liquid, e.g. water glass or oil. Close the
bottle
then
forcibly shake it to make the iron filings suspend evenly in the
liquid.
Place a strong magnet near the bottle. Note the distribution of the
iron
filings. You may save the distribution of the filings after it cools
and
solidifies if the liquid is transparent plastic in liquid state.
2. Use a suspension of
carbonyl
nickel powder in silicon oil as an indicator of magnetic field.
3. Make
a sandwich of iron filings in glycerine between two glass plates. Soft
iron bars extend the poles of a permanent magnet into a projection cell
with iron filings in an equal mixture of glycerine and alcohol.
4. Make
a 3-D view of magnetic fields by sprinkling iron filings on a series of
stacked glass plates.
29.2.1.9 Area of contact
If one end of a magnet 1 cm in diameter is reduced to 0.5 cm in
diameter,
the small end lifts a much larger piece of iron than the large end. An
electromagnet supports less weight when the face of the ring is against
the pole than when the curved edge is against the pole. A soft iron
truncated
cone will support less weight when the large end is in contact with the
face of an electromagnet.
29.2.1.10 Gap and field strength
Vary the gap of a magnet and measure the field with a gaussmeter (Name
in USA term for instrument that measures magnetic flux density.)
29.2.1.11 Shunting magnetic flux
Pick up a steel ball with a bar magnet then slide a soft iron bar along
the magnet towards the ball until it drops off.
29.2.1.12 Magnetic shielding, magnetic
screening
Slide sheets of copper aluminium and iron between an electromagnet
and an acrylic sheet separating nails from the magnet. Displace a
hanging
soft iron bar by attraction to a magnet then interpose a sheet of iron.
A test magnet is used to show the shielding properties of a soft iron
tube
with various magnetic field generators. Hold a magnet above a nail
attached
to the table by a string then interpose a sheet of iron. Two horizontal
sheets of glass separated by and air space intervene between an
electromagnet
and collection of nails being held up. Insert a sheet of iron into the
space and the nails drop. The following demonstration could be in the
Capacitors
and Dielectrics section: Place a compass in the gap of an electromagnet
and reverse the field at various rates, then use a sensitive
magnetometer.
29.2.1.13 Compare magnetic moments of two
bar
magnets using a deflection magnetometer (null method)
See diagram 29.4.5
Find the magnetic lengths of two bar magnets 2L1 and 2L1, using a
plotting
compass. Place the magnetometer so that the pointer indicates 0o at
one end and the arms lie magnetic east - west. Place magnet 1 at a
distance
d, to give a deflection of 30o. Adjust the position of
magnet
2 in the other arm of the magnetometer so that there is no deflection
of
the pivoted magnet P. Record d1 and d2. Repeat the experiment twice,
with
d1 so that magnet 1 alone gives initial deflections of 45o
and
60o and record d1 and d2. When there is no deflection of
the
pivoted magnet P, the magnetic fields at P from the two magnets, must
be equal and opposite. Taking each pair of readings of d1, and d2,
calculate
M1 / M2 each of the three cases from the formula above. Calculate the
mean
value M1 / M2. If the magnets are short and powerful so that L is small
compared with d, then L2 is negligible compared with d2.
29.2.1.14 Vibrator with a magnet
See diagram: 29.3.7
Place an U-shaped magnet with one pole up, the other pole down, at
the edge of the table. Put a needle or a razor blade on the pole that
is
down. The needle or razor blade will stand vertically between the two
poles.
Beat the needle in the centre of the magnetic field slightly by a
pencil
at right angles to the magnetic lines of force. Note how the needle
moves.
Move the needle up or down, i.e. change the length of the needle in the
magnetic field. Repeat the experiment, observe the variations of the
vibrating
frequency of the needle.
29.2.1.15 Substances magnetic lines of
force
can pass through
Put a bar magnet on the table and cover with a piece of paper. Put
different substances, e.g. wood, glass, copper, zinc, cardboard, paper,
plastic, iron, aluminium, on the paper over the bar magnet. Put iron
filings
on a piece of stiff white paper. Hold the paper over the substances and
tap the paper from the side until some pattern forms. You can
distinguish
which substances can allow magnetic lines of force to pass through them
by observing the pattern of iron filings on the paper. A magnetic field
acts though all these materials except iron.
29.2.2 Fields and Currents
29.2.2.1 Iron filings around a wire, parallel
wires, anti-parallel wires
Sprinkle iron filings around a vertical wire running through the centre
of a plexiglass sheet.
29.2.2.2 Magnetic field around a wire
Iron filings show the field of a wire passing through a sheet of
plexiglass.
Sprinkle iron filings around a vertical wire running through
plexiglass.
29.2.2.3 Magnetic fields around currents
Sprinkle iron filings around a current carrying wire loop, coil and
solenoid.
29.2.2.4 Fields around currents, uniform and
circular fields
Use iron filings to show the resultant of a vertical wire passing
through
a uniform field.
29.2.2.5 Show the right-hand rule,
force
on charges moving through magnetic field
See diagram 29.03
A positive charge crossing a magnetic field line experience a force
in the direction found by using the open right-hand rule: fingers point
in the direction of the magnetic field (north to south), thumb points
in
the direction of movement of the positive charge, palm of the hand
pushes
in direction of force on the positive charge. The size of the force on
the positive charge depends on the product of four factors: (a) size of
the charge, q (b) velocity of the charge, v, in m / s (c) strength of
the
magnetic field, B (d) angle between the direction of movement of the
positive
charge and the direction of the field lines, a. A positive charge
moving
parallel to field lines experiences no force. Move a compass around a
vertical
wire carrying a current, then reverse the current.
29.2.2.6 Biot-Savart law, Ampere's law,
Ampere-Laplace
law
The Biot-Savart law expresses the intensity of magnetic flux density
produced at a point at a distance from a current-carrying conductor. It
gives Ampere's law, Ampere-Laplace law, that expresses the force
between
parallel current-carrying conductors in free space.
29.2.2.7 Iron filings and a solenoid
A solenoid is wound through a piece of plexiglass for use with iron
filings on the overhead projector. Iron filings show the field of a
solenoid
wound through a sheet of plexiglass. Wind a solenoid through a piece of
plexiglass for use with iron filings on the overhead projector. Insert
into a solenoid a glass cylinder filled with iron filings in a solution
of glycerine and alcohol.
29.2.2.8 Length of a solenoid
Construct a large solenoid to make it easy to change the spacing of
turns and therefore the length. Use a magnetometer or coil to show
field
strength.
29.2.2.9 Small coils in a solenoid
Mount an array of small coils inside a large solenoid. Small springs
keep the small coils aligned randomly when no current is applied.
29.2.2.10 Demountable Helmholtz coils
Helmholtz coils are a pair of compact identical coaxial coils separated
by a distance equal to their radius. Use Helmholtz coils to generate a
large uniform magnetic field at a midway position.
29.2.2.11 Field of a toroid
A toroid is a coil in the shape of a ring, in geometry a torus. Iron
filings show the field of a toroid which is wound through a sheet of
plexiglass.
29.2.2.12 Iron filings on the overhead
projector
Use iron filings in a viscous liquid, e.g. castor oil, to show magnetic
field configurations. Sprinkle iron filings on plastic sheets that have
a single wire, parallel wires and a solenoid passing through holes.
29.2.2.13 Magnetic field round a bar magnet
with the axis in the magnetic meridian
See diagram 29.01
A magnetic field, B, exists where a charge experiences a force because
of its motion. You can detect a magnetic field by a compass
needle that
aligns itself in the direction of the magnetic field at that place. A
magnetic
field refers to where a magnetic force is found, i.e. magnetic flux is
present. The force found in a magnetic field has a direction at any
point
in the magnetic field found by putting small pieces of iron in the
magnetic
field. The direction is called a line of force. In a strong magnetic
field
many lines of force are found in a very small space, the flux density
is
high, magnetic field lines, lines of force, can be drawn to show the
direction
a compass needle would have at any place in the magnetic field. Assume
that the direction of the magnetic field is the direction of a compass
needle, so magnetic field lines leave north poles and enter south
poles.
Like magnetic poles repel, i.e. NN or SS. Unlike poles attract, i.e. NS
or SN. Magnetic field lines, lines of force, can be drawn to show the
direction
a compass needle would have at any place in the magnetic field. The
pattern
of a magnetic field about a bar magnet produced by sprinkling iron
filings
on a piece of paper over it. Lines of magnetic force are unbroken, pass
through the magnet, never cross and have the same strength A strong
magnetic
field has more lines of force in an area than a weak field. If you
bring
the north pole of one magnet close to the south pole of another magnet,
the two magnets will attract each other. If you bring the north pole of
one magnet near to the north pole of another magnet, these poles will
repel
each other. The lines of force repel each other, and the two magnets
push
each other away. If you turn around the magnets so that the two south
poles
brought together, the poles will repel each other. Like poles repel and
unlike poles attract.
29.2.3 Forces on Magnets
29.2.3.1 Magnets on a pivot
Place one magnet on a pivot and use the other to attract or repel the
first magnet. Place a magnet in a cradle then use a second magnet to
attract
and repel the first. Show interaction between bar magnets. Show
magnetic
attraction / repulsion. Snap the lines of force.
29.2.3.2 Levitation magnets, magnetic
suspension,
magnetized needle floats in air
See diagram 29.4.1: Hovering paper clip
1. Use a light thread to attach a paper clip to the desk with
adhesive tape. Hold a bar magnet above the paper clip and see it rise
up and hover in the air.
See diagram: 29.3.6
2. Place two ring magnets on an upright test-tube with like poles
facing. Suspend two disc magnets with like poles facing on an inverted
test-tube. Hold two notched bar magnets with like poles facing.
3. Suspend a threaded needle by
hanging on to the end of the thread. Hold
an U-shaped magnet below the needle so that one pole attracts the "eye"
end of the needle. Move the magnet horizontally so the needle drags
across
the pole until the sharp end of the needle separates from the pole.
Move
the magnet from below the needle to the place far from it. Move the
other
pole of the magnet below the sharp end of the needle. The magnetized
needle
floats in the air above the other pole of the magnet.
29.2.3.3 Hanging magnets and inverse square
law, pole strength of a bar magnet in the magnetic meridian using
neutral
points
See diagram 29.4.4 | See
diagram 29.4.3
1. The inverse square law of magnetism states that the force F between
two magnetic poles varies inversely as the square of the distance d
between
them, i.e. F is proportional to 1 / distance2. Rotate a
magnetometer
until the pointer indicates 0o at one end, and the arms lie
magnetic east - west. Clamp the ball-ended magnet at its centre so that
ball A lies vertically above, and ball B magnetic east of the pivoted
magnet
P to give a 35o deflection. Read both ends of the pointer
to
eliminate the error if the pivot is not at the centre of the circular
scale.
Record the distance d cm of the centre of B from the pivoted magnet P.
Repeat with ball B at the same distance from, and magnetic West of, the
pivoted magnet to eliminate the error if the pivot is not at the zero
marks
of the linear scales. Record the readings of both ends of the pointer.
Repeat the above procedure for values of d to give deflections between
30o and 60o.
| d cm |
B east of P, a1 |
B east of P, a2 |
B west of P, a3 |
B east of P, a4 |
Mean a |
tan a |
1 / d2 |
| - |
- |
- |
- |
- |
- |
- |
- |
Draw a graph of tan a (y axis) against 1 / d2 (x axis).
The poles of a ball-ended magnet are at the centre of each ball. Ball A
has
no influence on the needle since at P its field is vertical. Assuming
the inverse square law, magnetic intensity H in a horizontal direction
at P
caused by ball B = m / d2, m is the pole strength of B.
However,
H = Ho tan a, Ho is the horizontal component of the earth's field at P
and a is the angle of deflection of P. m / d2= H0 tan a, so
1 / d2= (H0 / m) tan a. However, Ho / m is a constant, so 1
/ d2
is
proportional to tan a, assuming the inverse square law. If the graph of
1 / d2 against tan a is a straight line passing through the
origin,
the inverse square law is verified.
Hang two magnets horizontally and parallel. Use the inverse square
law to compute the pole strength from the length of the suspension the
saturation and mass of the magnets.
2. Find the magnetic length 2L of
a weak bar magnet with known polarity. Draw the outline of the bar
magnet.
Put a plotting compass in several positions near one end and mark with
a pencil dot the position of each end of the compass needle. Repeat the
procedure at the other end of the magnet. Remove the magnet. Draw a
straight
line through each pair of dots, producing the lines to intersect over
two
small areas that are the poles of the magnet. The distance between
these
poles is the magnetic length 2L of the magnet.
3. Put the magnet in
the centre of the paper on the board with its south pole pointing
magnetic
north and its axis in the magnetic meridian. Plot lines of force in the
region of the neutral points P and Q. When the compass is placed on
these
points the needle does not set in any particular direction. Measure the
distances d1 and d2 from the centre of the magnet. If the pole strength
of the bar magnet is m and its magnetic length is 2L, then the field
strength
H at a point distance d from its centre and on its magnetic axis
produced
= 4mLd / (d2- L2)2. At the neutral
points
the field H caused by the magnet is equal and opposite to H0, the
earth's
horizontal component. So H0 = 4mLd / (d2- L2)2,d
= average distance of neutral points P and Q from centre of magnet.
29.2.3.4 Inverse square law, Inverse fourth
power, Inverse seventh power
Use a balance to measure the repulsion of two bar magnets. Make a
balance
out of a meter stick with a magnet on one end facing the pole of
another
similar magnet. Adjust the distance between the magnets and slide the
counterbalance
along the meter stick until equilibrium is reached. Use a bar magnet
brought
near a second bar magnet counterweighted and on a knife edge to roughly
verify the inverse square law. Use three simple variations of magnets
levitating
in a glass tube to show a force varying with the inverse of the
distance
squared. Apparatus shows the force between two dipoles varies as the
inverse
fourth power of the separation. Apparatus shows the force between a
magnet
and a piece of soft iron varies with the inverse seventh of the
separation.
29.2.4 Magnet / electromagnet Interaction
29.2.4.1 Interaction of magnet and magnetizing
coil
See diagram 2.166
| See diagram 2.167
Make a magnetizing coil by using a glass tube wound with close turns
of insulated copper wire to magnetize steel knitting needles. A
solenoid
on a pivot and a magnet on a pivot interact. A bar magnet is mounted in
a large flat coil. The deflection of a compass needle in the centre of
a large coil placed in the plane of the magnetic meridian is
proportional
to the tangent of the current.
29.2.4.2 Solenoid and bar magnet
A suspended solenoid reacts with a bar magnet only when the current
is on. A magnet oscillates in a coil proportional to the square of the
current in the coil. When a solenoid is energized an iron core is
violently
drawn into the coil.
29.2.4.3 Jumping magnet
Place a bar magnet in a vertical transformer and apply d.c. with a tap
switch.
29.2.4.4 Magnetically suspended globe,
unipolar
motor
Two magnetized knitting needles mounted as the legs of an H suspended
by a string rotate when a current flows upward through a rod.
29.2.4.5 Floating magnetic balls, float
magnetized
needles
See diagram: 29.3.5
Rub eight needles on one pole of a magnet to magnetize them and
make the sharp end of them being the same pole. Push each needle
through
a cork leaving only one cm length in the cork. Float the magnetized
needles
on the surface of water in a plastic bowl. Put one pole of a strong
magnet
above the floating magnet needles and the floating needles will change
their positions to form a certain picture. Increase or decrease the
numbers
of the magnet needles, change the poles of the magnet needles, change
the
distance from the pole to magnet needles, observe if the shape of the
picture
changes.
2. Thousands of small magnetic balls floating freely on
the
surface of water form hills and hollows when excited by an a.c.
magnetic
field.
29.2.4.6 Ampere's ants
An amusement park display where a pushbutton controlled magnetic
stirrer
is under a dish of iron filings.
29.2.5 Force on moving charges
29.2.5.1 Cathode ray tube, CRT
Deflect the beam in an open CRT with a magnet. A magnet or battery
connected to the plates
is used to deflect the beam of an open CRT.
29.2.5.2 e / m for electrons, measurement of
e / m
Deflect the beam in an open CRT with a magnet. Use the earth's field
to deflect the beam in an oscilloscope. Deflect the beam of an
oscilloscope
with large solenoids. Deflect the beam of an oscilloscope by current in
wires parallel to the axis of the tube.
29.2.5.3 Bending of an electron beam
An electron beam hitting a fluorescent screen in a tube is bent by
a magnet. A thin beam along a fluorescent screen is bent by a magnet or
charged rod. A thin electron beam made visible by a fluorescent screen
is bent when a magnet is brought near.
29.2.5.4 Crookes tube
The Crookes tube was an improved gas discharge tube, vacuum tube, that
showed a striped positive column, Faraday dark space, Crookes dark
space,
negative glow, cathode glow. Unwanted deflections of the beam in the
Crookes
tube are caused by induced charge.
29.2.5.5 CRT and earth's magnetic field
A CRT is mounted so it can be oriented in any direction and rotated
about its axis. Find the position that results in no deflection from
the
earth's field turn 90 degrees.
29.2.5.6 Forces on an electron beam,
Magnetic
deflection of cathode rays
A beam of free electrons is bent in a circle by large Helmholtz coils.
A beam from a lime spot cathode in a large bulb is made circular by
Helmholtz
coils.
29.2.5.7 Pinching mercury
(The
use of open surface mercury is illegal in some school systems!)
A thread of mercury in a glass tube is pinched in two by the
interaction
of the current and the conductor.
29.2.5.8 Magnetic pump, ion motor force on
conducting
field
copper (II) sulfate solution flows in a circle when placed between the
poles
of a magnet with a current from the centre to edge. An ion motor for
the
overhead projector with cork dust in a copper (II) sulfate solution.
Cork
dust
floating on a solution of zinc chloride in a circular container rotates
when current is passed through the solution in the presence of a
magnetic
field. Cork dust shows the motion of copper (II) sulfate an ion motor.
Salt
solution rotates when placed in a circular dish over a magnet with
electrodes
at the centre and edge.
29.2.6 Force on Current in Wires, parallel
conductors
29.2.6.1 Parallel conductors
(The
use of open surface mercury is illegal in some school systems!)
Long vertical parallel wires attract or repel depending on the current
direction. Use two heavy vertical wires 1 cm apart and pass 20
amps in
the same or opposite direction. Use rectangular loops of solid wire
hanging on
pivots
from two stands. Use parallel
wires with one being a loop free to turn in a pool of mercury. Radial
wires
(like clock hands) spring apart when current is passed through them.
29.2.6.2 Interacting coils
Two hanging loops attract or repel depending on current direction.
A narrow loop formed by hanging a flexible wire opens when current is
passed.
Two loops in proximity attract or repel depending on current direction.
29.2.6.3 Pinch effect
Six number 8 wires are connected loosely between two terminals. Pass
20 amps and the bundle is attracted. Six vertical parallel wires are
loosely
hung in a circular arrangement. Six wires in parallel attract when
current
passes through each in the same direction. Then sets of three wires
each
have current flowing in opposite directions. A high voltage capacitor
is
discharged through a cylinder of aluminium foil strips.
29.2.6.4 Filament and magnet with a.c. /
d.c.,
vibrating
lamp filament
A tube lamp with a straight filament on a.c. will vibrate when placed
between the poles of a magnet. A magnet is brought near carbon filament
lamps one powered by a.c. the other by D3. The images are projected.
29.2.6.5 a.c. / d.c. magnetic contrast
A magnet is brought near a carbon lamp filament powered by d.c. then
A3.
29.2.6.6 Dancing spring. jumping wire (The
use of open surface mercury is illegal in some school systems!)
Current is passed through a limp copper spring dangling in a pool of
mercury causing it to dance. A helix of fine wire hanging vertically
into
a pool of mercury contracts and breaks contact repeatedly. A wire is
placed
in a horseshoe magnet and connected to a battery. The wire jumps out of
the magnet. A wire is placed in a horseshoe magnet and connected to a
battery.
A large heavy wire clip rests in pools of mercury between the poles of
a strong magnet. An aluminium bar in a magnet has its ends in mercury.
Short the mercury pools to a storage battery and the aluminium bar hits
the ceiling. A wire hangs into a pool of mercury and between the poles
of a U shaped magnet. s current is passed through the wire it deflects
out of the mercury and breaks the circuit. A coil of wire wound around
one pole of a horseshoe magnet jumps off when energized. Run twenty
amps
through
a wire in a horseshoe magnet.
29.2.6.7 Current balance
In a current balance a balancing mass measures the force required to
prevent the movement of of one current-carrying coil in the magnetic
field
of a second coil carrying the same current. Current balance has a
rectangular
coil on knife edges and stationary windings with parallel conductors.
An
open rectangle of aluminium wire is balanced between the poles of a U
magnet
until current is passed through the part perpendicular to the field.
Hang
a triangular loop of wire from a spring scale in the mouth of an
electromagnet
and the current in the loop is varied.
29.2.6.8 Maxwell's rule
(The
use of open surface mercury is illegal in some school systems!)
Maxwell's rule states each part of an electric circuit the circuit
experiences a force causing it to tend to move in such a direction as
to
enclose the maximum possible magnetic flux. Show an electric
circuit
that can change shape to include the maximum possible magnetic flux. A
heavy wire connects two metal boats floating in mercury troughs with
electrodes
at one end.
29.2.6.9 Barlow's wheel
(The
use of open surface mercury is illegal in some school systems!)
A copper disc with current flowing from the centre to a pool of mercury
at the edge rotates when placed between the poles of a horseshoe
magnet.
A potential is applied from the axle of a wheel to a pool of mercury at
the rim while the wheel is between the poles of a magnet. Current
passes
from the bearings of a copper wheel mounted vertically to a pool of
mercury
at the base. A U shaped magnet is mounted so the current is
perpendicular
to the magnetic field. The copper disc in Barlow's wheel is replaced by
a cylindrical Alnico magnet with the field parallel to its axis.
For a variation
of Barlow's wheel, an Alnico disc magnetized in the direction of the
axis
rotates around the axis when a current is made to flow from the axis to
the rim.
29.2.6.10 Electromagnetic swing
Switch the current direction in a wire loop swing mounted above one
pole of a vertical bar magnet to build up a pendulum motion.
29.2.6.11 Magnetic grapevine
A very flexible wire suspended alongside a vertical bar magnet will
wrap itself around the magnet when there is a current in the wire.
29.2.6.12 Electromagnetic conical pendulum
(The
use of open surface mercury is illegal in some school systems!)
A vertical wire is suspended loosely from above a vertical solenoid
into a circular trough of mercury. As current is passed through the
wire
it rotates in the trough.
29.2.6.13 Ampere's motor, Ampere's frame
A coil on a reversing switch is placed between the poles of strong
magnets. A magnet is brought near and rotates a large current carrying
loop. A copper rod rolls along two electrified rails over ring magnets
sandwiched between steel plates. A wheel on electrified rails over a
large
vertical field produced by electromagnets rolls back and forth
depending
on the current direction. As the current is reversed in a rod rolling
horizontally
on a track between the poles of a strong magnet the direction of motion
reverses.
29.2.7 Torques on coils
29.2.7.1 Model galvanometers
Use a large working model of a galvanometer with a large coil and
magnet
to show the essentials. Construct a large model d'Arsonval
galvanometer
from a coil and a large U-shaped magnet.
29.2.7.2 Force on a current loop
A rectangular loop on wire aligns perpendicular to a magnetic field.
29.2.7.3 Interacting coils
A small free turning coil is mounted in a larger coil. Two horizontal
coaxial coils the inner stationary and the outer larger coil suspended
freely interact when currents are passed through in like or opposite
directions.
A solenoid attached to a battery is mounted in a large open Helmholtz
coils
assembly.
29.2.7.4 Interacting solenoids
(The
use of open surface mercury is illegal in some school systems!)
Two heavy copper horizontal solenoids pivot in mercury cups about a
vertical axis. Suspend a solenoid and show the effects of a bar magnet
on it. A vertical coil energized by a flashlight cell floats in a large
pan. Use a bar magnet to move the coil.
29.3.0 Earth's magnetic field, terrestrial
magnetism
principles, magnetic variation (magnetic declination, magnetic
deviation),
magnetic dip (magnetic inclination), Plotting Magnetic Fields, plotting
compass method, iron filings method, magnetic anomaly, geomagnetism,
palaeomagnetism,
measurement of the Earth's magnetic field strength by vector comparison
to a square coil field, terrestrial magnetism, magnetic anomaly,
magnetic
declination
The Earth has magnetic properties so you can imagine it containing a
huge bar magnet! The vertical plane containing the poles of a compass
needle
is called the magnetic meridian. The angle between the magnetic
meridian
and geographic meridian, the angle between true north and magnetic
north
is called the magnetic declination or magnetic deviation or angle of
declination.
It is recorded on all accurate maps, e.g. to tell you that magnetic
north
is 9o east of true north. Places with the same magnetic
declination
can be shown as isogonic lines. It may be shown as an angle at lookouts
that people visit to see surrounding countryside.
Magnetic declination
varies at different longitudes, and in the same place at different
times
of the year and over time.
A pocket compass has a compass needle made
of
magnetized steel in a nonmagnetic case.
A ship's compass contains a
disc
with parallel bar magnets attached underneath pivoted on a hard
bearing.
The compass case floats in a liquid and is suspended so that it always
remains horizontal. Most ports have two prominent reference points to
check
that the ship's compass is correctly pointing to magnetic north.
The phrase "to swing a ship" refers to checking the compass deviation
of a ship by swinging the ship in the smallest possible circle through
the points of the compass and taking sightings on objects with known
positions and comparing these sightings with the true bearings.
Sometimes the structure of the ship or even the cargo has its own
magnetic properties that affect the true reading of the ship's compass.
The cardinal points of a compass are due north, south, east and west,
i.e. in the direction of the poles, sunrise and sunset. To "box the
compass" is a nautical phrase meaning to name the 32 points of a ship's
compass in correct order. A wind that boxes the compass blow round from
every direction until the starting direction.
29.3.1 Simple compass needles, magnetic dip
angles,
magnetic inclination, magnetometer, magnetic needle, magnetic compass,
marching compass, magnetizing coil, simple compass, test-tube contains
iron filings, float magnetized razor blade, Make a card compass +
The potting compass consists of a small permanent magnetic needle
pivoted
at its centre so that it can swing freely in a horizontal plane inside
a glass case. The needle will turn in a magnetic field pointing along
the
direction of the field, i.e. the direction along which a north pole
would
be urged if it were free to move. A compass needle is a permanent bar
magnet
one end of which points towards the magnetic north pole and is called
the
north seeking or N pole. The other end is the south seeking or S
pole.
A compass needle is marked "N" at on end. As this end points towards
the
north Magnetic Pole, it is called the "north seeking pole" of the
magnet.
The other end is the "south seeking pole". Like poles repel each other
and unlike poles attract each other.
1. Make a simple compass needle. Stroke a sewing needle
many
times in the same direction. Push it sideways through a flat cork or
through the centre of a circular piece of paper.
Put the needle and cork or paper in a plastic bowl of water. The needle
turns
to a north south direction. Carefully turn the bowl in a circle. The
bowl turns but the needle keeps pointing in the north south direction.
2. See diagram 2.164A
Magnetize a sewing needle by stroking it with a bar magnet. Make a
simple compass by pushing the magnetized needle through cardboard and
suspending it on a thread.
Label the end of the magnet that tends to point north
with
an arrow. Use a magnetic needle on a stand or a 16 mm plotting compass
and compare the direction it points to the direction of the simple
compass
needles.
3. See diagram 2.164 B
Make another simple compass needle using two magnetized sewing needles
pushed through the holes of a large press stud. Balance it on the end
of a needle pushed into a
cork. Repeat the experiment with this stand and other magnetized
objects.
Use a half hemisphere-shaped metallic button with a smooth surface.
Place
it on a piece of smooth glass. Place a magnetized needle on the two
buttonholes.
Repeat the experiment with this stand and other magnetized objects.
4. See diagram 2.164C
Push
a magnetized needle through thin cardboard and suspending it on a
thread. Mark the end of the magnet that tends to point north. Use a
magnetic needle on a stand
or a 16 mm plotting compass and compare the direction it points to with
the direction of the simple compass needles. A compass needle is marked
"N" at on end. This
end points towards the north magnetic pole so you call it the
"north-seeking pole" of the magnet. The other end is the "south-seeking
pole".
5. See diagram 29.1.6d
Rub a piece of hacksaw blade,
a needle and a piece of razor blade with a pole of a bar magnet to
magnetize
them. Repeat the experiment by rubbing back and forth in a single
direction
or in different ways. Note whether direction of rubbing ways influences
the magnetization effect. Push a large iron nail through a cork and put
a small test-tube over the point of the nail. Place a magnetized
hacksaw
blade on the top of the test-tube. Adjust its position until it
balances
on the test-tube. Mark the balance point on the magnetized hacksaw
blade.
Put a drop of hot wax on the top of the test-tube then quickly place
the
hacksaw blade on the test-tube again so that you glue the hacksaw blade
on the test-tube firmly. Bring the north pole (N pole) of a small
permanent
magnet close to one end of the saw blade. If the end is attracted, it
must
be the S pole of the magnetized hacksaw blade. If the end is repelled,
it must be N pole of the magnetized hacksaw blade.
29.3.2 True north and magnetic north, magnetic
variation (magnetic declination, magnetic deviation), geomagnetism
1. Study maps, e.g. the International Chart Series (Admiralty charts)
from an hygrographic office or visit lookouts to see the
importance
of magnetic variation. Find where the direction of true north is shown,
then use a compass to measure the angle, D, between the horizontal
direction
it points, true north and the geographic meridian, magnetic north.
International
charts used by mariners show the magnetic variation curves for a
certain
base year in in degrees followed by the letter E or W to denote
east
or west. For example at Cabo Maguari at the mouth of the Amazon Rives
the
magnetic variation is 19oW (2'W), i.e. at Cabo Maguari a
magnetic
compass points 19o west of geographic north, in 1990, with a
further movement west of 2' per year since 1990.
2. Study a
pocket
compass
or a ship's compass and compare a compass reading between two points
with
the same two points shown on a map.
3. Hammer a soft iron bar
pointing
north and sloping downwards towards north. The bar becomes slightly
magnetic.
Some of its particles have become aligned with the earth's magnetic
field.
Hammer it again pointing east - west. The bar loses its magnetism.
4. The
study of palaeomagnetism records changes in the earth's magnetic field
in the past. The polarity of he earth's magnetic field has reversed
many
times. This information can be used to date old rocks.
29.3.3 Magnetic dip, measure magnetic dip
angles,
magnetic inclination
See diagram: 2.165
Dip is the angle in vertical plane between the Earth's magnetic field
and the horizontal. The Earth's magnetic north pole is at 67oS,
143oE. The Earth's magnetic south pole is at 75oN,
101oW. At the equator the dip is about 0o,
so at the equator a suspended bar magnet hangs horizontally. At the
earth's
magnetic poles the magnetic dip is 90o, so if you take an
aircraft
flight that passes over the north magnetic pole, a suspended bar magnet
will point vertically straight down! At London the dip from the
horizontal
is 67o. At New York the dip from the horizontal is 72o.
The earth's magnetic field is about 0.2 g.
1. Cut a 2 cm wide
rectangular
strip of sheet copper. Bend the strip into an U-shaped stand. Glue the
bottom of the copper stand to the middle of a piece of plywood. Glue a
protractor to the front of the stand. Insert a pin at the centre of
each
end of the cork. Insert a steel knitting needle through the centre of
the
cork. Place the cork on the stand supported by the two pins. Adjust the
lengths of the knitting needle on each side of the cork until the cork
balances horizontally, i.e. it is balancing about its centre of
gravity.
Take the cork off the stand and magnetize the knitting needle without
changing
its position in the cork. Put the cork with the magnetized needle on
the
copper stand again. When the knitting needle balances again, it
inclines
from the horizontal line. Measure the angle between the needle and the
horizontal line. This is the magnetic dip angle or dip.
2. Stroke two
pins many times with the north pole end of a magnet in the same
direction
so that their points attract each other. Push them into each end of a
thin
stick of foam plastic to make a dip needle. Push a sewing needle across
the middle of the dip needle to act as a pivot to balance between two
drinking
glasses. If you adjust the direction of movement to a north south
direction
the dip needle will dip down from the horizontal because it will be
parallel
to (tangent to) the earth's magnetic field lines. At the equator the
dip
needle will be about horizontal, 90o to the
vertical.
At the north or south pole it will point about straight down,
vertical.
3. Push a steel knitting needle through cylindrical cork at right
angles
to its long axis. Push a pin into the centre of each end of the cork to
act as an axle. Balance the cork through its axle of pins on knife
edges.
Magnetize the steel knitting needle using a magnetizing coil. Balance
the
cork again. One end of the needle is pulled downwards by the earth's
magnetic
field. Fix a spirit level, or a glass tube containing a bubble in
water,
above the knitting needle. Use a protractor to measure the angle of dip
between the horizontal spirit level and the knitting needle. At the
north
or south Magnetic Pole the needle should point straight down! At the
equator
the knitting needle will be about parallel to the spirit level.
29.3.4 North pole of magnet pointing magnetic
north
See diagram 29.4.2
1. Draw a line AB on paper fixed to a drawing board. Place a plotting
compass on AB and rotate the board until the line lies in the magnetic
meridian. Fix the position of the board with chalk marks. Remove the
plotting
compass and place a weak bar magnet of known polarity in the centre of
the board with its north pole pointing magnetic north and its axis over
the line AB. Draw the outline of the bar magnet. Put the plotting
compass
close to the north pole of the magnet and make pencil dots A and B at
the
south and north poles of the compass needle. Move the compass until its
south pole is over B. Tap it gently to prevent the needle sticking.
Draw
a dot C at the north end. Repeat this process until the line of dots
either
goes off the paper or finishes up at the south pole of the magnet.
Start
again at a slightly different point A1. Do this many times on both
sides
of the magnet. Remove the magnet. Join up the dots to give lines of
magnetic
force and show the directions of these lines with arrows.
2. South
pole
of magnet pointing magnetic north. The method used is the same as used
before except that the south pole of the magnet points towards magnetic
north.