physics-the magnetic power

PHYSICS-THE MAGNETIC POWER
MAKING A COMPASS
Experiment Objective:
Students will design and construct a simple compass and test it
outside. They will learn about the earth’s magnetic poles and
concepts of magnetism. The purpose of this activity is to
introduce students to the basics of the power of magnetism.
Learning Goals:
Students will learn the basic concepts of magnetism, magnetic fields, magnetic poles and
how compasses work. Students will be able to explore their own compass and orientation
outside. They will be able to understand the function and purpose of compass.
LESSON IMPLEMENTATION OUTLINE
Introduction:
Compasses have guided explorers and Earth scientists for
thousands of years. The magnetic compass has been used for
navigation for hundreds of years. At one time, it was the only
reliable means of direction-finding on days when the sun and stars were not visible. The
origin of the compass is hard to pin down - scientists in China understood that the Earth
had a magnetic field as far back as 2637 BC. However, it wasn't until 1190 AD when the
first written description of a compass and its uses in navigation appeared in Europe.
Explorers used the compass to aid in oceanic voyages, where sailors could get disoriented
from the lack of shorelines used as points of reference.
Nowadays, sophisticated equipment is available that enables
users to determine bearings accurately and to pinpoint
locations to within a few metres. However, such equipment
has not made the compass obsolete. It is still a very practical
tool for navigation for many small craft and for people on
foot. Even airplanes and ships equipped with more
sophisticated equipment often carry compasses as backups.
Today, compasses are more elaborate and technologically
advanced. The most common type of compass currently used
by geologists is the Brunton compass. This compass not only
gives accurate geographic directions down to the degree
(between 0 and 360), but it is also designed to allow
geologists to measure the inclination of a slanted bed of rocks - also known as the "dip"
of a rock formation. Many scientists now use an even more high-tech compass called a
Global Positioning System (GPS) to orient them and navigate around the world. A GPS
uses information from satellites to get the exact geographic location of an individual
anywhere in the world.
Lesson Background and Concepts:
A compass is an instrument used for navigation relative to
the earths magnetic poles. It's magnetized pointer aligns itself
to the earths magnetic field to calculate heading, allowing for
safer and more efficient travel. Compasses are often built as
stand alone instruments, sealed with a magnetized bar and
freely turning needle upon a pivot. Most compasses highlight
four cardinal directions or cardinal points of north, south,
east, and west.
Compasses come in a variety of shapes and sizes depending
on their intended use. The type of compass used on a ship or
aircraft is a complex electronic or mechanical device capable
of compensating for both the motion of the craft and its metallic structure. At the other
extreme are small pocket compasses of low precision intended for casual use.
Regardless of their intended purpose or the complexity of their construction, most
mechanical compasses operate on the same basic principle. A small, elongated,
permanently magnetized needle is placed on a pivot so that it may rotate freely in the
horizontal plane. The Earth's magnetic field exerts a force on the compass needle, causing
it to rotate until it comes to rest in the same horizontal direction as the magnetic field.
Over much of the Earth, this direction is roughly true north, which accounts for the
compass's importance for navigation.
How does a compass work?
The needle of a compass is a small magnet, one that is allowed
to pivot freely. When the needle experiences a magnetic field
either from a nearby magnet or the Earth's magnetic field it
moves. The reaction to this movement is the needle's preferred
alignment this magnetic field. The ‘north' end of the compass
needle is simply the north end of the magnet, and it is the end
of the compass needle that points in the general direction of the geographic north pole;
naturally, the ‘south' end of the compass needle is the south end of the magnet and it
points in the opposite direction, towards the general direction of the geographic southern
pole. Having said this, the preferred directionality of a compass can be affected by local
interference in the magnetic field, like those set up by (say) a near-by electrical system; a
compass can also be affected by local magnetization of the Earth's crust, particularly near
large igneous or volcanic rock deposits.
No matter where you stand on Earth, you can hold a compass in your hand and it will
point toward the North Pole. Imagine that you are in the middle of the ocean, and you are
looking all around you in every direction and all you can see is water, and it is overcast
so you cannot see the sun. Long before GPS satellites and other high-tech navigational
aids, the compass gave humans an easy and inexpensive way to orient themselves.
A compass is a simple device. A magnetic compass (as opposed to a gyroscopic
compass) consists of a small, lightweight magnet balanced on a nearly frictionless pivot
point. The magnet is generally called a needle. One end of needle is often marked "N,"
for north, or colored in some way to indicate that it points toward north.
What Is True North?
It is well known that the earth's magnetic poles and its axis of rotation are not at the same
geographical location. They are about 11.5° rotation from each other. This creates a
difference between the true north, or grid north, and the magnetic north, or direction a
magnetic compass will point. Simply it is the angular difference between the magnetic
and true north expressed as an Easterly or Westerly variation. This difference is defined
as the variation angle and is dependent on the compass short duration, making a magnetic
compass a useful navigation tool.
The types of compasses
There are two types of compasses: the mountaineering compass and card compass. The
former is also known as orienteering compass because the needle always points in the
direction of the north. The latter does not have a noticeable needle because there is a
plastic ball around it that rotates freely.
Area of Compass Unreliability
The horizontal force of the magnetic field, responsible for the direction in which a
compass needle is oriented, decreases in strength as one approaches the North Magnetic
Pole, where it is zero. Close to the pole, an area is reached where the frictional forces in
the pivot are comparable to the horizontal forces of the magnetic field. However, both
mechanical and electronic compasses are subject to another problem within the area of
compass unreliability - daily fluctuations in magnetic declination become increasingly
large as one approaches the North Magnetic Pole.
Area of compass unreliability. The compass becomes increasingly erratic (yellow zone)
and eventually unusable (red zone) as one approaches North Magnetic Pole (*)
How Do Electronic Compasses Work?
The electronic compass such as Wayfinder uses a patented magnetic sensor technology
that was first developed by PNI, Inc. for the U.S. military. This technology is called "
magneto-inductive" and is the largest advancement in compass technology since the
fulxgate was invented 60 years ago.
The magneto-inductive technology is able to electronically sense the difference in the
earth's magnetic field from a dusturbance caused by external elements such as ferromagnetic materials and the magnetic field generated by your automobile electrical
systems. WayFinder digital compass has an embedded microcontroller that subtracts
your automobile magnetic field (distortion) from the stronger earth magnetic fields
resulting in a highly accurate compass reading.
Compass Installation
The performance of a compass will greatly depend on its installation location. A
compass relies on the earth’s magnetic field to provide heading. Any distortions of
earth magnetic field by other sources such as your car massive iron components should
be compensated for in order to determine an accurate heading. Sources of magnetic
fields in your automobile include permanent magnets, motors, electric currents—either
dc or ac, and ferro-magnetic metals such as steel or iron. The influence of these sources
on compass accuracy can be greatly reduced by placing the compass far from them.
Some of the field effects can be compensated by way of calibrating the compass for a
defined location in terms of magnetic interference. However, it is not always possible to
compensate for time varying magnetic fields; for example, disturbances generated by
the motion of magnetic metals, or unpredictable electrical current in a nearby power
lines. Magnetic shielding can be used for large field disturbances from motors or audio
speakers. The best way to reduce disturbances is distance. Also, never enclose the
compass in a magnetically shielded metallic housing.
Compass Tilt Errors
Heading errors due to a tilt depend somewhat on geographic location. At the equator,
tilt errors are less critical since the earth's field is strictly in the horizontal plane. This
provides larger X and Y readings and little of the Z component correction near the
magnetic poles, tilt errors are extremely important—since there is less X,Y field and
more of the Z component. Tilt errors are also dependent on the heading
Magnetic Field Distortions
Nearby ferrous materials is another consideration for heading inaccuracy. Since
heading is based on direction of earth's horizontal field a digital compass must be able
to measure this field with influence from nearby
magnetic sources or disturbances.
The amount of disturbance depends on the material
content of the platform and connectors as well as
ferrous objects moving nearby. When a ferrous object is
placed in a uniform magnetic field it will create
disturbances as shown in this figure. This object could be a steel bolt or bracket near the
compass or an iron door latch close to the compass. The net result is a characteristic
distortion, or anomaly to the earth’s magnetic field that is unique to the shape of object.
Magnetic distortions can be categorized as two types—hard iron and soft iron effects.
Hard iron distortions arise from permanent magnets and magnetized iron or steel on the
compass platform. These distortions will remain constant and in a fixed location
relative to the compass for all heading orientations. Hard iron effects add a constant
magnitude field component along each axes of the sensor output. This appears as a shift
in the origin of the circle equal to the hard iron disturbance in the Xh and Yh axis
To compensate for hard iron distortion is usually done by rotating compass and
platform in a circle and measure enough points on circle to determine this offset. Once
found, (X,Y) offset can be stored in memory and subtracted from every reading. The
net result will be to eliminate the hard iron disturbance from heading calculation.
The soft iron distortion arises from the interaction of the earth’s magnetic field and any
magnetically soft material surrounding the compass. Like the hard iron materials, the
soft metals also distort the earth’s magnetic field lines. The difference is the amount of
distortion from the soft iron depends on the compass orientation.
Compass Calibration
Each calibration is associated with a specified physical movement of compass platform
in order to sample magnetic space surrounding compass. The hard and soft iron
distortions will vary from location to location within same platform. The compass has
to be mounted permanently to its platform to get a valid calibration.
A particular calibration is only valid for that location of the compass. If the compass is
re-oriented in same location, a new calibration is required. It is possible to use a
compass without calibration if the need is only for repeatability and not accuracy.
Tips in using the compass
1. No matter what kind of compass you are using, the
red end always points to North. A good way to test if
the compass is working is to stand facing the sun
during lunchtime. If the red end points at North and
you and other end points at the sun, then your
compass is in good working condition. This applies
when you are north of equator. However, if you are
down under, then the red point should be facing sun.
2. When you are reading your compass, make sure
that you are holding it steadily in your hand in a way
that the baseplate is level. You must hold your compass on a comfortable manner, which
is between your face and your waist with your elbow bent. The compass should also be
read while holding it close to your stomach.
3. When you are using your compass, use your common sense. Be aware that if you
heading towards the sun, there is no way that you are going to the direction of north,
northwest, or northeast.
4. Never use a compass near metal objects such as knife, flashlight or keychain because it
can give you a false reading.
Maps & Map Reading
A map is a two-dimensional representation of the three-dimensional world you'll be
hiking in. All maps will have some basic features in common and map reading is all
about learning to understand their particular "language." You'll end up using a variety of
maps to plan and run your trip but perhaps the most useful map is a topographic map. A
topographic map uses markings such as contour lines (see page 00) to simulate the threedimensional topography of the land on a two-dimensional map. In the U.S. these maps
are usually U.S. Geological Survey (USGS) maps. Other maps that you'll find helpful are
be local trail maps which often have more accurate and up-to-date information on
specific trails than USGS maps do. Here's a brief overview of the basic language of maps.
Latitude and Longitude:
Maps are drawn based on latitude and longitude lines.
Latitude lines run east and west and measure the
distance in degrees north or south from the equator
(0° latitude). Longitude lines run north and south
intersecting at the geographic poles. Longitude lines
measure the distance in degrees east and west from
the prime meridian that runs through Greenwich,
England. The grid created by latitude and longitude
lines allows us to calculate an exact point using these
lines as X axis and Y axis coordinates.
Both latitude and longitude are measured in degrees (°).
1° = 60 minutes
1 minute = 60 seconds
Lab Activity Instructions:
Students will perform the compass making experiment in pairs or groups of three.and can
share ideas among each other. Instructors will explain the materials for the experiment
and model a quick demonstration of a real compass. Students are given the opportunity to
venture outside and test their compass for their accuracy. Allow one instructor for
demonstration and other instructors as assistants around the classroom.
Materials: medium paper clips, small bar magnet or refrigerator magnet, a small piece of
cork (like a wine bottle cork), scissors, a cup with water to float the cork and needle.
Procedure:
1. Hold one end of the pin, and wipe the magnet along
the pin, always in the same direction. When you get
to the end of the pin, lift the magnet off and move it
back to the top of the pin. Do NOT wipe the magnet
back up the pin. It is important to rub it in the same
direction, and not back and forth, so that one end of
the needle is magnetized. Repeat at least 20 times.
Your compass will work better if you first run a magnet over the needle a few
times, always in the same direction. This action 'magnetizes' is to some extent.
2. Tape the needle to one side of the cork with a small piece of clear tape. Using a
pair of pliers, push the magnetized needle through the cork so that equal portions
of the needle are sticking out on either side of the cork. This is the compass. Cork
from wine bottles works well.
3. Fill bowl with water. Put the pin and cork on the water. Float the cork + needle in
your cup of water so the floating needle lies roughly parallel to surface of water.
4. Place your 'compass' on a still surface and watch what happens. The needle
should come to point towards the nearest magnetic pole. Place the cup of water
with the compass inside on a flat surface and watch the magnetized end of the
needle point either towards the North Pole if in the Northern Hemisphere or
towards the South Pole if in the Southern Hemisphere.
5. If you want to experiment further, try placing a magnet near your compass and
watch what happens. How close/far does can the magnet be to cause any effects?
Hints: Make sure there the bowl isn't made of metal and there is no metal around the
bowl (like a knife or pair of scissors). Move your compass away from other magnetic
fields, like the TV.
Expected Outcome:
The earth produces a magnetic field. This field, although weak, is sufficient to align iron
and other paramagnetic compounds such as your needle within it. By floating the needle
on the cork, you let it rotate freely so it can orient itself within the earth's magnetic field,
to point toward the north or south poles of the planet.
Checking for student understanding:
Ask students questions about magnetic poles and fields and how they can use a compass
to orient. Encourage them to think about the mechanism of a real compass and how it is
relate to the one they are making for the experiment. Ask them to observe their compass
and their peers’.
Extensions and connections:
Students can relate their compasses to real compasses. Ask them to state the similarities
and differences between them. Question students to think about situations when they
need a compass and how they can use compass to solve various problems that they may
encounter under those circumstances.
CURRICULUM CONCEPTS
Physics-Magnetism and Earth’s Magnetic Field and Poles
References:
http://www.madsci.org/experiments/archive/860218908.Es.html
http://www.youthonline.ca/crafts/makingacompass.shtml
http://www.creativekidsathome.com/science/magnet.html
http://www.how-things-work-science-projects.com/second-grade-science-projects.html
http://www.runnerduck.com/kc_a_compass.htm
http://www.howstuffworks.com/compass.htm
http://www.physics.sjsu.edu/becker/physics51/mag_field.htm
http://www.safety-devices.com/how_compass_works.htm
http://gsc.nrcan.gc.ca/geomag/field/compass_e.php
http://www.chinesecompass.org/tips_in_using_a_compass.html