BACKGROUND:
Geophysicists theorize that circulation of molten
metal in the Earth’s outer core generates the Earth’s magnetic field. At
the present time, the field is oriented so that the north end is positive,
and the south end is negative. These are called, respectively, the North and
South Magnetic Poles. They are not the same as the geographic poles, which
mark then ends of the Earth’s rotational axis. The magnetic poles actually
move around constantly. It is easiest to say that the Magnetic North Pole is
near the geographic North Pole.
The compass is a device that helps us locate north and
south. A compass consists of a small needle which is free to swing
horizontally. A small magnet is attached on one end of the needle and the other
end is balanced so the needle swings easily. The magnet pulls the needle to
line up with the Earth’s magnetic field. One end of the needle (often
colored red) thus points north, while the other end points south. Once the
needle points to north, you move the North position on the compass to
coincide with the needle. Then you can see the directions easily.
PROCEDURE:
- Ask the students which way is north. Many will say
"up," as if the north pole is in the sky. Describing the
direction of the north pole is difficult, because north, in and of
itself, describes location.
Give each student or student group an inflatable
globe. Have them put a "post it" sticker with a "N"
on it on the North Pole and an sticker with an "S" on the
South Pole.
Ask them again if north is up. It should now be
clearer to them that north and south are directions on the Earth’s
surface.
- Give each student or student group a compass.
Explain how the compass works. Review the notation on the compass. Let
them experiment with them briefly, learning that the needle always
points the same direction.
- Give each student or group a bar magnet, and have
them experiment with the compass and magnet. Tell them which side of the
magnet is north and which is south. The needle will line up with the
north end of the magnet. This works on the same physical principle as
the Earth’s magnetic field. However, the local magnetic field of the
magnet is stronger than the Earth’s magnetic field, so the compass
"feels" it more and lines up with it. Discuss why this happens
with the class.
- Now that the students have found North and South,
tell them that East is half way between North and South on the right
side of the compass, and that West is half way between North and South
on the left side.
Have the students put the compass on the worksheet
where it is labeled "compass." Have the students put the bar
magnet at each of the lines, with the North end point outward. Go over
what happens, and why, at each position.
- Ask the students from what direction the Sun
appears to rise (the East). Ask them if they can use this knowledge
to find North, West, and South. Go over this carefully with the
students. West would be directly opposite east. North would be left of
east-west line and south would be right of the east-west line. Ask the
class in what direction the Sun sets (the West). Ask them what direction
the Sun moves in the sky (East to West).
- You may want to go outside to find where north and
south are located from the school. Ask students, in what direction the
Sun rises? That is the first clue of east. Where does the Sun set? In
the west. They can figure north and south from these two pieces of
observation. So students can locate the general direction without a
compass.
Note: be sure to store the compass and magnets
separately, so as not to demagnetize the compasses. A compass works best
outside, when there is no interference of electricity or metal.
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