Lecture 22

Earth Magnetism

Question:  I was wondering, if the tip of a compass always points 
north, even when it is held in the southern hemisphere.  Is the magnetic 
pull greater, lesser, or equal depending on your geographic location?   
At first, I thought that the compass's attraction to the north
would be weaker if it was located in the southern hemisphere 
because it would have a greater distance to go, but then I was 
thinking, well, perhaps, that greater distance makes for a stronger 
attraction somehow.  And finally, I decided that perhaps I just don't 
fully understand the concept of the Earth's magnetic forces, and that 
maybe the geographic location has absolutely no bearing on the 
strength of a compasses positioning.


Answer:  The north end of the compass needle will always point in 
the direction of the Earth's geographic north pole which located 
near the earth's south magnetic pole.  The one place where this 
will not happen is when the compass is directly over the earth's 
magnetic south pole which is located in upper Canada or when the 
compass is over the Artic Ocean which is between the geographic 
north pole and magnetic south pole.   Like any force field, magnetic 
forces do depend on the distance from the magnet.  This means that 
in space the earth's magnetic field becomes appreciable weaker 
with distance from the Planet.  However, on the surface of the earth 
the magnetic field is approximately of equal strength everywhere.  
Of course, there are second-order variations in strength depending 
on the local geology and position relative to the magnetic pole. 
(11-6-96)


Question:  I came across the topic on magnetic reversals.  I felt 
that this material was pretty interesting, but I didn't understand 
why or how these reversals occurred.  It mentioned in the reading 
that, "today's North was once yesterday's South."  My question for 
you is, "Why do these reversals happen, and what would happen if 
they did not occur?"

Answer:  Geologists believe that convection in the liquid core of 
the Earth changes with time.  This causes a reversal in the electrical 
current passing through the core.  With current traveling in the 
opposite direction, the earth's magnetic field would reverse. (11-6-96)


Question: Not long ago I read that the earth's magnetic field is just 
starting to change again, meaning the south pole will become the north
pole and vice-versa.  Apparently it is a cycle that coincides with the 
axis of the earth and the way that the axis completes a full rotation 
every 26,000 years. Have you heard about this? and if it is true wouldn't 
that mean that there will be world catastrophic events that go with it?  

Answer:  Is it true that the Earth's magnetic field is losing strength at 
the rate of 6% in about 150 years.  This implies that the magnetic field 
will reverse in about 2000 years.  The magnetic reversals happen less 
frequently than every 26,000 years, so I doubt there is a strong connection.  
In fact, there was a period during the Cretaceous in which there were no 
reversals for roughly 40 million years.  Rocks from this age are part of
 the Cretaceous quiet zone.  But, the magnetic field does change position 
relative to the geographic north pole on this time scale.  (11-7-96)


Question:  I was always taught that the reason why the north pole was 
called the north pole was because of it's magnetic pull, as the north pole 
of a giant magnet.  This is why I am a little confused.  If the North Pole 
is really the magnetic South Pole, then how do regular compasses work.  
I was taught that the metal arrow points towards the North pole because 
of it's magnetism, but if what you say is true, wouldn't everyone be going 
south instead of north?

Answer:  The north end of one magnetic will be attracted by the south end 
of another compass.  The only reason that a compass points north is that 
its north end is attracted by the south end of the earth's magnetic field 
which happens to be near the geographic north pole.  (11-7-96)


Question:  In class, you discussed why your compass has a copper weight 
on the needle.  You said is was to counter-balance inclination.  I am 
experienced in using a compass but never understood what declination is.  
What is declination and why do I have to take it into account when I use 
a compass.  Does it have anything to do with inclination?

Answer:  Declination is the difference between the direction of the geographic 
north pole and the Earth's magnetic south pole (often called magnetic north).  
Because the two are not located at the same point on earth, topographic 
maps record both true north and declination.  Here in central Pennsylvania 
declination is roughly 7¡ with the true north east of the magnetic south 
pole (located in northern Canada).  (11-7-96)


Question:  Today in class we were discussing what was contained in the 
cores of a few planets. You said that scientists know that the core of 
Jupiter is liquid hydrogen. How did they figure out what is contained in 
the center of a planet so far away from earth.

Answer:  The evidence for a liquid hydrogen core is based of several 
observations.  Jupiter is a gas giant that does not have any of the heavy 
elements including iron.  Jupiter has a density consistent with a hydrogen 
core.  Jupiter does have a large amount of gases including methane, 
ammonia, and hydrogen.  Jupiter has a powerful magnetic field.  Under 
the pressure equivalent to that found in the core of Jupiter, hydrogen 
is liquid.  Liquid hydrogen is a superconducting material.  Hence, the 
conditions are right for a magnetic field.  (11-13-96)