Download Physics 1 notes 4-11-13 NOVA earth`s magnetic field

Physics 1
Daniel Chang, Max Tripolsky
Pennington
C°
4/11/13
Page 1
NOVA “Magnetic Storm” Video
 Is earth’s magnetic field weakening? Will it disappear in a few thousand years?
 Earth as a molten iron/nickel core that generates a magnetic field. But in recent years,
the magnetic field seems to be fading and may disappear in about one thousand years.
 Magnetism seems like magic attraction or repulsion.
 Gravity is obvious and visible; magnetic force cannot be seen.
 The field protects earth from sun’s cosmic rays and radiation from other stellar sources.
The field deflects these rays/radiation around the earth. Some rays are dragged
northward or southward toward the poles, visible as the Aurora Borealis.
 What would happen if the earth’s magnetic field went away? NASA’s mission Mars –
Mars Global Surveyor, 1996 -- was to determine the extent of Mars’s magnetic field.
The results: no magnetic field around Mars today. But there is evidence that Mars once
had a magnetic field because rocks in the crust showed evidence of magnetism. Yet
some parts of Mars at impact craters showed no trace of magnetism. This suggests that
about four billion years ago, Mars lost its magnetic field. Without the protection of that
field, Mars’ oceans dried up.
 Other evidence: ancient pottery that records the magnetic field when the pottery was
made. Clay contains magnetite, which in its raw state has no overall magnetic field. On
firing, the heat realigns the magnetite to form a weak magnet, locking in the earth’s
magnetic field at that time. It’s possible to plot a graph of the field strength over time.
In the past 300 years, the field strength has declined.
 Scientists model the earth’s core with liquid sodium, which is highly conductive, like
molten iron.
 Magnetic field generates electric current that generates a magnetic field – a
feedback loop.
 Flowing molten material is like an electrical dynamo. Once the core cools, the
motion slows. This may have been what happened to Mars – it cooled quickly,
shutting down its dynamo.
 Earth’s field is weakening too fast to be explained by core cooling.
 Other clues in volcano eruptions and lava produced – specifically lava on Hilo, Hawaii.
Each layer of lava contains a record of the earth’s magnetic field. As lava cools, it leaves
a record of the strength and direction of the magnetic field.
 Today – field points north
 7,000 years ago – field pointed south.
 Deeper samples showed that field reverses itself about every 200,000 years. Could
this be why field is getting weaker?
 Computer modeling confirmed this finding. The reversals occurred as the field
weakened from magnetic anomalies in the core metal flow.
Physics 1
Daniel Chang, Max Tripolsky

Pennington
C°
4/11/13
Page 2
NOVA “Magnetic Storm” Video
19th
century British sailors were obsessed with magnetic field; compasses pointed to
magnetic north, not the actual north pole. The magnetic pole varied over time. True
north was measured astronomically by the position of the sun at sunrise and its apex.
This was compared to the compass direction. This was essential for good navigation.
 Magnetic anomalies began to appear in 1850s in the South Atlantic Ocean. There,
the reversed field has cancelled the main field, suggesting that the field is about to
flip.