17.2 Notes: Earth’s Magnetism

Main Points:All magnets have a north & a south pole.

Opposite poles attract.

Like poles repel each other (ie south repels south, north

repels north).

Types: Permanent magnets & electromagnets

SN

S S

Electromagnet: Can only display magnetism when there is an electric current source.

They are found in motors, hard drives, and tons of other devices.

Permanent Magnet:

The magnetic field is generated and maintained by the internal structure of the material itself.

Earth’s Magnetosphere

Magnetic Field Lines:

Describe:These allow you to figure out which way a magnet will align.Lines exit south pole & enter at north pole.

Magnetic North Pole

Main Points

It’s not the permanent geographic north pole!

It shifts about 10km/year.

Declination

Define

The angle of difference between the geographic (G-N) & magnetic north (M-N) poles.

20 degrees

M-N

G-N

Pole Reversals

Length of Cycle

The poles REVERSE approx every 250,000 years. Last reversal was more than twice as long: about 740,000 yrs ago!

If you were alive about 800,000 years ago, and facing what we call north with a magnetic compass in your hand, the needle would point to 'south.'

Ocean floor evidence of Pole Reversals

Location:

Areas where crust is actively being formed at mid-oceanic ridges.

Describe:

Location:

Lava pushes its way up thru seafloor & cools & solidifies.This reveals the alignment of magnetic minerals w/in the sea floor.

Scientists can see a banding pattern.

Young rocks are nearer the ridge, olderRocks are further away.

This banding also reveals pole reversals that have occurred.

Earth’s Liquid Outer Core

Composition: IRON!

Motion: Constantly moving in convection cells around the inner core!

Solid Inner Core:

Composition: IRON & Nickel!

Describe Theory

The liquid outer core swirls around the inner core.

Iron in the outer core acts as a conductor & creates a magnetic field.