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Notes, Exercises, Videos, Tests and Things to Remember on Earth’s Magnetic Field

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## Note on Earth’s Magnetic Field

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• Things to remember
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#### Terrestrial Magnetism

The earth’s magnetism is called terrestrial magnetism. Declination, dip and the horizontal component of earth’s magnetic field are elements of terrestrial magnetism which helps to locate correct direction.

##### Evidences of terrestrial magnetism
• A freely suspended magnet always point N-S direction by the influence of terrestrial magnetism on it.
• Some iron ores possess magnetism, which may be due to the effect of earth’s magnetism.
• Angle of declination and angle of dip are due to the effect of earth’s magnetism.
• Neutral points are obtained on the magnetic field of a magnet which is due to the combined effects of its magnetic field and earth’s magnetism.

##### Earth's Magnetic Field

A freely suspended bar magnet always comes to rest in a north-south direction. This shows that earth behaves as a huge magnet. The n-pole of a compass needle points towards geographical north showing that there is earth’s magnetic South Pole. Similarly, S-pole of the compass needle points towards geographical south showing that there is earth’s magnetic North pole.

The possible causes of earth’s magnetic field are given below:

• There is a permanent magnet inside the earth. This idea is ruled out. This is because the earth’s interior is at very high temperature and in this state nothing is magnetic.
• Earth’s crust contains magnetic substances, which may have produced the earth’s magnetism. This idea was also rejected because the earth’s crust does not contain sufficient magnetic substances.
• According to the latest theory, the magnetic field of the earth is caused by the magnetic current circulating in its core due to convection currents, arising from radioactive heating inside the earth.
##### Magnetism meridian and Geographic meridian

The vertical plane passing through the axis of a freely suspended magnet at a place is called the magnetic meridian.

The vertical plane passing through a place and the geographical north and south poles is called the geographic meridian at that place.

##### Magnetic elements of earth’s magnetic fields

The quantities that are required to specify the magnetic field of the earth completely is called the magnetic elements of the earth.

Angle of declination: The angle between the magnetic meridian and geographic meridian at a place is called angle of declination at that place.

Angle of dip or inclination: Angle of dip is the angle made by a freely suspended magnet at rest with the horizon.

• A freely suspended bar magnet always comes to rest in a north-south direction.
• Angle of declination and angle of dip are due to the effect of earth’s magnetism.
• Neutral points are obtained on the magnetic field of a magnet which is due to the combined effects of its magnetic field and earth’s magnetism.
• The vertical plane passing through the axis of a freely suspended magnet at a place is called the magnetic meridian
• The vertical plane passing through a place and the geographical north and south poles is called the geographic meridian at that place.
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### Very Short Questions

• They attract magnetic substances
• Like poles repel, unlike poles attract
• A freely suspended magnet points north-south direction
• Poles of magnet always exists in pairs.

The materials which are attracted by a magnet are called magnetic materials. The materials which are not attracted by a magnet are called non magnetic materials.

The magnetic field of a magnet is defined as the region around the magnet in which magnet or magnetic substance experiences a force The magnetic lines of force are the curves around the magnet along which a unit north pole would move if it was free to move.

They are:

• They move from N-pole to S-pole outside the magnet and from S-pole to N-pole internally.
• The magnetic lines of force do not intersect each other.

The neutral point is the point where the earth's magnetic field and the field due to the bar magnet becomes equal and opposite making the resultant field zero. At this point, magnetic compass does not show any fixed direction.

It is also considered that the earth is a huge magnet in which North pole lies at geographical south pole and south pole lies at the geographical north pole. When a bar magnet is freely suspended, north pole of a bar magnet is attracted by earth's magnetic south pole as opposite poles attract each other and turns towards north pole.

They are:

• Earth's crust contains magnetic substances, which may have produced earth's magnetism.
• The magnetic current circulating in its core due to convection currents, arising from radioactive heating inside the earth.

• A freely suspended magnet always points N-S direction.
• Neutral points are obtained on the magnetic field of a magnet which is due to combined effects of its magnetic field and earth's magnetic.

The angle madeby the intersection of the line joining the magnetic north-south (magnetic meridian) and the line joining the geographical north-south (geographical meridian) at a place is called angle of declination of that place. It is necessary while flying airplane during nights as well as sailingships in the ocean.

The angle made by the freely suspended magnet with the horizontal axis called the angle of dip or angle of inclination.
To measure the angle of dip, dip circle is used.

The figure shows the angle of dip at the poles. Its value is 900at this place because south pole of the dip needle becomes perpendicular to the geographical north pole and vice versa.

The value of angle of dip at the magnetic equator is 00 and 900 at the magnetic north pole.

The maximum value of angle of dip on the earth's surface is 900 which is at the magnetic poles.

The angle of dip at Kathmandu is 420 means that a dip needle will be inclined to horizontal axis by 420 in Kathmandu.

The angle of declination in Kathmandu valley is 220E means that N-pole of horizontal compass needle will point 220 east to the geographical north and south direction.

 Angle of declination Angle of dip It is the angle between magnetic meridian and geographical meridian at a place. It is the angle made by a freely suspended magnet with the horizontal. Its maximum value is less than 900. Its maximum value is 900.

The component of earth's magnetic field along the horizontal direction is called the horizontal component of earth's magnetic field.

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• ### Which is not an evidence for earth's magnetic field?

Neutral points are obtained when the lines of force of earth and bar magnet are drawn
When current is passed through a metallic rod, it turns into a magnet
A freely suspended magnet always rests pointing to the north-south of the earth
An iron piece buried in the earth becomes a weak magnet

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North Pole
South Pole
America
Equator
• ### If a freely suspended needle has a angle of dip equal to 90o where in earth is it's position?

Not enough information

North Pole

South Pole

Antarctica

• ### What is the angle in degree between magnetic meridian and geographical meridian?

It varies place to place
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