IEMDaily - Video Lecture Notes

Definition

Compass needle at the poles

At the poles, the magnetic field lines are converging or diverging vertically so that the horizontal component is negligible.If the needle is only capable of moving in a horizontal plane, it can point along any direction,rendering it useless as a direction finder. What one needs in such a case is a dip needle which is a compass pivoted to move in a vertical plane containing the magnetic field of the earth. The needle of the compass then shows the angle which the magnetic field makes with the vertical. At the magnetic poles such a needle will point straight down.

Definition

Dynamo effect

The magnetic field of the earth is thought to arise due to electrical currents produced by convective motion of metallic fluids (consisting mostly of molten iron and nickel) in the outer core of the earth. This is known as the dynamo effect.

Definition

Earth as a magnetic dipole

The magnetic field lines of the earth resemble that of a (hypothetical) magnetic dipole located at the centre of the earth. The axis of the dipole does not coincide with the axis of rotation of the earth but is presently titled by approximately 11.3 with respect to the later. In this way of looking at it, the magnetic poles are located where the magnetic field lines due to the dipole enter or leave the earth. The location of the north magnetic pole is at a latitude of 79.74 N and a longitude of 71.8 W, a place some where in north Canada. The magnetic south pole is at 79.74 S, 108.22 E in the Antarctica.The pole near the geographic north pole of the earth is called the north magnetic pole. Likewise, the pole near the geographic south pole is called the south magnetic pole.

Definition

Earth's magnetic field and dipole moment

Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth's interior out into space, where it meets the solar wind, a stream of charged particles emanating from the Sun.
The earths magnetic field at the equator is approximately

0.4 G

. Estimate the earths dipole moment.
Solution: The equatorial magnetic field is,

B_{E} = \frac{μ_{0} m}{4 π r^{3}}

We are given that

B_{E} \sim 0.4 G = 4 \times 10^{- 5} T

. For

r

, we take the radius of the earth

6.4 \times 10^{6} m

. Hence,

m = \frac{4 \times 10^{- 5} \times {(6.4 \times 10^{6})}^{3}}{μ_{0} / 4 π} = 1.05 \times 10^{23} A m^{2}

This is close to the value

8 \times 10^{22} A m^{2}

quoted in geomagnetic texts.

Definition

Define and discuss terms related to magnet in earth's magnetic field

At any location, the Earth's magnetic field can be represented by a three-dimensional vector. A typical procedure for measuring its direction is to use a compass to determine the direction of magnetic North. Its angle relative to true North is the declination (

D

) or variation. Facing magnetic North, the angle the field makes with the horizontal is the inclination (

I

) or magnetic dip. The intensity (

F

) of the field is proportional to the force it exerts on a magnet.

Definition

Magnetic Meridian

A magnetic meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the north pole and the south pole, connecting points of equal longitude. The position of a point along the meridian is given by its latitude indicating how many degrees north or south of the equator the point is

Definition

Elements of Earth's Magnetic Field

Angle of declination - The acute angle between the magnetic meridian and geographic meridian at a place is called angle of declination at that place.
Angle of dip - If a magnetic needle is pivoted on a horizontal axis through its centre of gravity and itis free to rotate in vertical plane, then such a needle is called dip needle.

Definition

Geographic and magnetic meridian

Consider a point on the earths surface. At such a point, the direction of the longitude circle determines the geographic north-south direction, the line of longitude towards the north pole being the direction of true north. The vertical plane containing the longitude circle and the axis of rotation of the earth is called the geographic meridian. In a similar way, one can define magnetic meridian of a place as the vertical plane which passes through the imaginary line joining the magnetic north and the south poles.This plane would intersect the surface of the earth in a longitude like circle.

Definition

Magnetic declination

The angle between the true geographic north and the north shown by a compass needle is called the magnetic declination or simply declination.The declination is greater at higher latitudes and smaller near the equator. The declination in India is small, it being 041 E at Delhi and 058 W at Mumbai.

Definition

Angle of dip or inclination

If a magnetic needle is perfectly balanced about a horizontal axis so that it can swing in a plane of the magnetic meridian, the needle would make an angle with the horizontal. This is known as the angle of dip (also known as inclination).Thus, dip is the angle that the total magnetic field