Earth’s Magnetic Field

A compass needle is a thin magnetic needle that is freely balanced on a pivot point and can rotate in a horizontal plane.

Purpose:

1. It is used to detect the direction of the Earth's magnetic field.
2. When spun, the needle comes to rest pointing along the North-South direction.

Magnetic Field Lines:

1. Around a bar magnet, the lines of force form closed loops.
2. These lines emerge from the North Pole and enter the South Pole of the magnet.
3. On Earth, these field lines form a similar pattern.

1. Let $B_0$ represent the horizontal component of the Earth's magnetic field.
2. Let $B$ represent the magnetic field strength from a bar magnet.

When both fields interact:

A neutral point forms where $B = B_0$ but in the opposite direction, so they cancel each other.

When the North Pole of a bar magnet points towards Earth's North Pole

1. The bar magnet's field opposes the Earth's field in front of it.
2. Neutral points appear on either side of the bar magnet, at equal distances.

When the South Pole of a bar magnet points towards Earth's North Pole

1. The fields oppose each other behind the bar magnet.
2. Neutral points again appear, but their location shifts depending on the orientation.

When the north pole of a bar magnet points towards the north pole of the earth.

Fixing the Paper: Attach a white sheet of paper onto a drawing board using brass pins to keep it flat.

Finding the Earth's Magnetic Meridian: a) Place the compass needle at the center of the paper. b) Mark the ends of the needle: one for North (N), the other for South (S). c) Join these points with a straight line. This line represents the Earth's magnetic meridian.

Indicating Geographical Directions: Draw geographical N, S, E, W directions at one corner of the paper for reference.

Showing the Earth's Horizontal Magnetic Field $B_0$: Draw an arrow from geographic South to geographic North (on the right side of the paper) to show the direction of $B_0$ (horizontal component of Earth's magnetic field).

Placing the Bar Magnet: Put a bar magnet at the center of the paper, with its North Pole pointing towards Earth's geographic North.

Tracing Magnetic Field Lines with Compass: a) Place the compass at the North Pole of the bar magnet.

1. Mark the position of the North end of the compass needle.
2. Shift the compass so the South end of the needle touches the previous mark.
3. Again mark the North end of the needle.
4. Continue this step until you reach the South Pole of the magnet.
5. Join all the marked points with a smooth curve—this is one magnetic field line.

Repeating from Other Points: Repeat the above tracing method starting from other points around the North Pole of the magnet to get multiple magnetic field lines.

1. The magnetic field lines emerge from the North Pole of the magnet and enter the South Pole.
2. The density of the lines indicates the strength of the magnetic field (closer lines = stronger field).
3. The field lines never cross each other.
4. The field lines are influenced by the Earth's magnetic field, especially along the magnetic meridian.

The Angle of Inclination (dip) and Angles of Declination Angle of dip is the angle between the direction of the earth's magnetic flux and the horizontal.

The angle of dip (or magnetic inclination) is the angle made by the Earth's magnetic field lines with the horizontal direction at a given point.

Key Facts:

1. It varies from place to place across the Earth's surface.
2. It is 0° at the magnetic equator and 90° at the magnetic poles.

Method of Measurement:

1. Suspend a thin iron rod using a thread so that it can rotate freely in a vertical plane.
2. Let the rod align itself according to the Earth's magnetic field.
3. Measure the angle between the axis of the rod and the horizontal using a protractor.
4. This measured angle is the angle of dip.

The angle of declination is the angle between the geographic meridian (true North-South) and the magnetic meridian (direction of a compass needle).

Key Facts:

1. It also varies by location on Earth.
2. It is positive when the magnetic North is east of true North.
3. It is negative when the magnetic North is west of true North.
4. Declination is shown on maps for navigation purposes.

1. Indicating poles of unknown magnets; The Earth's magnetic field helps determine the North and South poles of a bar magnet by observing which pole aligns with the North direction.
2. Use of magnetic needle; A magnetic compass needle aligns with the Earth's magnetic field, allowing it to indicate direction (N-S orientation) for navigation, mapping, and geophysical surveys.