Magnetic strips are installed in the door seal

(gaskets)

Because, they help keep the door tightly

closed.

This prevents leakage of cold air (in a

fridge) or microwaves (in a microwave

oven) and improve efficiency and safety

Suppose a region is to be shielded from

magnetic effects, it could be enclosed with

ring made up of soft iron. The field lines

could follow the iron ring around the

shielded part

3.3 EARTH’S MAGNETIC FIELD

The earth behaves as if it has a short bar

magnet inside it. This apparent magnet is

inclined at a small angle to the Earth’s axis

of rotation, with a south pole pointing in the

northern hemisphere

Example 04

Explain four ways in which the magnets are

used in daily life

Answers

(i)

Electric appliances

Magnets are used in electric fans,

mixers, washing machines, and

generators to convert electrical

energy into other forms of

energies

(ii)

Loudspeakers and headphones

Magnets help convert electrical

signals into sound

(iii)

Refrigerator door seals

Magnets are used in fridge doors

to keep them tightly closed and

prevent cold air from escaping

(iv)

Navigation (compass)

A magnetic compass uses a

magnet to show direction by

aligning with earth’s magnetic

field

ORIGIN OF EATH’S MAGNETISM

Earth’s magnetism is produced by electric

currents created by the motion of molten

iron in the outer core, driven by heat and

shaped by Earth’s rotation

Example 05

The earth’s rotation leads to the rotation of

metallic iron fluid that make up the large

portion of the interior of the Earth

In which part of the fridge or microwave

oven are magnetic strips installed? Why?

Answers

Direction of the Earth’s magnetic field

The earth’s field around the equator consists

of parallel lines that point towards the north

The magnetic field direction (outside the

Earth) moves from geographic South to the

geographic north

This a “uniform field” in which the direction

and the strength of the lines of force is

constat.

This is because, the earth’s magnetic north

pole is located near the geographic south

while the earth’s magnetic south pole is

located near the geographic north

The north end of the compass needle points

towards the geographic north pole because,

the region is actually the magnetic south

pole (MSP)

Fig: Earth’s field lines

Earth’s magnetic lines of force about the

bar magnet

The Earth’s magnetic lines of force about a

bar magnet are a resultant of two magnetic

fields, one due to the bar magnet and the

other due to the Earth

For this reason, the magnetic north pole of a

compass needle is called a north seeking

pole and the general direction indicated by

the needle is called the “magnetic north”

Example 06

Does the suspended bar magnet always point

in one direction? Why?

At a certain location, the Earth’s field

neutralizes the field due to a bar magnet.

This results into neutral points (X)

Answers

Yes, the suspended bar point to the N – S

direction, because as it turns, align with the

earth’s magnetic field

Nature of the earth’s magnetic field

angle between the two axes is called the

angle of declination, D or angle of

deviation

(ii)

Additionally, the earth’s

magnetic field changes slow

overtime so that MNP moves

The angle of deviation (D) also

indicates whether the magnetic

south is east or west of the

geographic north

(iii)

(iv)

Also, the angle of declination can

be explained in terms of

geographic and magnetic

meridian

Angle of declination is the angle formed

between the magnetic and geographic

meridian

Angle of declination (D)

Angle of declination is the angle between

the Geographic North Pole (GNP) and

Magnetic North Pole (MNP)

This angle is also called the angle of

deviation

Magnetic meridian is a vertical plane

containing magnetic axis of a freely

suspended magnet at rest under the action of

the earth’s field

Geographic meridian is a plane containing

the place and the earth’s axis of rotation

Note:

(i)

The earth’s Magnetic North Pole

(MNP) and Geographic North

Pole (GNP) are not in the same

plane

Angle of inclination (I)

Angle of inclination is the angle between the

resultant magnetic field direction and the

horizontal component of the field

The Earth’s axis of rotation does not

coincide with the magnetic axis. The

It also, called dip angle.

The angle of inclination (dip angle) can be

measured by using a dip needle.

A dip needle consists of a magnetised needle

pivoted to rotated freely in a vertical plane

A compass needle is used to align the dip

needle with the horizontal components of

the magnetic field, and the needle rotates

vertically to align with the total field

Points to be considered

If a compass needle is rotated, it aligns with

the earth’s magnetic field. The angle formed

between the earth’s magnetic field and the

horizontal is known as dip angle

Applications of the earth’s magnetic field

The Earth’s magnetic field has very

important applications in our daily life

(i)

It is used by map – readers to

find locations of different places

It gives useful information in the

search of minerals

Reversals of the earth’s magnetic

field provide the basis for dating

rocks and sediments

(ii)

(iii)

(iv)

(v)

Map – readers and magnetic

compass can locate direction of

the magnetic north using the

compass

It protects the earth from cosmic

radiations (potentially damaging

high energy particles from the

sun)

Determination of dip angle