Overview…Overview…• Properties of magnetsProperties of magnets• Magnetic & non-magnetic materials Magnetic & non-magnetic materials • Testing of a magnetTesting of a magnet• Induced magnetismInduced magnetism• Theory of magnetismTheory of magnetism• Methods of magnetisation & Methods of magnetisation &

demagnetisationdemagnetisation• Plotting / Drawing of magnetic field linesPlotting / Drawing of magnetic field lines

Testing of MagnetTesting of Magnet

SpecimenOne end of specimen

brought near to N pole

The other end of specimen brought

near to N pole

Bar Magnet Attraction occurs(or repulsion occurs)

Repulsion occurs(or attraction occurs)

Soft Iron Rod (magnetic material)

Attraction occurs Attraction occurs

Wooden Rod Nothing happens Nothing happens

Testing of magnetTesting of magnet

ThusThus

• If attraction occurs for both ends of the If attraction occurs for both ends of the material, then it must be a material, then it must be a soft iron (or any soft iron (or any magnetic material)magnetic material)

• If repulsion occurs for one side of its end, then it must be a magnet.magnet.

•Only repulsion repulsion between a specimen and a magnet allows us to conclude that the specimen is a magnet

Induced MagnetismInduced Magnetism• When a piece of When a piece of unmagnetised magneticunmagnetised magnetic material material

(iron or steel) touches or is brought near to a (iron or steel) touches or is brought near to a permanent magnet, it is attracted and becomes a permanent magnet, it is attracted and becomes a magnetmagnet itself. The material is said to have itself. The material is said to have magnetismmagnetism induced into it.induced into it.

• Materials that has magnetism Materials that has magnetism inducedinduced into it are called into it are called induced magnetinduced magnet..

• The process by which the unmagnetised magnetic The process by which the unmagnetised magnetic materials become magnets is called materials become magnets is called magnetisationmagnetisation..

Induced MagnetismInduced Magnetism

Induced MagnetismInduced Magnetism

Temporary. They lose their magnetism once the permanent magnet is drawn away from it.

Theory of MagnetismTheory of Magnetism

• In an unmagnetized state, the domains In an unmagnetized state, the domains all point in different directions.all point in different directions.

•Their magnetic effects cancel each other.Their magnetic effects cancel each other.

Background Background knowledgeknowledge

Theory of MagnetismTheory of Magnetism

•All the domains of a fully magnetized All the domains of a fully magnetized material point in the same direction.material point in the same direction.

Background Background knowledgeknowledge

Theory of MagnetismTheory of Magnetism

S N

Method of magnetizationMethod of magnetization

• The process of magnetization The process of magnetization convertsconverts a piece of a piece of steelsteel into a into a permanentpermanent magnet. magnet.

• Note: Methods of magnetisation Note: Methods of magnetisation produces permanent magnet, unlike produces permanent magnet, unlike induced magnets. Thus, induced magnets. Thus, steelsteel must must be used for all magnetisation and not be used for all magnetisation and not ironiron

Method of magnetizationMethod of magnetization

• Magnetization by strokingMagnetization by stroking Single stroke methodSingle stroke method Double touch methodDouble touch method

• Magnetization using an electric Magnetization using an electric current.current.

S

N

S

N

S

N

S

N

S

N

S

Steel Bar

The pole produced at the end of the magnetized steel is of the opposite polarityopposite polarity of the stroking pole.

N

What are the poles induced in the steel bar above?

Click on either of the diagrams below.

Steel Bar

N S NS

N

S

S

N

SNThe steel bar to be

magnetized is placed inside a solenoid.When a direct direct current (d.c.)current (d.c.) is passed through the solenoidsolenoid and then turned off, the steel steel barbar becomes magnetizedmagnetized when removed from the solenoid.

Use the Right-Hand Grip Rule to determine the polarity of the magnetized steel.

Ways of increasing the Ways of increasing the MagnetismMagnetism

• 1) Increase the current (ie Use a 1) Increase the current (ie Use a stronger battery)stronger battery)

• 2) Increase the number of coils / turns in 2) Increase the number of coils / turns in the solenoidthe solenoid

Methods of DemagnetizationMethods of Demagnetization• The methods of demagnetisation The methods of demagnetisation

removesremoves all magnetism from a all magnetism from a permanent magnet.permanent magnet.– HeatingHeating– Hammering (east-west direction)Hammering (east-west direction)– Using an A.C current.Using an A.C current.

• Note:Note:

We only We only demagnetiseddemagnetised permanent permanent magnets. So all specimen described here magnets. So all specimen described here are either permanent bar magnets or are either permanent bar magnets or magnetised steel and not ironmagnetised steel and not iron

Hammer magnet vigorously.

By heating, the molecules By heating, the molecules vibrate vibrate vigorously vigorously thus causing the magnetic thus causing the magnetic alignment to be lost. This is a very quick way alignment to be lost. This is a very quick way to remove magnetismto remove magnetism

Heat magnet to red-hot. Then cool.

As in heating, the molecules are set to As in heating, the molecules are set to vibration, causing the magnetic alignment to vibration, causing the magnetic alignment to be lost.be lost.

• Place a magnet or magnetized steel inside a solenoid connected to an alternating current (a.c)

solenoid

12 V

a.c.

• Turn on the current and slowly remove the magnet 2 to 3 metres away from the solenoid.

Magnetic FieldMagnetic Field

Magnetic field lines were introducedMagnetic field lines were introduced

by Michael Faraday (1791 – 1867)by Michael Faraday (1791 – 1867)

who named them who named them lines of forcelines of force..

Plotting Magnetic Field PatternPlotting Magnetic Field Pattern

Use of compass.

Magnetic FieldMagnetic Field

It is a It is a regionregion around a magnet around a magnet where other magnetic objects where other magnetic objects experience a experience a magnetic forcemagnetic force..

Lines used to represent the direction of direction of magnetic field magnetic field pattern.pattern.

Magnetic field lines are Magnetic field lines are similarsimilar to to

electric field lineselectric field lines

1)1) The lines can never The lines can never crosscross each each

other.other.

2)2) The more The more closely spacedclosely spaced the lines, the lines,

the stronger the force and the the stronger the force and the furtherfurther

apart the lines, the weaker the force.apart the lines, the weaker the force.

Magnetic Field LinesMagnetic Field Lines

Magnetic Field Lines (I)Magnetic Field Lines (I)

How would the magnetic field patterndiffer if there are two magnets withlike poles facing each other?

X is known as the neutral point.No magnetic field lines at X.

It is the region between two magnetstwo magnets where there are nono magnetic field lineslines.

This is because the field due to one magnet cancels outcancels out that due to the other.

Magnetic Field Line (II)Magnetic Field Line (II)

X X

Magnetic Field Line (II)Magnetic Field Line (II)

Magnetic Properties of Iron and Magnetic Properties of Iron and SteelSteel

IronIron SteelSteel

It gets It gets easilyeasily magnetizedmagnetized

It is It is hardhard to get to get magnetizedmagnetized

It gets It gets easilyeasily demagnetizeddemagnetized

It is It is hardhard to get to get demagnetizeddemagnetized

Used for Used for temporarytemporary magnets such as magnets such as induced and induced and electromagnetselectromagnets

Used to make Used to make permanent permanent magnetsmagnets

Known as Known as soft-soft-magnetic materialmagnetic material

Known as Known as hard hard magnetic materialmagnetic material