Magnetism and Electromagnetic Induction

Magnet

A magnet is a material or object that can attract magnetic materials such as iron, nickel, and cobalt.

Magnets are widely used in electrical, electronic, industrial, and household devices.

Properties of a Magnet

A magnet has the following important properties:

Every magnet has two poles:
- North Pole
- South Pole
Like poles repel each other.
Unlike poles attract each other.
A freely suspended magnet generally aligns itself approximately in the north-south direction.
A magnet can induce magnetism in suitable magnetic materials placed near it.
Magnetic poles always occur in pairs. A single isolated north or south pole is not normally obtained by simply breaking a magnet.

Natural and Artificial Magnets

Natural Magnet

A naturally occurring magnetic material is called a natural magnet.

A well-known natural magnet is:

Lodestone

Artificial Magnet

Magnets manufactured by humans are called artificial magnets.

They may be made from magnetic materials and alloys.

Examples of common magnet shapes include:

Bar Magnet
Horseshoe Magnet
Ring Magnet
Circular Magnet

Uses of Magnets

Magnets are useful in daily life.

They can be used for:

Separating iron from mixtures
Electric Motors
Loudspeakers
Electric Bells
Magnetic Compasses
Generators
Electronic Devices
Toys

Magnetism

Magnetism is the phenomenon associated with magnetic attraction and repulsion.

Magnetism is closely related to moving electric charges and electric current.

A moving electric charge can produce a magnetic field.

Types of Magnets

The source discusses three major types of magnets:

Temporary Magnet
Permanent Magnet
Electromagnet

1. Temporary Magnet

A temporary magnet behaves like a magnet mainly while it is under the influence of an external magnetic field.

When the external field is removed, it loses most or all of its magnetism.

Examples may include:

Soft iron
Iron nails
Paper clips

2. Permanent Magnet

A permanent magnet retains its magnetic properties for a long time after being magnetized.

Examples include:

Bar Magnets
Horseshoe Magnets
Permanent Speaker Magnets

3. Electromagnet

An electromagnet is a magnet whose magnetic field is produced by electric current.

It commonly consists of:

A coil of insulated wire
A soft iron core

When current passes through the coil, a magnetic field is produced.

The electromagnet can be switched on or off by controlling the current.

Magnetic and Non-Magnetic Materials

Materials can be classified according to their magnetic behaviour.

Magnetic Materials

Magnetic materials are materials that respond noticeably to a magnetic field and, in many school-level contexts, are described as materials that can be attracted by a magnet.

Examples include:

Iron
Nickel
Cobalt
Steel

Types of Magnetic Behaviour

The source discusses:

Diamagnetic Materials
Paramagnetic Materials
Ferromagnetic Materials

Diamagnetic Materials

Diamagnetic materials are weakly repelled by an external magnetic field.

Examples mentioned in the source include:

Gold
Silver

Paramagnetic Materials

Paramagnetic materials are weakly attracted by a magnetic field.

Examples include:

Aluminium
Platinum

Ferromagnetic Materials

Ferromagnetic materials are strongly attracted by magnets and can be strongly magnetized.

Examples include:

Iron
Steel
Nickel
Cobalt

Non-Magnetic Materials

In basic classification, non-magnetic materials are materials that are not noticeably attracted by ordinary magnets.

Examples include:

Wood
Plastic
Rubber

Important Magnetic Terminologies

The important magnetic quantities discussed in this chapter are:

Magnetic Field
Magnetic Field Intensity
Magnetic Flux
Magnetic Flux Density

Magnetic Field

A magnetic field is the region around a magnet, current-carrying conductor, or moving electric charge where magnetic effects can be experienced.

It is represented as a vector field.

Magnetic Field Intensity

Magnetic Field Intensity, also called magnetic field strength, indicates the magnetizing force at a point in a magnetic field.

It is represented by:

Its SI unit is:

Ampere per metre (A/m)

Magnetic Flux

Magnetic flux represents the total magnetic field passing through a given surface.

It is represented by:

Φ

Its SI unit is:

Weber (Wb)

Magnetic Flux Density

Magnetic Flux Density is the magnetic flux passing normally through a unit area.

It is represented by:

Mathematically:

B = Φ / A

Where:

B = Magnetic Flux Density
Φ = Magnetic Flux
A = Area

Its SI unit is:

Tesla (T)

Since:

1 T = 1 Wb/m²

Magnetic Effect of Electric Current

When electric current flows through a conductor, a magnetic field is produced around the conductor.

This is called the magnetic effect of electric current.

A simple demonstration is obtained by bringing a compass near a current-carrying wire.

The compass needle deflects because the electric current produces a magnetic field.

Applications of Magnetic Effect of Current

The magnetic effect of current is used in:

Electric Bells
Electric Motors
Loudspeakers
Electric Fans
Toys
Telephone Equipment
Relays
Electromagnets

Electric Motor

An electric motor converts electrical energy into mechanical energy.

When current flows through a coil placed in a magnetic field, magnetic forces can produce rotation.

Electric motors are used in:

Fans
Household Appliances
Electric Vehicles
Industrial Machines
Pumps
Toys

Transformer

A transformer is an electrical device used to change the voltage level of alternating current.

It generally contains:

Primary Coil
Secondary Coil
Magnetic Core

A changing current in the primary coil creates a changing magnetic field.

This changing magnetic flux induces an EMF in the secondary coil.

Transformers operate on the principle of electromagnetic induction.

Magnetic Resonance Imaging (MRI)

MRI systems use strong magnetic fields and radio-frequency signals to produce detailed images of structures inside the human body.

Magnetic fields play an important role in the operation of MRI technology.

Principle of Electromagnetism

Electromagnetism

Electromagnetism is the branch of physics that studies electric fields, magnetic fields, and their interaction.

Some important ideas are:

Electric charges produce electric fields.
Electric current produces magnetic fields.
Changing magnetic fields can produce electric fields.
Changing electric fields can produce magnetic fields.

Electricity and magnetism are therefore closely related.

Electromagnet

An electromagnet produces a magnetic field when electric current passes through a coil.

The strength of an electromagnet depends on factors such as:

Amount of current
Number of turns in the coil
Core material

Placing a soft iron core inside the coil can greatly increase magnetic field strength.

Advantages of Electromagnets

Can be switched on and off
Magnetic strength can be controlled
Can produce strong magnetic fields
Useful in many electrical devices

Applications of Electromagnets

Electromagnets are used in:

Electric Bells
Relays
Motors
Loudspeakers
Magnetic Cranes
Electromagnetic Locks

Electromagnetic Induction

Electromagnetic induction is the production of an electromotive force (EMF) in a conductor or coil when the magnetic flux linked with it changes.

An induced current flows if the circuit is closed.

Electromagnetic induction can occur when:

A magnet moves relative to a coil
A coil moves through a magnetic field
The magnetic field surrounding a stationary coil changes

Faraday’s Experiment

Michael Faraday demonstrated electromagnetic induction using:

A Coil
A Magnet
A Galvanometer

The coil is connected to the galvanometer.

Case 1: Magnet at Rest

If the magnet remains stationary relative to the coil:

No change in magnetic flux
        ↓
No induced EMF
        ↓
No current

The galvanometer does not show a continuous deflection.

Case 2: Magnet Moving Toward the Coil

When the magnet moves toward the coil:

Magnetic flux through the coil changes.
An EMF is induced.
Current flows if the circuit is closed.
The galvanometer needle deflects.

Case 3: Magnet Moving Away from the Coil

When the magnet moves away:

Magnetic flux changes in the opposite manner.
EMF is induced in the opposite direction.
The galvanometer deflects in the opposite direction.

Effect of Speed of Magnet

If the magnet is moved faster, the magnetic flux changes more rapidly.

Therefore, a larger induced EMF is produced.

Hence:

Faster change of magnetic flux → Greater induced EMF

Faraday’s Laws of Electromagnetic Induction

Faraday proposed two important laws.

Faraday’s First Law

Whenever the magnetic flux linked with a conductor or coil changes, an EMF is induced in it.

If the circuit is closed, this EMF causes an induced current.

Faraday’s Second Law

The magnitude of induced EMF is directly proportional to the rate of change of magnetic flux linkage.

Mathematically:

|E| = N |dΦ/dt|

Including the direction given by Lenz’s Law:

E = -N(dΦ/dt)

Where:

E = Induced EMF
N = Number of turns
Φ = Magnetic Flux
dΦ/dt = Rate of change of magnetic flux

Lenz’s Law

Lenz’s Law gives the direction of the induced EMF or induced current.

It states:

The induced current flows in such a direction that the magnetic effect produced by it opposes the change that caused the induction.

The negative sign in Faraday’s equation represents Lenz’s Law:

E = -N(dΦ/dt)

The induced effect opposes the change in magnetic flux that produces it.

Types of Induced EMF

The source discusses two broad types:

Dynamically Induced EMF
Statically Induced EMF

Dynamically Induced EMF

Dynamically induced EMF is produced when there is physical relative motion between a conductor and a magnetic field.

It may occur when:

A conductor moves in a stationary magnetic field.
A magnetic field or magnet moves relative to a stationary conductor.

In both cases, the conductor experiences changing magnetic flux and an EMF is induced.

Applications of Dynamically Induced EMF

The principle is used in:

Electric Generators
Dynamos
Alternators

Statically Induced EMF

Statically induced EMF occurs when the conductor or coil remains physically stationary but the magnetic flux linked with it changes.

This can happen because the current producing the magnetic field changes.

Statically induced EMF can be:

Self-Induced EMF
Mutually Induced EMF

Self-Induced EMF

Self-induced EMF is the EMF induced in a coil or circuit because of a change in its own current.

When current changes:

The magnetic field of the coil changes.
The magnetic flux linked with the same coil changes.
An EMF is induced in the same coil.

The self-induced EMF is commonly expressed as:

E = -L(di/dt)

Where:

E = Self-Induced EMF
L = Self-Inductance
di/dt = Rate of change of current

The negative sign indicates opposition to the change according to Lenz’s Law.

Self-Inductance

Self-inductance is the property of a coil by which a change in its own current produces an induced EMF in the same coil.

Its SI unit is:

Henry (H)

Mutually Induced EMF

Mutually induced EMF is the EMF induced in one coil because of a changing current in another nearby coil.

Suppose there are two coils:

Coil A — Primary Coil
Coil B — Secondary Coil

When current in Coil A changes:

Magnetic field produced by Coil A changes.
Magnetic flux linked with Coil B changes.
An EMF is induced in Coil B.

Formula for Mutually Induced EMF

A common expression is:

E₂ = -M(di₁/dt)

Where:

E₂ = EMF induced in the second coil
M = Mutual Inductance
di₁/dt = Rate of change of current in the first coil

Mutual Inductance

Mutual inductance is the property of two nearby coils by which a change in current in one coil induces an EMF in the other.

Its SI unit is:

Henry (H)

Characteristics of Mutually Induced EMF

It occurs between two magnetically linked coils.
It exists when current in one coil changes.
The changing current produces changing magnetic flux.
The changing flux induces EMF in the second coil.
The induced EMF opposes the change causing it according to Lenz’s Law.
If current becomes constant, the flux becomes constant and the induced EMF becomes zero.

Difference Between Self and Mutual Induction

Self Induction	Mutual Induction
EMF is induced in the same coil.	EMF is induced in another nearby coil.
Caused by change in its own current.	Caused by changing current in another coil.
Uses self-inductance `L`.	Uses mutual inductance `M`.
Formula: `E = -L(di/dt)`	Formula: `E₂ = -M(di₁/dt)`

Difference Between Permanent Magnet and Electromagnet

Permanent Magnet	Electromagnet
Retains magnetism without continuous electric current.	Requires electric current to produce its magnetic field.
Cannot normally be switched off easily.	Can be switched on or off.
Strength is relatively fixed.	Strength can be varied by changing current or coil turns.
Used in compasses and permanent-magnet devices.	Used in relays, bells, cranes, and electrical equipment.

Quick Revision

Magnet

A magnet attracts magnetic materials.

Main Poles

North
South

Magnetic Pole Rule

Like Poles → Repel
Unlike Poles → Attract

Types of Magnet

Temporary Magnet
Permanent Magnet
Electromagnet

Magnetic Materials

Diamagnetic
Paramagnetic
Ferromagnetic

Magnetic Flux

Symbol:

Φ

Unit:

Weber (Wb)

Magnetic Flux Density

B = Φ/A

Unit:

Tesla (T)

Magnetic Effect of Current

A current-carrying conductor produces a magnetic field.

Electromagnet

A current-carrying coil produces a magnetic field.

An iron core can increase its strength.

Electromagnetic Induction

Changing magnetic flux produces an induced EMF.

Faraday’s First Law

A change in magnetic flux linkage induces EMF.

Faraday’s Second Law

E = -N(dΦ/dt)

Lenz’s Law

The induced effect opposes the change that causes it.

Types of Induced EMF

Dynamically Induced EMF
Statically Induced EMF

Statically Induced EMF

Self-Induced EMF
Mutually Induced EMF

Self-Induced EMF

E = -L(di/dt)

Mutually Induced EMF

E₂ = -M(di₁/dt)

Important Formulae

Magnetic Flux Density

B = Φ/A

Faraday’s Law

E = -N(dΦ/dt)

Self-Induced EMF

E = -L(di/dt)

Mutually Induced EMF

E₂ = -M(di₁/dt)

Important Exam Points

A magnet has north and south poles.
Like poles repel and unlike poles attract.
Lodestone is a natural magnet.
Temporary magnets lose magnetism easily.
Permanent magnets retain magnetism.
Electromagnets produce magnetic fields using electric current.
Ferromagnetic materials are strongly attracted by magnets.
Magnetic field describes the region of magnetic influence.
Magnetic flux is measured in webers.
Magnetic flux density is measured in tesla.
B = Φ/A.
Electric current produces a magnetic field.
Electromagnets are used in bells, relays, motors, and magnetic cranes.
Electromagnetic induction occurs when magnetic flux changes.
Faraday demonstrated electromagnetic induction using a magnet, coil, and galvanometer.
A stationary magnet relative to a coil does not produce continuous induced EMF.
Moving a magnet toward or away from a coil induces EMF.
Faster flux change produces greater induced EMF.
Faraday’s First Law describes the condition for induction.
Faraday’s Second Law relates EMF to the rate of change of magnetic flux.
E = -N(dΦ/dt).
Lenz’s Law determines the direction of induced EMF.
Dynamically induced EMF involves relative motion.
Statically induced EMF occurs without physical movement of the coil.
Self-induced EMF occurs in the same coil.
Mutually induced EMF occurs between two magnetically linked coils.
Self-inductance and mutual inductance are measured in henries.