PrepYodhaClass Notes Β· Physics
Physics Β· Chapter 12

Magnetism

Magnetism is the science of magnets β€” the materials that attract iron, the invisible field that surrounds them, and the deep link between electricity and magnetism. The sections below begin with what a magnet is and how its field behaves, move through its properties, the Earth's own magnetism, the three classes of magnetic substances, and finish with electromagnetic induction and the machines (generators, motors, transformers) that it makes possible.

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Topic 01

What is Magnetism?

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A magnet is any material that can attract iron and a few other metals; magnets occur both in nature and as man-made bars.

⭐
Key Point
A magnet is a material that can attract iron objects and a few other metals.
Basic facts about magnets
  • A natural magnet is an ore of iron with formula Fe₃Oβ‚„, called magnetite or lodestone.
  • A magnet prepared artificially is called an artificial magnet (e.g. bar magnet, horseshoe magnet).
  • A freely suspended magnet always aligns itself in the North–South direction β€” this is why a compass works.
  • Like magnetic poles repel and unlike magnetic poles attract each other.
πŸ“ Quick self-test 2 MCQs Β· 2 fill-ups

A natural magnet is an ore of iron with which formula?

  1. Feβ‚‚O₃
  2. Fe₃Oβ‚„
  3. FeO
  4. FeSβ‚‚
βœ” B. Fe₃Oβ‚„ β€” The natural magnet magnetite (lodestone) has the formula Fe₃Oβ‚„.

In which direction does a freely suspended magnet always align itself?

  1. North–South
  2. East–West
  3. North–East
  4. South–West
βœ” A. North–South β€” A freely suspended magnet aligns North–South, which is why a compass works.

A natural magnet, an ore of iron with formula Fe₃Oβ‚„, is called magnetite or .

βœ” lodestone

Like magnetic poles repel and magnetic poles attract each other.

βœ” unlike
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Topic 02

Magnetic Field around a Magnet

Every magnet has two poles and is wrapped in an invisible field that can be mapped with field lines.

⭐
Key Point
A bar magnet has a North (N) pole and a South (S) pole; the poles always occur together.
Field around a bar magnet
  • Magnetic field lines run from N to S outside the magnet and from S to N inside it, forming closed loops.
  • The field is strongest at the poles, where the lines are most crowded.
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Where is a magnet's field strongest?

  1. At the centre
  2. At the poles
  3. Midway between the poles
  4. Everywhere equally
βœ” B. At the poles β€” The field is strongest at the poles, where the lines are most crowded.

Outside a bar magnet, magnetic field lines run in which direction?

  1. From N to S
  2. From S to N
  3. In no fixed direction
  4. In straight parallel lines
βœ” A. From N to S β€” Field lines run from N to S outside the magnet and from S to N inside it.

Magnetic field lines form closed , running from N to S outside the magnet and S to N inside.

βœ” loops

A bar magnet has a North (N) pole and a (S) pole, which always occur together.

βœ” South
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Topic 03

Electromagnet

An electromagnet is a temporary magnet made by passing current through a coil wound on a soft iron core.

⭐
Key Point
A current-carrying coil wound on a soft iron core is called an electromagnet.
Electromagnet β€” make and uses
  • The magnetism lasts only as long as the current flows, so it can be switched on and off.
  • An electromagnet is used in the electric bell, telegraph receiver, telephone diaphragm, transformer, dynamo and many other devices.
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An electromagnet is made by winding a current-carrying coil on a core of which material?

  1. Steel
  2. Soft iron
  3. Copper
  4. Bismuth
βœ” B. Soft iron β€” An electromagnet is a current-carrying coil wound on a soft iron core.

How long does the magnetism of an electromagnet last?

  1. Only as long as the current flows
  2. Permanently
  3. For one hour after switching off
  4. Only when heated
βœ” A. Only as long as the current flows β€” The magnetism lasts only as long as the current flows, so it can be switched on and off.

A current-carrying coil wound on a soft iron core is called an .

βœ” electromagnet

An electromagnet is used in the electric , telegraph receiver, telephone diaphragm, transformer and dynamo.

βœ” bell
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Topic 04

Permanent and Temporary Magnets

The choice of material decides whether a magnet is permanent or temporary.

⭐
Key Point
Permanent magnets are made of steel and temporary magnets (electromagnets) are made of soft iron.
Permanent vs temporary magnets
MagnetMaterialBehaviour
Permanent magnetsteelhard to magnetise, but once magnetised holds it for long
Temporary magnet (electromagnet)soft ironeasy to magnetise and easy to demagnetise
  • Steel cannot be magnetised easily, but once magnetised it cannot be demagnetised easily.
  • Soft iron can be magnetised or demagnetised easily, which is why it is used as the core of electromagnets.
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Permanent magnets are made of which material?

  1. Soft iron
  2. Steel
  3. Copper
  4. Aluminium
βœ” B. Steel β€” Permanent magnets are made of steel, which holds magnetism for long once magnetised.

Why is soft iron used as the core of electromagnets?

  1. It can be magnetised and demagnetised easily
  2. It is a permanent magnet
  3. It cannot be magnetised
  4. It is very hard to magnetise
βœ” A. It can be magnetised and demagnetised easily β€” Soft iron can be magnetised or demagnetised easily, making it ideal for electromagnet cores.

Steel cannot be magnetised easily, but once magnetised it cannot be easily.

βœ” demagnetised

Temporary magnets (electromagnets) are made of soft .

βœ” iron
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Topic 05

Properties of a Magnet

A magnet shows a few fixed properties that examiners ask about repeatedly.

⭐
Key Point
Attractive property: a magnet attracts small pieces of magnetic substances like iron, steel, cobalt and nickel; the attraction is maximum at the poles.
Key properties
  • Unlike poles attract and like poles repel each other.
  • When two bar magnets are brought close: N facing S attracts, while N facing N (or S facing S) repels.
  • A freely suspended magnet is a directive property, always settling along North–South.
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The attraction of a magnet is maximum at its:

  1. Centre
  2. Poles
  3. Edges
  4. Surface
βœ” B. Poles β€” A magnet's attractive property is maximum at the poles.

Which of these is attracted by a magnet?

  1. Copper
  2. Cobalt
  3. Water
  4. Bismuth
βœ” B. Cobalt β€” A magnet attracts magnetic substances like iron, steel, cobalt and nickel.

A freely suspended magnet always settling along North–South shows its property.

βœ” directive

When two bar magnets are brought close, N facing N (or S facing S) .

βœ” repels
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Topic 06

Magnetic Poles Exist in Pairs

Poles can never be separated β€” there is no isolated single pole.

⭐
Key Point
Magnetic poles always exist in pairs; an isolated single pole (monopole) does not exist.
No magnetic monopole
  • If a bar magnet is cut into two parts, each piece becomes a complete magnet with its own N and S pole.
  • However many times you cut it, every fragment still has both a North and a South pole.
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What happens when a bar magnet is cut into two parts?

  1. Each piece becomes a complete magnet with N and S poles
  2. One piece is N and the other is S
  3. Both pieces lose their magnetism
  4. Only one piece remains a magnet
βœ” A. Each piece becomes a complete magnet with N and S poles β€” If a bar magnet is cut, each piece becomes a complete magnet with its own N and S pole.

Which statement about magnetic poles is correct?

  1. An isolated single pole exists
  2. Magnetic poles always exist in pairs
  3. A magnet can have three poles
  4. Poles can be fully separated
βœ” B. Magnetic poles always exist in pairs β€” Magnetic poles always exist in pairs; an isolated monopole does not exist.

An isolated single magnetic pole, called a , does not exist.

βœ” monopole

However many times a magnet is cut, every fragment still has both a North and a pole.

βœ” South
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Topic 07

Magnetic Field

The magnetic field is the region of influence around a magnet or a current.

⭐
Key Point
The space around a magnet (or a current-carrying conductor) in which its magnetic effect can be felt is called the magnetic field.
Definition
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The magnetic field is the region around a magnet in which:

  1. Its magnetic effect can be felt
  2. There is no force
  3. Electricity cannot flow
  4. Light cannot travel
βœ” A. Its magnetic effect can be felt β€” The magnetic field is the space around a magnet in which its magnetic effect can be felt.

Besides a magnet, what else can produce a magnetic field?

  1. A current-carrying conductor
  2. A stationary charge
  3. An insulator
  4. A dry cell that is disconnected
βœ” A. A current-carrying conductor β€” The field also exists around a current-carrying conductor.

The space around a magnet in which its magnetic effect can be felt is called the magnetic .

βœ” field

A magnetic field can also surround a current-carrying .

βœ” conductor
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Topic 08

Magnetic Lines of Force

Field lines are imaginary curves that picture the field's direction and strength.

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Key Point
A magnetic line of force is an imaginary line along which a free North pole would move in the field.
Lines of force and flux
  • A tangent drawn at any point on a line of force gives the direction of the magnetic field at that point.
  • The magnetic flux through a surface equals the total number of lines of force passing normally through it; its unit is the weber.
  • Two lines of force never cross each other.
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What is the SI unit of magnetic flux?

  1. Henry
  2. Weber
  3. Tesla
  4. Pascal
βœ” B. Weber β€” Magnetic flux is measured in weber.

Two magnetic lines of force:

  1. Always cross at the poles
  2. Never cross each other
  3. Cross only inside the magnet
  4. Cross at the centre
βœ” B. Never cross each other β€” Two lines of force never cross each other.

A tangent drawn at any point on a line of force gives the of the magnetic field at that point.

βœ” direction

A magnetic line of force is an imaginary line along which a free pole would move in the field.

βœ” North
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Topic 09

Earth's Magnetism

The Earth itself behaves like a giant magnet, which is what guides every compass.

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Key Point
The Earth has its own magnetic field, as if a huge bar magnet lay inside it.
The Earth as a magnet
  • The magnetic pole near the geographic North is the magnetic North pole, and the pole near the geographic South is the magnetic South pole.
  • The Earth's magnetic field deflects charged particles coming from space towards the poles, shielding living beings from harm.
  • A magnetic compass is a magnetic needle that always points North–South; its dial is marked N, E, W, S.
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The Earth's magnetic field deflects charged particles from space towards the:

  1. Equator
  2. Poles
  3. Tropics
  4. Oceans
βœ” B. Poles β€” The Earth's magnetic field deflects charged particles from space towards the poles, shielding living beings.

The magnetic pole near the geographic North is the:

  1. Magnetic North pole
  2. Magnetic South pole
  3. Neutral pole
  4. Equatorial pole
βœ” A. Magnetic North pole β€” The magnetic pole near the geographic North is the magnetic North pole.

The Earth behaves as if a huge bar lay inside it.

βœ” magnet

A magnetic is a magnetic needle that always points North–South.

βœ” compass
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Topic 10

Magnetic Storm

A magnetic storm is a sudden disturbance of the Earth's field caused by the Sun.

⭐
Key Point
Local disturbances in the Earth's magnetic field that can damage telecommunication are called a magnetic storm.
Magnetic storm
  • They are caused by bursts of charged particles emanating from the Sun: Sun β†’ charged particles β†’ disturbance in the magnetic field.
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A magnetic storm is caused by:

  1. Bursts of charged particles from the Sun
  2. Rainfall
  3. Volcanic eruptions
  4. Ocean currents
βœ” A. Bursts of charged particles from the Sun β€” Magnetic storms are caused by bursts of charged particles emanating from the Sun.

Local disturbances in the Earth's magnetic field can damage:

  1. Telecommunication
  2. Ocean tides
  3. Plant growth
  4. Rock formation
βœ” A. Telecommunication β€” Magnetic storms are local disturbances that can damage telecommunication.

Local disturbances in the Earth's magnetic field that can damage telecommunication are called a magnetic .

βœ” storm

Magnetic storms are caused by charged particles emanating from the .

βœ” Sun
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Topic 11

Geomagnetic Storm

A geomagnetic storm is the large-scale version of the same effect across the whole planet.

⭐
Key Point
A geomagnetic storm is caused by a lump of charged particles (solar wind) from the Sun disturbing the Earth's magnetic field.
Geomagnetic storm
  • The chain is: Sun β†’ solar wind β†’ Earth's magnetic field is disturbed.
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A geomagnetic storm is caused by a lump of charged particles from the Sun known as:

  1. Solar wind
  2. Cosmic dust
  3. Sunspots
  4. Ozone
βœ” A. Solar wind β€” A geomagnetic storm is caused by solar wind β€” charged particles from the Sun β€” disturbing the Earth's field.

What does a geomagnetic storm disturb?

  1. The Earth's magnetic field
  2. The ozone layer only
  3. The Earth's core
  4. The Moon's orbit
βœ” A. The Earth's magnetic field β€” The solar wind disturbs the Earth's magnetic field.

A geomagnetic storm is caused by a lump of charged particles called the solar from the Sun.

βœ” wind

The chain of a geomagnetic storm is: Sun β†’ solar wind β†’ Earth's magnetic field is .

βœ” disturbed
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Topic 12

Aurora

When solar particles strike the upper atmosphere near the poles, they produce glowing auroras.

The two auroras
RegionNameCommon name
Arctic Circle (north)Aurora Borealisnorthern lights
Antarctic Circle (south)Aurora Australissouthern lights
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The aurora seen near the Arctic Circle is called:

  1. Aurora Australis
  2. Aurora Borealis
  3. Southern lights
  4. Solar wind
βœ” B. Aurora Borealis β€” Aurora Borealis (northern lights) occurs in the Arctic Circle.

Aurora Australis is commonly known as the:

  1. Northern lights
  2. Southern lights
  3. Polar wind
  4. Sunspots
βœ” B. Southern lights β€” Aurora Australis, seen near the Antarctic Circle, is the southern lights.

Aurora Borealis is commonly called the lights.

βœ” northern

The aurora seen near the Antarctic Circle is called Aurora .

βœ” Australis
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Topic 13

Moving Coil Galvanometer

A galvanometer is the basic instrument for detecting electric current.

⭐
Key Point
A moving coil galvanometer detects the presence and the direction of current in a circuit.
Galvanometer
  • It works on the torque experienced by a current-carrying coil placed between the N and S poles of a magnet.
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A moving coil galvanometer is used to detect:

  1. The presence and direction of current
  2. The mass of a body
  3. The temperature of a wire
  4. The volume of a gas
βœ” A. The presence and direction of current β€” A moving coil galvanometer detects the presence and the direction of current in a circuit.

A galvanometer works on the torque experienced by a current-carrying coil placed between:

  1. Two capacitors
  2. The N and S poles of a magnet
  3. Two resistors
  4. Two batteries
βœ” B. The N and S poles of a magnet β€” It works on the torque on a current-carrying coil placed between the N and S poles of a magnet.

A moving coil galvanometer detects the presence and the of current in a circuit.

βœ” direction

A galvanometer works on the experienced by a current-carrying coil in a magnetic field.

βœ” torque
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Topic 14

Ammeter and Voltmeter

A galvanometer can be adapted into either an ammeter or a voltmeter.

⭐
Key Point
An ammeter measures current and is always connected in series; the resistance of an ideal ammeter is zero.
Ammeter vs voltmeter
InstrumentMeasuresConnectedIdeal resistanceConverted from galvanometer by
Ammeterelectric currentin serieszeroadding low resistance in parallel
Voltmeterpotential differencein parallelinfinityadding high resistance in series
  • A galvanometer is converted into an ammeter by connecting a low resistance in parallel.
  • A voltmeter measures the potential difference between two points and is always connected in parallel; the resistance of an ideal voltmeter is infinity.
  • A galvanometer is converted into a voltmeter by connecting a high resistance in series.
  • A small resistance connected in parallel to divert part of the current is called a shunt.
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An ammeter is always connected:

  1. In series
  2. In parallel
  3. Across the battery
  4. In a shunt loop only
βœ” A. In series β€” An ammeter measures current and is always connected in series.

The ideal resistance of a voltmeter is:

  1. Zero
  2. Infinity
  3. One ohm
  4. Equal to the circuit resistance
βœ” B. Infinity β€” The resistance of an ideal voltmeter is infinity.

A galvanometer is converted into an ammeter by connecting a low resistance in .

βœ” parallel

A small resistance connected in parallel to divert part of the current is called a .

βœ” shunt
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Topic 15

Magnetic Substances

All materials fall into three magnetic classes by how they respond to a field.

⭐
Key Point
There are three types of magnetic substances: diamagnetic, paramagnetic and ferromagnetic.
The three types of magnetic substances
TypeBehaviour in a magnet's fieldExamples
Diamagneticweakly repelled by a magnetbismuth, copper, water, gold, silver
Paramagneticweakly attracted by a magnetaluminium, platinum, chromium, manganese, oxygen
Ferromagneticstrongly attracted by a magnetiron, cobalt, nickel
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How many types of magnetic substances are there?

  1. Two
  2. Three
  3. Four
  4. Five
βœ” B. Three β€” There are three types: diamagnetic, paramagnetic and ferromagnetic.

Which type of substance is strongly attracted by a magnet?

  1. Diamagnetic
  2. Paramagnetic
  3. Ferromagnetic
  4. Non-magnetic
βœ” C. Ferromagnetic β€” Ferromagnetic substances (iron, cobalt, nickel) are strongly attracted by a magnet.

Substances that are weakly repelled by a magnet, such as bismuth and copper, are called .

βœ” diamagnetic

Aluminium, platinum and chromium are examples of substances that are weakly attracted.

βœ” paramagnetic
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Topic 16

Paramagnetic Substances

Paramagnetic materials are feebly attracted and magnetise weakly along the field.

⭐
Key Point
Substances that are feebly magnetised in the direction of the field when placed in a strong field are paramagnetic.
Paramagnetic substances
  • Examples β€” aluminium, platinum, chromium, manganese, salt solutions of iron and nickel, and oxygen.
  • In a non-uniform field they move towards the stronger part of the field.
  • Their magnetism decreases as temperature increases.
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In a non-uniform field, paramagnetic substances move towards the:

  1. Stronger part of the field
  2. Weaker part of the field
  3. Centre of the field
  4. Edge of the field
βœ” A. Stronger part of the field β€” Paramagnetic substances move towards the stronger part of a non-uniform field.

What happens to the magnetism of a paramagnetic substance as temperature increases?

  1. It increases
  2. It decreases
  3. It stays constant
  4. It becomes zero suddenly
βœ” B. It decreases β€” The magnetism of a paramagnetic substance decreases as temperature increases.

Substances feebly magnetised in the direction of a strong field are called .

βœ” paramagnetic

Aluminium, platinum, chromium, manganese and are examples of paramagnetic substances.

βœ” oxygen
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Topic 17

Diamagnetic Substances

Diamagnetic materials are feebly repelled and magnetise opposite to the field.

⭐
Key Point
Substances that are feebly magnetised opposite to the field when placed in a strong field are diamagnetic.
Diamagnetic substances
  • Examples β€” gold, silver, zinc, copper, mercury, water, alcohol, air, hydrogen and bismuth.
  • In a non-uniform field they move towards the weaker part of the field.
  • Their (very weak) magnetism does not change appreciably with temperature.
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Diamagnetic substances are magnetised:

  1. In the direction of the field
  2. Opposite to the field
  3. Perpendicular to the field
  4. Randomly
βœ” B. Opposite to the field β€” Diamagnetic substances are feebly magnetised opposite to the field.

In a non-uniform field, diamagnetic substances move towards the:

  1. Stronger part of the field
  2. Weaker part of the field
  3. Poles
  4. Centre
βœ” B. Weaker part of the field β€” Diamagnetic substances move towards the weaker part of a non-uniform field.

Gold, silver, water and are examples of diamagnetic substances.

βœ” bismuth

The weak magnetism of diamagnetic substances does not change appreciably with .

βœ” temperature
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Topic 18

Ferromagnetic Substances

Ferromagnetic materials are strongly attracted and are the ones used to make magnets.

⭐
Key Point
Substances that are strongly magnetised in the direction of the field are ferromagnetic.
Ferromagnetic substances
  • Examples β€” iron, nickel and cobalt.
  • Their magnetism decreases with rising temperature and is lost completely at the Curie point (Curie temperature).
  • Above the Curie point a ferromagnetic substance turns paramagnetic.
  • The Curie temperature for iron is about 770Β°C and for nickel is about 358Β°C.
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Above the Curie point, a ferromagnetic substance becomes:

  1. Diamagnetic
  2. Paramagnetic
  3. Antiferromagnetic
  4. Non-magnetic permanently
βœ” B. Paramagnetic β€” Above the Curie point a ferromagnetic substance turns paramagnetic.

Which of these is a ferromagnetic substance?

  1. Copper
  2. Aluminium
  3. Cobalt
  4. Bismuth
βœ” C. Cobalt β€” Iron, nickel and cobalt are ferromagnetic substances.

A ferromagnetic substance loses its magnetism completely at the point.

βœ” Curie

The Curie temperature for iron is about Β°C.

βœ” 770
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Topic 19

Magnetic Ordering and Spin Alignment

The deeper reason for these classes is how the atomic spins line up below a transition temperature.

Spin alignment (below transition temperature Tc / TN)
TypeSpin alignment
Ferromagneticspins aligned parallel in magnetic domains, below Tc
Antiferromagneticspins aligned antiparallel and cancel out, below TN
Ferrimagneticspins antiparallel but unequal, so they do not cancel, below Tc
Paramagneticspins randomly oriented (above Tc or TN)
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In a ferromagnetic substance, the atomic spins are aligned:

  1. Parallel in magnetic domains
  2. Antiparallel and cancelling
  3. Randomly oriented
  4. Perpendicular
βœ” A. Parallel in magnetic domains β€” Ferromagnetic spins are aligned parallel in magnetic domains below Tc.

In an antiferromagnetic substance, the spins are:

  1. Aligned parallel
  2. Aligned antiparallel and cancel out
  3. Randomly oriented
  4. Antiparallel but unequal
βœ” B. Aligned antiparallel and cancel out β€” Antiferromagnetic spins are aligned antiparallel and cancel out below TN.

In a ferrimagnetic substance, spins are antiparallel but unequal, so they do not .

βœ” cancel

In a paramagnetic substance, the spins are oriented above Tc or TN.

βœ” randomly
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Topic 20

Electromagnetic Induction (EMI)

A changing magnetic field can itself create electricity β€” the principle behind generators and transformers.

⭐
Key Point
Whenever the magnetic flux linked with a circuit changes, an emf is induced in it; this is electromagnetic induction.
EMI
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Electromagnetic induction occurs whenever:

  1. The magnetic flux linked with a circuit changes
  2. A steady current flows
  3. A resistor is added
  4. A magnet is kept still near a coil
βœ” A. The magnetic flux linked with a circuit changes β€” Whenever the magnetic flux linked with a circuit changes, an emf is induced in it.

What is induced in a circuit during electromagnetic induction?

  1. A resistance
  2. An emf
  3. A capacitance
  4. A magnetic pole
βœ” B. An emf β€” A changing flux induces an emf in the circuit.

Whenever the magnetic flux linked with a circuit changes, an is induced in it.

βœ” emf

The production of an emf due to a changing magnetic flux is called electromagnetic .

βœ” induction
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Topic 21

Faraday's Laws of EMI

Faraday gave the rule for when and how long an induced emf appears.

⭐
Key Point
A changing magnetic flux through a circuit induces an emf in it.
Faraday's laws
  • The induced emf lasts only as long as the flux keeps changing.
  • The size of the induced emf is proportional to the rate of change of flux.
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According to Faraday, the induced emf lasts:

  1. Only as long as the flux keeps changing
  2. Forever
  3. Only when flux is constant
  4. Only at high temperature
βœ” A. Only as long as the flux keeps changing β€” The induced emf lasts only as long as the flux keeps changing.

The size of the induced emf is proportional to the:

  1. Total flux
  2. Rate of change of flux
  3. Resistance of the coil
  4. Number of poles
βœ” B. Rate of change of flux β€” The induced emf is proportional to the rate of change of flux.

A changing magnetic flux through a circuit induces an in it.

βœ” emf

The size of the induced emf is proportional to the rate of change of .

βœ” flux
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Topic 22

Lenz's Law

Lenz's law fixes the direction of the induced current.

⭐
Key Point
The induced emf (or current) always opposes the very change that produces it.
Lenz's law
  • It is a statement of the conservation of energy; detected with a sensitive galvanometer when a bar magnet moves near a coil.
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Lenz's law states that the induced emf always:

  1. Aids the change that produces it
  2. Opposes the change that produces it
  3. Is zero
  4. Doubles the change
βœ” B. Opposes the change that produces it β€” The induced emf always opposes the very change that produces it.

Lenz's law is a statement of the:

  1. Conservation of energy
  2. Conservation of charge
  3. Conservation of momentum
  4. Law of gravitation
βœ” A. Conservation of energy β€” Lenz's law is a statement of the conservation of energy.

The induced current always the very change that produces it.

βœ” opposes

Lenz's law is detected with a sensitive when a bar magnet moves near a coil.

βœ” galvanometer
πŸ”Œ
Topic 23

Eddy Current

Eddy currents are swirling currents set up in solid metal by a changing field.

⭐
Key Point
When a metal is placed in a varying magnetic field (or spun in a field), looping currents called eddy currents are induced in it.
Eddy current
  • They are also known as Foucault's currents.
  • Eddy currents are used in diathermy for deep-heat treatment of the body.
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Eddy currents are also known as:

  1. Foucault's currents
  2. Faraday's currents
  3. Lenz's currents
  4. Ohm's currents
βœ” A. Foucault's currents β€” Eddy currents are also known as Foucault's currents.

Eddy currents are used in which medical treatment?

  1. Radiotherapy
  2. Diathermy
  3. Dialysis
  4. Endoscopy
βœ” B. Diathermy β€” Eddy currents are used in diathermy for deep-heat treatment of the body.

Looping currents induced in a metal placed in a varying magnetic field are called currents.

βœ” eddy

Eddy currents are used in for deep-heat treatment of the body.

βœ” diathermy
πŸ”Œ
Topic 24

Uses of Eddy Current

Despite causing energy loss, eddy currents are put to good use in several machines.

⭐
Key Point
Eddy currents are used in the dead-beat galvanometer, induction furnace, induction motor and the speedometer of automobiles.
Uses of eddy current
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Eddy currents are used in the speedometer of:

  1. Automobiles
  2. Aircraft altimeters
  3. Clocks
  4. Telephones
βœ” A. Automobiles β€” Eddy currents are used in the speedometer of automobiles.

Which of these uses eddy currents?

  1. Dead-beat galvanometer
  2. Voltmeter
  3. Barometer
  4. Thermometer
βœ” A. Dead-beat galvanometer β€” The dead-beat galvanometer uses eddy currents.

Eddy currents are used in the dead-beat galvanometer, induction furnace, induction and the speedometer of automobiles.

βœ” motor

Eddy currents are used in the induction , which produces intense heat for melting metals.

βœ” furnace
πŸ”Œ
Topic 25

Self and Mutual Induction

Induction can occur within a single coil or between two neighbouring coils.

⭐
Key Point
Self induction is the production of emf in a circuit due to a change in its own current.
Self vs mutual induction
TypeCauseUnit
Self inductionchange of current in the coil's own circuitHenry (H)
Mutual inductionchange of flux from a neighbouring circuitHenry (H)
  • Mutual induction is the production of emf in a circuit due to changing flux in a neighbouring circuit.
  • The unit of both self and mutual induction is the Henry (H).
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The unit of both self and mutual induction is the:

  1. Weber
  2. Henry
  3. Tesla
  4. Farad
βœ” B. Henry β€” The unit of both self and mutual induction is the Henry (H).

Self induction is the production of emf in a circuit due to:

  1. A change in its own current
  2. A neighbouring circuit's current
  3. A steady current
  4. A magnetic pole
βœ” A. A change in its own current β€” Self induction is the production of emf due to a change in the circuit's own current.

The production of emf in a circuit due to changing flux in a neighbouring circuit is called induction.

βœ” mutual

The unit of both self and mutual induction is the .

βœ” Henry
πŸ”Œ
Topic 26

Alternating Current

Alternating current reverses direction many times each second and powers our homes.

⭐
Key Point
An electric current whose magnitude and direction change continuously is called alternating current.
Alternating current (AC)
  • The frequency of AC supply in India is 50 Hz.
  • The mean (average) value of AC over one complete cycle is zero.
  • The rms value of AC is Irms = Iβ‚€/√2.
  • AC ammeters and AC voltmeters read the rms value of current and voltage respectively.
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The frequency of AC supply in India is:

  1. 60 Hz
  2. 50 Hz
  3. 100 Hz
  4. 25 Hz
βœ” B. 50 Hz β€” The frequency of AC supply in India is 50 Hz.

The mean (average) value of AC over one complete cycle is:

  1. Zero
  2. Equal to peak value
  3. Iβ‚€/√2
  4. Infinite
βœ” A. Zero β€” The mean value of AC over one complete cycle is zero.

An electric current whose magnitude and direction change continuously is called current.

βœ” alternating

The rms value of AC is given by Irms = Iβ‚€/√ .

βœ” 2
πŸ“˜
Topic 27

AC Generator or Dynamo

A dynamo turns motion into electricity using induction.

⭐
Key Point
An AC generator (dynamo) converts mechanical energy into alternating current.
AC generator / dynamo
  • Its working is based on electromagnetic induction.
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An AC generator (dynamo) converts:

  1. Mechanical energy into alternating current
  2. Electrical energy into mechanical energy
  3. Heat into light
  4. Sound into electricity
βœ” A. Mechanical energy into alternating current β€” An AC generator converts mechanical energy into alternating current.

The working of an AC generator is based on:

  1. Electromagnetic induction
  2. Ohm's law
  3. Pascal's law
  4. Archimedes' principle
βœ” A. Electromagnetic induction β€” Its working is based on electromagnetic induction.

An AC generator is also called a .

βœ” dynamo

An AC generator converts energy into alternating current.

βœ” mechanical
πŸ“˜
Topic 28

DC Motor

A motor is the reverse machine β€” it turns electricity into motion.

⭐
Key Point
A DC motor converts electrical energy into mechanical energy.
DC motor
  • It works because a current-carrying coil in a magnetic field experiences a torque; its parts include the commutator, rotor coils, shaft, brushes and stator magnets.
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A DC motor converts:

  1. Electrical energy into mechanical energy
  2. Mechanical energy into electrical energy
  3. Heat into electricity
  4. Light into heat
βœ” A. Electrical energy into mechanical energy β€” A DC motor converts electrical energy into mechanical energy.

A DC motor works because a current-carrying coil in a magnetic field experiences a:

  1. Torque
  2. Resistance
  3. Capacitance
  4. Buoyant force
βœ” A. Torque β€” A current-carrying coil in a magnetic field experiences a torque, which drives the motor.

A DC motor converts electrical energy into energy.

βœ” mechanical

The parts of a DC motor include the , rotor coils, shaft, brushes and stator magnets.

βœ” commutator
πŸ”Œ
Topic 29

Transformer

A transformer steps voltage up or down without changing the frequency.

⭐
Key Point
A transformer changes a low-voltage current into a high-voltage current and vice-versa, at constant frequency.
Transformer
  • Its working is based on mutual induction.
  • A step-up transformer converts low voltage into high voltage.
  • It has a primary winding (Np turns) and a secondary winding (Ns turns) linked by magnetic flux Ξ¦ through the transformer core.
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The working of a transformer is based on:

  1. Mutual induction
  2. Self induction
  3. Ohm's law
  4. Lenz's law only
βœ” A. Mutual induction β€” A transformer works on the principle of mutual induction.

A step-up transformer converts:

  1. Low voltage into high voltage
  2. High voltage into low voltage
  3. AC into DC
  4. DC into AC
βœ” A. Low voltage into high voltage β€” A step-up transformer converts low voltage into high voltage.

A transformer changes voltage at constant .

βœ” frequency

A transformer has a primary winding and a winding linked by magnetic flux.

βœ” secondary
πŸ”Œ
Topic 30

Step-down Transformer

The step-down transformer does the opposite job.

⭐
Key Point
A step-down transformer converts high voltage into low voltage.
Step-down transformer
  • Here the primary (Np turns) carries the high voltage and the secondary (Ns turns) gives the low voltage.
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A step-down transformer converts:

  1. High voltage into low voltage
  2. Low voltage into high voltage
  3. AC into DC
  4. Current into resistance
βœ” A. High voltage into low voltage β€” A step-down transformer converts high voltage into low voltage.

In a step-down transformer, the primary winding carries the:

  1. High voltage
  2. Low voltage
  3. Zero voltage
  4. Reversed voltage
βœ” A. High voltage β€” In a step-down transformer the primary carries the high voltage and the secondary gives the low voltage.

A step-down transformer converts high voltage into voltage.

βœ” low

In a step-down transformer, the gives the low voltage.

βœ” secondary
πŸ”Œ
Topic 31

Uses of Transformer

Transformers appear in everything from power grids to phone chargers.

⭐
Key Point
Transformers are used in power stations, electrical appliances, mobile chargers, adapters, rectifiers, radios and TVs.
Uses
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Transformers are used in all of these EXCEPT:

  1. Power stations
  2. Mobile chargers
  3. Adapters
  4. Refrigerator compressors
βœ” D. Refrigerator compressors β€” Transformers are used in power stations, appliances, mobile chargers, adapters, rectifiers, radios and TVs.

Which everyday device uses a transformer?

  1. Mobile charger
  2. Bicycle
  3. Pressure cooker
  4. Umbrella
βœ” A. Mobile charger β€” Mobile chargers use transformers.

Transformers are used in power stations, electrical appliances, mobile , adapters, rectifiers, radios and TVs.

βœ” chargers

Transformers are used in stations to change voltage for transmission.

βœ” power
🎯
Recap

Quick Revision

⭐
Key Point
Like poles repel, unlike poles attract; a freely suspended magnet points North–South.
  • Magnetite (Fe₃Oβ‚„) is the natural magnet, also called lodestone.
  • Magnetic poles always exist in pairs β€” no isolated monopole; cutting a magnet gives smaller complete magnets.
  • Field lines run from N to S outside the magnet and form closed loops; the field is strongest at the poles.
  • Soft iron makes temporary magnets / electromagnets; steel makes permanent magnets.
  • Diamagnetic = weakly repelled (bismuth, copper, water); paramagnetic = weakly attracted (aluminium, platinum).
  • Ferromagnetic = strongly attracted (iron, cobalt, nickel) β€” magnetism lost above the Curie point.
  • Ammeter: low resistance, in series, ideal R = 0; voltmeter: high resistance, in parallel, ideal R = ∞.
  • EMI: changing flux induces emf (Faraday); induced current opposes the change (Lenz).
  • Eddy currents (Foucault's currents) are used in speedometers, induction furnaces and dead-beat galvanometers.
  • Frequency of AC in India is 50 Hz; rms value Irms = Iβ‚€/√2; the unit of inductance is the Henry.
  • Dynamo converts mechanical β†’ electrical (AC); DC motor converts electrical β†’ mechanical; transformer works on mutual induction.

Test Yourself

Take 5 questions at a time β€” tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.