Why use AC for transmission and DC for batteries?

Chapter 12 of 13

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CBSE Class 10 Science★ 10-12 marks · CBSE Class 10 Science (Chapter 12)

Magnetic Effects of Electric Current

Magnetic Effects of Electric Current — magnetic field, Oersted's discovery, magnetic field due to current-carrying conductors (straight wire, loop, solenoid), force on current-carrying conductor in magnetic field, electric motor, electromagnetic induction, electric generator, AC and DC. CBSE Class 10 Science Chapter 12.

⏱ 24 min readMedium difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Understand magnetic field of currents
2Apply Right-Hand Thumb Rule
3Apply Fleming's Left and Right Hand rules
4Understand motor and generator principles
5Distinguish AC and DC

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Why this chapter matters

Electromagnetism powers modern world. Motors, generators, MRI, transformers — all use principles from this chapter.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Electricity

"Where there is electric current, there is magnetism. Where there is changing magnetism, there is electric current. The two are eternally linked."

1. About the Chapter

This chapter completes Class 10 Physics by exploring electromagnetism:

Magnetic field of currents
Force on conductor in magnetic field (motor principle)
Electromagnetic induction (generator principle)
AC vs DC
Domestic electrical circuits

Why Important

Electric motors EVERYWHERE (fans, mixers, cars, trains)
Power generation (all electricity made by generators)
Modern technology depends on this

2. Magnetic Field

Definition

Region around a magnet where its magnetic force is experienced.

Field Lines

Imaginary lines showing direction of magnetic field.

Properties:

Always go from NORTH to SOUTH outside magnet
Closed loops (form complete circuits)
Never intersect
Closer lines = stronger field

SI Unit

TESLA (T) — named after Nikola Tesla

3. Oersted's Discovery (1820)

Experiment

Hans Christian Oersted noticed that a magnetic compass needle near a CURRENT-CARRYING WIRE got DEFLECTED.

Conclusion

Electric current produces magnetic field around it.

This was the BEGINNING of electromagnetism — one of physics' greatest discoveries.

4. Magnetic Field around Current-Carrying Conductors

Straight Wire

Right-Hand Thumb Rule:

Hold wire with right hand
THUMB points in direction of CURRENT
CURLED FINGERS show direction of MAGNETIC FIELD

Field lines are CONCENTRIC CIRCLES around wire.

Circular Loop

When current flows in a circular loop:

Magnetic field at centre is PERPENDICULAR to plane of loop
Field is stronger if more current or more turns

Solenoid

A coil of insulated wire wrapped tightly.

When current flows:

Strong magnetic field INSIDE — uniform and parallel
Like a BAR MAGNET — has North and South poles
Direction determined by RIGHT-HAND THUMB RULE

Electromagnet

A SOLENOID with a SOFT IRON CORE inside.

When current ON: behaves as STRONG MAGNET
When current OFF: magnetism DISAPPEARS

Uses:

Electric bells, doorbells
Lifting magnets in junkyards
MRI machines
Speakers

5. Force on Current-Carrying Conductor

When a current-carrying conductor is placed in a magnetic field, it EXPERIENCES a FORCE.

Magnitude

F = BIL

where:

F = force
B = magnetic field strength
I = current
L = length of conductor

Direction — Fleming's Left-Hand Rule

Stretch left hand: thumb, index, middle finger MUTUALLY PERPENDICULAR.

THUMB: direction of FORCE (motion)
INDEX (Fore): direction of MAGNETIC FIELD
MIDDLE: direction of CURRENT

(Used for ELECTRIC MOTORS.)

6. Electric Motor

Principle

Converts ELECTRICAL energy → MECHANICAL energy.

Working

A current-carrying coil placed in magnetic field rotates due to force on it.

Components

Coil (armature): wire wound on core
Magnetic field: provided by permanent magnets
Brushes: maintain electrical contact
Split-ring commutator: reverses current direction every half cycle (keeps rotation continuous)
Axle: rotates

How It Works

Current flows through coil → forces on each side (opposite directions by Fleming's left-hand rule)
Forces cause coil to ROTATE
After half turn, commutator reverses current
Forces continue to act in correct direction
Continuous rotation!

Uses

Fans, water pumps, refrigerators
Mixers, washing machines
Electric vehicles (Tata, Mahindra EVs)
Indian Railways (electric trains)

7. Electromagnetic Induction

Faraday's Discovery (1831)

Michael Faraday discovered that CHANGING magnetic field produces EMF (electromotive force) and CURRENT in a coil.

Two Cases

Case 1: Move a magnet near a coil → induced current Case 2: Change current in one coil → induces current in nearby coil

Direction of Induced Current — Fleming's Right-Hand Rule

For GENERATORS:

THUMB: direction of motion (force)
INDEX: direction of magnetic field
MIDDLE: direction of induced current

(Compare with LEFT-HAND for motors.)

8. Electric Generator

Principle

Converts MECHANICAL energy → ELECTRICAL energy (OPPOSITE of motor).

Working

A coil rotated in magnetic field induces current (Faraday's law).

Components (Similar to Motor, but...)

Coil rotated EXTERNALLY (by water, steam, etc.)
Slip rings (instead of commutator) for AC
Split-rings for DC

How Power Plants Work

Water/steam/wind turns turbine
Turbine spins generator
Generator produces electricity
Distributed via wires

Indian Power Plants

Coal: Singrauli, Korba (50% of Indian electricity)
Hydro: Bhakra Nangal, Sardar Sarovar
Solar: Bhadla Solar Park
Wind: Tamil Nadu, Gujarat coasts
Nuclear: Tarapur, Kaiga
All use generator principle.

9. AC vs DC

Direct Current (DC)

Flows in ONE DIRECTION only
Constant magnitude
Example: BATTERIES (cells)

Alternating Current (AC)

Direction REVERSES periodically
Magnitude varies sinusoidally
INDIAN supply: 220V AC, 50 Hz (50 cycles per second)
Generated by power plants

Why AC for Long Distance?

Can be EASILY STEPPED UP/DOWN with transformers
High voltage → less loss in transmission lines

Why DC for Batteries?

Constant direction needed for chemical cells
Smartphone batteries, car batteries store DC

10. Domestic Electrical Circuits

Indian Home Wiring

Three Wires:

LIVE (red/brown): 220V from main supply
NEUTRAL (black/blue): return path
EARTH (green/yellow): for safety

Why Earth Wire?

If short circuit in appliance, current flows to ground via earth wire (not through user).

Parallel Connection

All home appliances connected in PARALLEL → each gets full 220V independently.

Fuses and MCBs

Fuse: thin wire melts when current too high → breaks circuit
MCB (Miniature Circuit Breaker): switch that trips on overload — reusable

Short Circuit

When live and neutral wires touch → very high current → fire risk → fuse blows

Overloading

Too many appliances using too much current → wires heat up → fire risk

11. Worked Examples

Example 1: Direction

A current flows EASTWARD in a wire. Find direction of magnetic field BELOW wire.

Using right-hand thumb rule:

Thumb points EAST
Fingers curl: NORTH (above wire) to SOUTH (below wire)
BELOW wire: direction is NORTHWARD

Example 2: Force

A wire of length 5 cm carries current 2 A in a magnetic field of 0.5 T perpendicular to wire. Find force.

F = BIL = 0.5 × 2 × 0.05 = 0.05 N

Example 3: Motor Principle

What does an electric motor do?

Converts ELECTRICAL energy → MECHANICAL energy (rotation).
Used in fans, pumps, mixers.

Example 4: AC frequency

What is the frequency of Indian AC supply?

50 Hz (cycles per second)
220V AC

12. Common Mistakes

Confusing motor and generator
- MOTOR: electricity → motion
- GENERATOR: motion → electricity
Fleming's hand rules
- LEFT hand: for MOTORS
- RIGHT hand: for GENERATORS
- Mnemonic: 'LEFT-MOTOR'
Magnetic field around straight wire
- CIRCULAR (concentric circles), not straight.
AC vs DC
- AC: alternates (homes, factories). DC: direct (batteries).
Earth wire purpose
- SAFETY only. Doesn't carry current normally.

13. Indian Context

Indian Heritage

Jagadish Chandra Bose: early experiments with EM waves
C.V. Raman: Nobel for scattering of light

Modern Indian

Indian Railways: increasing electric trains (>95% electrified)
Tata Power, NTPC, Adani: major power producers
Bhadla Solar Park (Rajasthan): world's largest single solar park (~2,245 MW)

Indian Goals

500 GW renewable by 2030
Net-zero emissions by 2070
Electric vehicles push (Tata, Mahindra, Ola)

14. Conclusion

Electromagnetism POWERS the modern world:

Oersted, Faraday, Tesla discovered the principles
Magnetic fields from currents
Force on conductors = MOTOR
Induced current = GENERATOR
AC vs DC for different uses
Home wiring with safety

Master:

Right-hand thumb rule (for field direction)
Fleming's left-hand rule (motor)
Fleming's right-hand rule (generator)
Motor and generator principles
AC vs DC differences
Domestic safety

Practice 15+ problems. This is HIGH-MARK chapter (~10-12 marks).

Electromagnetism: the partnership of electricity and magnetism powering all of modern technology.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Oersted (1820)

Current produces magnetic field

Foundation of electromagnetism

Right-Hand Thumb Rule

Thumb = current; fingers curl = field direction

For straight wire

Force on conductor

F = BIL

B=field, I=current, L=length

Fleming's Left-Hand (motor)

Thumb=force, Index=field, Middle=current

Fleming's Right-Hand (generator)

Thumb=motion, Index=field, Middle=induced current

Faraday's Law

Changing magnetic flux induces EMF

Generator principle

Indian AC

220V AC at 50 Hz

⚠️

Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing motor and generator

✓ MOTOR: electricity → motion (Fleming's LEFT). GENERATOR: motion → electricity (Fleming's RIGHT).

WATCH OUT

✗ Field lines straight near wire

✓ Magnetic field around STRAIGHT wire is CONCENTRIC CIRCLES, not straight lines.

WATCH OUT

✗ Earth wire carries current

✓ Earth wire is for SAFETY (carries current only in case of fault). Normally NO current flows.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Oersted

Who discovered the magnetic effect of current?

▶ Show solution

✦ Answer: Hans Christian Oersted in 1820. He observed that a compass needle near a current-carrying wire got deflected.

Q2EASY· AC/DC

What is the frequency of Indian AC supply?

▶ Show solution

✦ Answer: 50 Hz (50 cycles per second). Voltage: 220V AC.

Q3MEDIUM· Motor

Explain the principle of an electric motor.

▶ Show solution

Step 1 — Underlying principle.
  An electric motor uses the FORCE ON A CURRENT-CARRYING CONDUCTOR in a magnetic field.

Step 2 — Setup.
  • Coil placed between two magnets (creating field)
  • Current flows through coil

Step 3 — Force on coil sides.
  Apply Fleming's Left-Hand Rule:
  • One side experiences force UPWARD
  • Other side experiences force DOWNWARD (since current direction opposite)
  • These forces create TORQUE → rotation

Step 4 — Continuous rotation.
  After half-rotation, current direction reverses (split-ring commutator does this automatically).
  Forces continue in same rotational direction.
  Coil keeps rotating.

Step 5 — Energy conversion.
  Electrical energy → Mechanical energy (rotation)

Step 6 — Applications.
  Fans, mixers, washing machines, electric vehicles, water pumps.

✦ Answer: Electric motor works on principle that a CURRENT-CARRYING COIL in a MAGNETIC FIELD experiences a FORCE (Fleming's left-hand rule). Forces on opposite sides of coil create torque → rotation. Split-ring commutator reverses current every half cycle to maintain continuous rotation. Converts electrical energy → mechanical energy.

Q4HARD· Generator

Explain electromagnetic induction and how it is used in an electric generator.

▶ Show solution

Step 1 — Electromagnetic Induction (Faraday's Law).
  When the MAGNETIC FLUX through a coil CHANGES, an EMF (and hence current) is INDUCED in the coil.
  Discovered by Michael Faraday in 1831.

Step 2 — Ways to change flux.
  • Move a magnet near a coil
  • Move a coil in magnetic field
  • Change current in one coil → changes magnetic field → induces current in nearby coil

Step 3 — Direction of induced current.
  Fleming's RIGHT-HAND RULE:
  • THUMB: direction of motion
  • INDEX: direction of magnetic field
  • MIDDLE: direction of induced current

Step 4 — Electric Generator (Dynamo).
  Converts MECHANICAL energy → ELECTRICAL energy.
  Opposite of motor.

Step 5 — Components of generator.
  • Coil placed between magnets
  • Coil is mechanically ROTATED (by water, steam, wind)
  • Slip rings (AC) or commutator (DC) for external circuit connection

Step 6 — How generator works.
  As coil rotates in magnetic field, flux through it CHANGES.
  By Faraday's law, EMF is induced → current flows in external circuit.
  Each half-rotation, induced current reverses direction → AC generator.

Step 7 — Power plants use generators.
  Coal/water/wind turns turbine.
  Turbine spins generator shaft.
  Generator produces electricity.
  Distributed via transmission lines.

Step 8 — Indian power plants.
  • Coal: Singrauli, Korba
  • Hydro: Bhakra Nangal, Sardar Sarovar
  • Solar: Bhadla
  • Wind: Tamil Nadu, Gujarat
  All ultimately use generators.

Step 9 — AC vs DC generators.
  AC generator: slip rings (current reverses each half cycle).
  DC generator: commutator (current maintained in one direction).
  Indian supply: 220V AC at 50 Hz from generators.

✦ Answer: Electromagnetic induction (Faraday's law): changing magnetic flux through a coil induces EMF/current. Used in GENERATOR — mechanical energy → electrical energy. Coil rotates in magnetic field; flux changes; current induced. Slip rings give AC, commutator gives DC. All Indian power plants (coal, hydro, solar+wind, nuclear) use generators. Output: 220V AC at 50 Hz for India.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Oersted (1820): current produces magnetic field
•Right-hand thumb rule: thumb=current, fingers curl=field
•Magnetic field around wire: concentric circles
•Solenoid: like bar magnet when current flows
•Electromagnet: solenoid + iron core
•Force on conductor: F = BIL
•Fleming's LEFT-hand rule: MOTOR
•Fleming's RIGHT-hand rule: GENERATOR
•Faraday's law: changing flux induces EMF
•Motor: electricity → motion (uses split-ring commutator)
•Generator: motion → electricity (uses slip rings for AC)
•Indian supply: 220V AC, 50 Hz
•Home wiring: Live (red), Neutral (black), Earth (green)
•Fuse: safety device that melts on overload
•Indian power: coal 50%, solar 15%, hydro 12%, wind, nuclear

CBSE marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 10-12 marks

Question type	Marks each	Typical count	What it tests
MCQ	1	3	Rules, definitions
Short	2-3	2	Motor/generator principles
Long	5	1	Detailed working of motor or generator

Prep strategy

Memorise Fleming's rules (LEFT motor, RIGHT generator)
Master right-hand thumb rule
Understand motor and generator
AC vs DC distinction

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Indian Railways

95%+ electric trains. Use AC traction motors at high voltage.

Electric Vehicles

Tata, Mahindra, Ola Electric all use electric motors based on Class 10 principles.

Indian power grid

1,000+ GW grid; all electricity generated by generators based on Faraday's principle.

MRI machines

Strong electromagnets create field for medical imaging. Indian hospitals widely use them.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Memorise hand rules (left=motor, right=generator)
Master motor and generator working
AC vs DC clearly
Domestic safety (earth wire)

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Detailed Maxwell's equations
Transformers (turns ratio)
Electromagnetic waves (light)
Superconductivity

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

CBSE Class 10 BoardVery High

Science OlympiadVery High

JEE/NEETVery High

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

AC can be EASILY stepped up/down using transformers. High voltage transmission reduces I²R losses over long distances. Then stepped down to 220V for homes. Batteries store energy CHEMICALLY which is naturally DC. AC would damage chemistry. So: AC for transmission, DC for storage (batteries).

Practice more, ask doubts, find a tutor

CBSE Class 10 mock tests

CBSE Class 10 PYQs

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Science flashcards

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Acids, Bases and Salts — definitions, properties, indicators, pH scale, neutralisation, common acids and bases, salts and their uses (washing soda, baking soda, bleaching powder, plaster of paris). CBSE Class 10 Science Chapter 2.

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Metals and Non-metals — physical and chemical properties, reactivity series, ionic compounds, extraction of metals from ores, corrosion and prevention. CBSE Class 10 Science Chapter 3.

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Magnetic Effects of Electric Current

By the end of this chapter you'll be able to…

Before you start — revise these

Magnetic Effects of Electric Current — Class 10 Science

1. About the Chapter

Why Important

2. Magnetic Field

Definition

Field Lines

SI Unit

3. Oersted's Discovery (1820)

Experiment

Conclusion

4. Magnetic Field around Current-Carrying Conductors

Straight Wire

Circular Loop

Solenoid

Electromagnet

5. Force on Current-Carrying Conductor

Magnitude

Direction — Fleming's Left-Hand Rule

6. Electric Motor

Principle

Working

Components

How It Works

Uses

7. Electromagnetic Induction

Faraday's Discovery (1831)

Two Cases

Direction of Induced Current — Fleming's Right-Hand Rule

8. Electric Generator

Principle

Working

Components (Similar to Motor, but...)

How Power Plants Work

Indian Power Plants

9. AC vs DC

Direct Current (DC)

Alternating Current (AC)

Why AC for Long Distance?

Why DC for Batteries?

10. Domestic Electrical Circuits

Indian Home Wiring

Why Earth Wire?

Parallel Connection

Fuses and MCBs

Short Circuit

Overloading

11. Worked Examples

Example 1: Direction

Example 2: Force

Example 3: Motor Principle

Example 4: AC frequency

12. Common Mistakes

13. Indian Context

Indian Heritage

Modern Indian

Indian Goals

14. Conclusion

Key formulas & results

Common mistakes & fixes

NCERT exercises (with solutions)

Practice problems

5-minute revision

CBSE marks blueprint

Where this shows up in the real world

Exam strategy

Going beyond the textbook

Where else this chapter is tested

AI helpers for this chapter

Generate 5 more questions

Explain at a different level

📝 My notes

Questions students ask