What is the difference between an electric motor and an electric generator?

Electric motor: converts electrical energy into mechanical energy. Works on the principle that a current-carrying conductor in a magnetic field experiences a force (Fleming's Left Hand Rule). Electric generator: converts mechanical energy into electrical energy. Works on the principle of electromagnetic induction (Fleming's Right Hand Rule).

What is the function of fuse in a domestic circuit? Why should it be connected in series?

A fuse is a thin wire of low melting point metal that melts and breaks the circuit when excess current flows, protecting appliances from damage. It must be connected in series so that when it melts, the circuit is broken completely. It is always connected to the live wire.

What is the difference between AC and DC?

DC (Direct Current): current flows in one direction only. Example: battery, torch. AC (Alternating Current): current periodically reverses direction. Example: mains supply (50 Hz in India — reverses 50 times per second). AC is used for power transmission because its voltage can be easily stepped up/down using transformers.

CBSE Class 10 Science Chapter 12 Notes — Magnetic Effects of Electric Current

Q: What is electromagnetic induction? State Faraday's law.

Electromagnetic induction is the production of an electric current (EMF) in a conductor when it is placed in a changing magnetic field. Faraday's law states that the magnitude of the induced EMF is proportional to the rate of change of magnetic flux. The direction of induced current is given by Fleming's Right Hand Rule or Lenz's Law.

Magnetic Field and Field Lines

Magnetic Field

The region around a magnet where the force of the magnet is felt is called the magnetic field.

Magnetic field lines show the direction and strength of a magnetic field:

They emerge from the north pole and enter the south pole outside the magnet
They run from south to north inside the magnet
Field lines never intersect
Closer lines = stronger field; farther apart lines = weaker field

Magnetic Field Due to a Straight Current-Carrying Conductor

When current flows through a wire, it creates a magnetic field in concentric circles around the wire.

Right-Hand Thumb Rule: If you hold a current-carrying wire with the right hand so the thumb points in the direction of current, the fingers curl in the direction of the magnetic field lines.

Factors affecting strength of magnetic field around a wire:

Current: More current → stronger field
Distance: Farther from wire → weaker field

Magnetic Field Due to Coils

Circular Loop

A circular current-carrying loop creates a magnetic field. At the centre of the loop, field lines are straight. The two faces of the loop act like north and south poles.

Solenoid

A solenoid is a coil of many turns of insulated wire wound in the form of a helix (spring shape).

Properties of solenoid's magnetic field:

Inside: uniform, parallel field lines (like a bar magnet)
Outside: similar to bar magnet's field
One end acts as North pole, other as South pole

How to find which end is N pole: Look at the face where current appears anticlockwise → that is the North pole.

Electromagnet

An electromagnet consists of a solenoid with a soft iron core inside.

Advantages of electromagnet over permanent magnet:

Can be switched on/off
Strength can be varied (by changing current or number of turns)
Polarity can be reversed (by reversing current)

Applications: Electric cranes, loudspeakers, electric bells, MRI machines

Force on a Current-Carrying Conductor in Magnetic Field

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is the principle behind electric motors.

Factors affecting the force:

Magnitude of current (more current → more force)
Strength of magnetic field (stronger B → more force)
Length of conductor
Angle between current and field (maximum when perpendicular, zero when parallel)

Fleming's Left Hand Rule

Stretch the thumb, index finger, and middle finger of the LEFT hand mutually at right angles:

First finger (index) → direction of Field (B)
Centre finger (middle) → direction of Current (I)
ThuMB → direction of Thrust (Force on conductor)

Used for: Electric motor (finding direction of force on a motor coil)

Electric Motor

An electric motor converts electrical energy → mechanical energy.

How it Works

1. A rectangular coil ABCD is placed between poles of a magnet 2. Current flows through the coil (supplied via brushes and split-ring commutator) 3. By Fleming's Left Hand Rule, force acts on AB upward and force on CD downward 4. This creates a turning effect (torque) → coil rotates 5. After half rotation, the commutator reverses the current direction → force directions remain the same → continuous rotation

Split-ring commutator: Reverses current direction every half rotation to keep the motor spinning continuously.

Uses of electric motor: Electric fans, washing machines, refrigerators, electric vehicles, water pumps, mixers

Electromagnetic Induction

Faraday's Discovery

When a magnet is moved in/out of a coil, a current is induced in the coil — this is electromagnetic induction.

Current is induced when:

Magnet moves toward or away from a coil
Coil moves toward or away from a magnet
AC current in one coil changes (mutual induction)

Faraday's Law: The magnitude of induced EMF is proportional to the rate of change of magnetic flux.

More current is induced when:

Magnet is moved faster
Stronger magnet is used
More turns of wire in the coil

Direction of Induced Current

Fleming's Right Hand Rule: Stretch thumb, index finger, and middle finger of the RIGHT hand mutually at right angles:

Index finger → direction of magnetic Field (B)
Thumb → direction of Motion of conductor
Middle finger → direction of induced Current

Lenz's Law: The direction of induced current is such that it opposes the change that causes it.

Electric Generator

An electric generator converts mechanical energy → electrical energy.

Working

A rectangular coil rotates in a magnetic field (driven by turbine, windmill, etc.):

As coil rotates, the flux through it changes continuously
This induces an alternating EMF → AC is produced

AC Generator: Uses slip rings → produces alternating current (AC) DC Generator: Uses split-ring commutator → produces direct current (DC)

AC vs DC:

DC	AC
Current in one direction	Current alternates direction
Battery, solar cell	Power plant generators
Cannot be step up/down easily	Can be transformed easily
Used in electronics	Used for power transmission

In India: AC supply is 220 V, 50 Hz (reverses 50 times per second)

Domestic Electric Circuits

Household Wiring

Three wires in domestic supply:

Live wire (L): Red/Brown — carries high voltage (~220 V)
Neutral wire (N): Black/Blue — at 0 V (connects to earth at power station)
Earth wire (E): Green/Yellow-green — connects metal bodies of appliances to earth; safety wire

How Appliances are Connected

All appliances in a house are connected in parallel (not series) so that:

Each appliance gets the full supply voltage (220 V)
One appliance failing doesn't affect others
Different appliances can be switched on/off independently

Safety Devices

Fuse:

Thin wire of low melting point (usually Sn-Pb alloy)
Connected in series with the live wire
When excess current flows (short circuit or overload), fuse wire melts and breaks the circuit
Protects appliances and wiring from fire

MCB (Miniature Circuit Breaker):

Modern replacement for fuse — uses electromagnet/bimetallic strip
Trips (opens the circuit) when excess current flows
Can be reset (switched back on) — unlike fuse which must be replaced

Earthing:

Metal bodies of appliances are connected to earth
If live wire accidentally touches the metal body, current flows to earth (not through the person)
Prevents electric shock

Electric Power in Domestic Circuits

Electrical units are measured in kilowatt-hour (kWh) — the "unit" on your electricity bill. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J

Worked numerical: A 100 W bulb runs for 10 hours. How many units of electricity does it consume? Energy = Power × Time = 100 W × 10 h = 1000 Wh = 1 kWh = 1 unit

Board Exam Tips

6–8 marks from this chapter
Fleming's Left Hand Rule vs Right Hand Rule: Left for motor (force direction), Right for generator (induced current direction) — remember: Motor = Left, Generator = Right
Draw electric motor diagram with labels: magnet, coil ABCD, brushes, commutator, axle
Electromagnetic induction: know Faraday's law (rate of change of flux = induced EMF)
Domestic circuit diagram — label live, neutral, earth wires, fuse, earth connection
"Why are appliances connected in parallel?" — state both reasons: same voltage + independent operation
Fuse must be on the live wire — if placed on neutral wire, appliance stays at high voltage even when fuse blows

📚 Lakhmir Singh Science Class 10 — Magnetism Chapter

📝 Oswaal CBSE Question Bank — Magnetic Effects Board Questions

NCERT Exercise Solutions

Intext Questions

Page 224, Q1: Why does a compass needle get deflected when brought near a bar magnet?

AnswerA compass needle is itself a small bar magnet. A bar magnet produces a magnetic field around it. When the compass needle is brought near the bar magnet, it experiences a magnetic force — the unlike poles attract and like poles repel. This force exerts a torque on the needle, causing it to rotate and align itself along the field lines of the bar magnet. The needle deflects from its original north-pointing position toward the poles of the bar magnet.

Page 228, Q1: What is the pattern of the magnetic field around a straight current-carrying conductor?

AnswerThe magnetic field around a straight current-carrying conductor forms concentric circles centred on the wire. Key properties:

The circles are in a plane perpendicular to the wire
The spacing between circles increases with distance (field weakens farther from the wire)
Direction: determined by the Right-hand thumb rule — if the thumb points in the direction of current, the fingers curl in the direction of the magnetic field
Increasing current → stronger field (closer circles); increasing distance → weaker field (wider circles)

Chapter-End Exercise

Q1. Which of the following correctly describes the magnetic field near a long straight wire? (a) Field is in the direction of current (b) Field is perpendicular to the current (c) Field forms closed concentric circles around the wire

Answer(c) Field forms closed concentric circles around the wire — this correctly describes the magnetic field around a long straight current-carrying conductor.

Q2. The magnetic field in a given region is uniform. Draw a diagram to represent it. Write the forces experienced when a current-carrying conductor is placed in it.

AnswerA uniform magnetic field is represented by parallel, equidistant field lines.

When a current-carrying conductor is placed in this field:

The conductor experiences a magnetic force (F)
Direction of force: given by Fleming's Left-Hand Rule — Hold left hand with index finger in field direction, middle finger in current direction → thumb points in force (motion) direction

FormulaF = BIl sin θ where B = magnetic field, I = current, l = length, θ = angle between current and field

Maximum force when conductor is perpendicular to field (θ = 90°)
Zero force when conductor is parallel to field (θ = 0°)

Q3. Name some devices that use current-carrying conductors and magnetic fields.

Answer1. Electric motor — converts electrical energy to mechanical energy (uses electromagnetic force) 2. Electric generator — converts mechanical energy to electrical energy (uses electromagnetic induction) 3. Loudspeaker — voice coil in magnetic field moves cone to produce sound 4. Microphone — reverses loudspeaker principle 5. Galvanometer/Ammeter/Voltmeter — measure current/voltage using deflection of coil in magnetic field 6. Transformer — uses changing magnetic fields to change AC voltage

Q4. State Fleming's Left-Hand Rule.

AnswerFleming's Left-Hand Rule (for motors / force on current-carrying conductor):

Hold the left hand with the:

Index finger pointing in the direction of the magnetic field (B)
Middle finger pointing in the direction of the electric current (I)
Thumb pointing in the direction of the force (motion) on the conductor (F)

Note: Fleming's Right-Hand Rule is for generators (motion of conductor → direction of induced current).

Q5. What is electromagnetic induction? State Faraday's law.

AnswerElectromagnetic induction is the process of generating an electric current in a conductor by changing the magnetic flux through it. Discovered by Michael Faraday (1831).

Faraday's Law: The magnitude of the induced EMF (electromotive force) in a circuit is directly proportional to the rate of change of magnetic flux through the circuit.

This occurs when:

A magnet is moved into or out of a coil
A current-carrying coil moves in a magnetic field
The current in a nearby coil changes

Faraday's work led directly to the invention of the electric generator.

Q6. What are the three safety measures used in domestic electric circuits?

Answer1. Earthing (Grounding): The metallic body of every appliance is connected to the earth (ground) via an earth wire. If insulation fails and the body becomes live, current flows safely to earth instead of through a person → prevents electric shock.

2. Fuse: A thin wire of low melting point connected in series with the circuit. If current exceeds the safe limit (overloading/short circuit), the fuse wire melts → breaks the circuit → prevents fire and damage to appliances. Must be placed in the live (phase) wire.

3. MCB (Miniature Circuit Breaker): An automatic switch that trips (opens circuit) when current exceeds the rated value. Unlike fuses, MCBs can be reset (switched back on) after the fault is corrected. Safer and more convenient than fuses.

Frequently Asked Questions

Fleming's Left Hand Rule: Stretch the thumb, index finger, and middle finger of the left hand mutually at right angles. If the index finger points in the direction of the magnetic field (B) and the middle finger points in the direction of current (I), then the thumb points in the direction of the force (F) on the conductor. Used for electric motors.