Chapter 13: Magnetic Effects of Electric Current
NCERT Class 10 Science Exercise Solutions
How to Use This Exercise Widget
Each question block contains a "Show Answer" button. Click it to reveal the detailed answer. Only one answer is shown at a time — opening a new answer will hide the previous one. These solutions cover magnetic fields, electromagnetism, electric motors, generators, electromagnetic induction, and domestic circuits.
Question 1
Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.
Answer: (d) The field consists of concentric circles centred on the wire.
Explanation:
When electric current flows through a long straight conductor, it produces a magnetic field with the following characteristics:
Key properties:
1. Concentric circles around the wire
2. Plane perpendicular to the wire
3. Direction given by right-hand thumb rule
4. Strength decreases with distance from wire
Right-hand thumb rule:
• Iron filings around current-carrying wire arrange in concentric circles
• Compass needle placed near wire deflects tangentially to circles
Why other options are incorrect:
• (a) Straight lines perpendicular: This describes electric field pattern
• (b) Straight lines parallel: No magnetic field parallel to current
• (c) Radial lines: This describes electric field from point charge
Mathematical fact: Magnetic field strength B ∝ I/r where I = current, r = distance from wire
Explanation:
When electric current flows through a long straight conductor, it produces a magnetic field with the following characteristics:
Key properties:
1. Concentric circles around the wire
2. Plane perpendicular to the wire
3. Direction given by right-hand thumb rule
4. Strength decreases with distance from wire
Right-hand thumb rule:
Grasp the wire with right hand
Thumb points in direction of current
Fingers curl in direction of magnetic field lines
Experimental verification:Thumb points in direction of current
Fingers curl in direction of magnetic field lines
• Iron filings around current-carrying wire arrange in concentric circles
• Compass needle placed near wire deflects tangentially to circles
Why other options are incorrect:
• (a) Straight lines perpendicular: This describes electric field pattern
• (b) Straight lines parallel: No magnetic field parallel to current
• (c) Radial lines: This describes electric field from point charge
Mathematical fact: Magnetic field strength B ∝ I/r where I = current, r = distance from wire
Question 2
The phenomenon of electromagnetic induction is:
(a) the process of charging a body.
(b) the process of generating magnetic field due to a current passing through a coil.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.
Answer: (c) producing induced current in a coil due to relative motion between a magnet and the coil.
Explanation:
Electromagnetic induction is the production of an electromotive force (emf) or induced current in a conductor when there is a change in magnetic flux through it.
Key aspects of electromagnetic induction:
1. Relative motion between magnet and coil
2. Change in magnetic field through coil
3. Induced current appears only during change
4. Direction given by Fleming's right-hand rule
Faraday's experiments demonstrated:
• Moving magnet into coil → current induced
• Moving magnet out of coil → current induced in opposite direction
• Changing current in nearby coil → current induced
• No current when magnet stationary inside coil
Why other options are incorrect:
• (a) Charging a body: Electrostatics, not electromagnetism
• (b) Generating magnetic field: Electromagnetism (Oersted's discovery)
• (d) Rotating coil in motor: Conversion of electrical to mechanical energy
Applications: Electric generators, transformers, induction cooktops, wireless charging
Explanation:
Electromagnetic induction is the production of an electromotive force (emf) or induced current in a conductor when there is a change in magnetic flux through it.
Key aspects of electromagnetic induction:
1. Relative motion between magnet and coil
2. Change in magnetic field through coil
3. Induced current appears only during change
4. Direction given by Fleming's right-hand rule
Faraday's experiments demonstrated:
• Moving magnet into coil → current induced
• Moving magnet out of coil → current induced in opposite direction
• Changing current in nearby coil → current induced
• No current when magnet stationary inside coil
Why other options are incorrect:
• (a) Charging a body: Electrostatics, not electromagnetism
• (b) Generating magnetic field: Electromagnetism (Oersted's discovery)
• (d) Rotating coil in motor: Conversion of electrical to mechanical energy
Applications: Electric generators, transformers, induction cooktops, wireless charging
Question 3
The device used for producing electric current is called a:
(a) generator.
(b) galvanometer.
(c) ammeter.
(d) motor.
Answer: (a) generator.
Explanation:
A generator converts mechanical energy into electrical energy by electromagnetic induction.
How generator works:
1. Coil rotates in magnetic field
2. Changing magnetic flux induces emf
3. Current flows through external circuit
4. Mechanical rotation → electrical energy
Comparison of devices:
Types of generators:
1. AC generator (alternator): Produces alternating current
2. DC generator (dynamo): Produces direct current
Key difference from motor:
• Generator: Mechanical input → Electrical output
• Motor: Electrical input → Mechanical output
Real-world examples: Power plants (hydro, thermal, wind), car alternators, bicycle dynamos
Explanation:
A generator converts mechanical energy into electrical energy by electromagnetic induction.
How generator works:
1. Coil rotates in magnetic field
2. Changing magnetic flux induces emf
3. Current flows through external circuit
4. Mechanical rotation → electrical energy
Comparison of devices:
| Device | Function | Energy Conversion |
|---|---|---|
| Generator | Produces electric current | Mechanical → Electrical |
| Galvanometer | Detects small currents | Electrical → Mechanical (needle deflection) |
| Ammeter | Measures current | Electrical → Mechanical (measurement) |
| Motor | Produces rotation | Electrical → Mechanical |
Types of generators:
1. AC generator (alternator): Produces alternating current
2. DC generator (dynamo): Produces direct current
Key difference from motor:
• Generator: Mechanical input → Electrical output
• Motor: Electrical input → Mechanical output
Real-world examples: Power plants (hydro, thermal, wind), car alternators, bicycle dynamos
Question 4
The essential difference between an AC generator and a DC generator is that:
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Answer: (d) AC generator has slip rings while the DC generator has a commutator.
Explanation:
The fundamental difference lies in the collecting arrangement that determines the type of current output.
Comparison table:
Why other options are incorrect:
• (a) Both can use electromagnets or permanent magnets
• (b) & (c) Voltage depends on design, not type of generator
Working principle:
Explanation:
The fundamental difference lies in the collecting arrangement that determines the type of current output.
Comparison table:
| Feature | AC Generator | DC Generator |
|---|---|---|
| Collecting arrangement | Slip rings (two continuous rings) | Commutator (split ring) |
| Current output | Alternating current (AC) | Direct current (DC) |
| Current reversal | External circuit | Internal (by commutator) |
| Applications | Power stations, homes, industries | Battery charging, some motors |
Why other options are incorrect:
• (a) Both can use electromagnets or permanent magnets
• (b) & (c) Voltage depends on design, not type of generator
Working principle:
Slip rings (AC): Two continuous rings → current reverses naturally in external circuit → AC output
Commutator (DC): Split ring reverses connections every half rotation → current in one direction → DC output
Visual difference:Commutator (DC): Split ring reverses connections every half rotation → current in one direction → DC output
AC Generator: ○○ (two separate slip rings)
DC Generator: ◑ (split ring commutator)
Practical note: Most modern power generators produce AC because it's easier to transform to different voltages.
DC Generator: ◑ (split ring commutator)
Question 5
At the time of short circuit, the current in the circuit:
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Answer: (c) increases heavily.
Explanation:
During a short circuit, the current increases dramatically because the resistance becomes nearly zero.
What is short circuit?
A direct connection between live and neutral wires (or between phases in 3-phase), bypassing the normal load resistance.
Why current increases:
According to Ohm's law: I = V/R
• Normally: R = load resistance (appliance resistance)
• Short circuit: R ≈ 0 (only wire resistance)
• Therefore: I → very large (theoretically infinite)
Example calculation:
1. Overheating due to I²R heating (Joule's law)
2. Fire hazard from hot wires
3. Equipment damage
4. Electric shock risk
Protection devices:
1. Fuses: Melt when current exceeds rating
2. Circuit breakers: Automatically switch off
3. MCBs (Miniature Circuit Breakers): Modern protection
Causes of short circuit:
• Damaged insulation
• Loose connections
• Water entering electrical parts
• Faulty appliances
Explanation:
During a short circuit, the current increases dramatically because the resistance becomes nearly zero.
What is short circuit?
A direct connection between live and neutral wires (or between phases in 3-phase), bypassing the normal load resistance.
Why current increases:
According to Ohm's law: I = V/R
• Normally: R = load resistance (appliance resistance)
• Short circuit: R ≈ 0 (only wire resistance)
• Therefore: I → very large (theoretically infinite)
Example calculation:
Normal: V = 220V, R_appliance = 44Ω → I = 220/44 = 5A
Short circuit: V = 220V, R_wire = 0.1Ω → I = 220/0.1 = 2200A
Dangers of short circuit:1. Overheating due to I²R heating (Joule's law)
2. Fire hazard from hot wires
3. Equipment damage
4. Electric shock risk
Protection devices:
1. Fuses: Melt when current exceeds rating
2. Circuit breakers: Automatically switch off
3. MCBs (Miniature Circuit Breakers): Modern protection
Causes of short circuit:
• Damaged insulation
• Loose connections
• Water entering electrical parts
• Faulty appliances
Question 6
State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.
Answer:
(a) An electric motor converts mechanical energy into electrical energy.
False - An electric motor converts electrical energy into mechanical energy.
• Motor: Electrical → Mechanical (rotation)
• Generator: Mechanical → Electrical (current)
(b) An electric generator works on the principle of electromagnetic induction.
True - Generators operate on Faraday's law of electromagnetic induction.
• Rotating coil in magnetic field induces emf
• Mechanical rotation → Electrical energy
• Discovered by Michael Faraday (1831)
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
True - Inside a solenoid (long coil), magnetic field lines are parallel and uniform.
• Field strength: B = μ₀nI (n = turns per unit length)
• Direction: Along axis, given by right-hand rule
• Similar to bar magnet's field
(d) A wire with a green insulation is usually the live wire of an electric supply.
False - Green insulation indicates the earth wire, not live wire.
Standard color coding (India):
• Red/Brown: Live wire (phase)
• Black/Blue: Neutral wire
• Green/Green-Yellow: Earth wire (safety)
Summary table:
(a) An electric motor converts mechanical energy into electrical energy.
False - An electric motor converts electrical energy into mechanical energy.
• Motor: Electrical → Mechanical (rotation)
• Generator: Mechanical → Electrical (current)
(b) An electric generator works on the principle of electromagnetic induction.
True - Generators operate on Faraday's law of electromagnetic induction.
• Rotating coil in magnetic field induces emf
• Mechanical rotation → Electrical energy
• Discovered by Michael Faraday (1831)
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
True - Inside a solenoid (long coil), magnetic field lines are parallel and uniform.
• Field strength: B = μ₀nI (n = turns per unit length)
• Direction: Along axis, given by right-hand rule
• Similar to bar magnet's field
(d) A wire with a green insulation is usually the live wire of an electric supply.
False - Green insulation indicates the earth wire, not live wire.
Standard color coding (India):
• Red/Brown: Live wire (phase)
• Black/Blue: Neutral wire
• Green/Green-Yellow: Earth wire (safety)
Summary table:
| Statement | True/False | Correction/Explanation |
|---|---|---|
| (a) Motor: mech → elec | False | Motor: elec → mech |
| (b) Generator principle | True | Electromagnetic induction |
| (c) Field in solenoid | True | Parallel uniform lines |
| (d) Green wire = live | False | Green = earth wire |
Question 7
List three sources of magnetic fields.
Answer:
Three sources of magnetic fields are:
1. Permanent Magnets
• Natural: Lodestone (magnetite - Fe₃O₄)
• Artificial: Bar magnets, horseshoe magnets, disc magnets
• Materials: Iron, cobalt, nickel, and their alloys (Alnico, ferrite)
• Properties: North and south poles, magnetic field lines from N to S
2. Current-carrying Conductors
• Straight wire: Concentric circular field lines
• Circular loop: Field similar to small bar magnet
• Solenoid: Strong uniform field inside, like bar magnet
• Principle: Oersted's discovery (1820) - electricity produces magnetism
3. Electromagnets
• Construction: Soft iron core + current-carrying coil
• Advantage: Magnetic field can be controlled (on/off, strength)
• Strength factors: Number of turns, current magnitude, core material
• Applications: Electric bells, relays, MRI machines, cranes
Additional sources:
• Earth itself: Acts as giant magnet (geodynamic effect)
• Moving charges: Any moving charged particle creates magnetic field
• Atomic magnets: Electron spin and orbital motion
Comparison table:
Three sources of magnetic fields are:
1. Permanent Magnets
• Natural: Lodestone (magnetite - Fe₃O₄)
• Artificial: Bar magnets, horseshoe magnets, disc magnets
• Materials: Iron, cobalt, nickel, and their alloys (Alnico, ferrite)
• Properties: North and south poles, magnetic field lines from N to S
2. Current-carrying Conductors
• Straight wire: Concentric circular field lines
• Circular loop: Field similar to small bar magnet
• Solenoid: Strong uniform field inside, like bar magnet
• Principle: Oersted's discovery (1820) - electricity produces magnetism
3. Electromagnets
• Construction: Soft iron core + current-carrying coil
• Advantage: Magnetic field can be controlled (on/off, strength)
• Strength factors: Number of turns, current magnitude, core material
• Applications: Electric bells, relays, MRI machines, cranes
Additional sources:
• Earth itself: Acts as giant magnet (geodynamic effect)
• Moving charges: Any moving charged particle creates magnetic field
• Atomic magnets: Electron spin and orbital motion
Comparison table:
| Source | Field Control | Strength | Example |
|---|---|---|---|
| Permanent magnet | Fixed | Constant | Refrigerator magnet |
| Current-carrying wire | By current | Weak (usually) | Power line field |
| Electromagnet | Fully controllable | Very strong possible | MRI scanner |
Question 8
How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Answer:
Part 1: How solenoid behaves like a magnet
A solenoid (long coil of wire) carrying current behaves like a bar magnet because:
1. Magnetic field pattern: Identical to bar magnet
2. Definite poles: One end behaves as north, other as south
3. Field lines: Emerge from one end, enter at other end
4. External field: Same as equivalent bar magnet
5. Internal field: Strong, uniform, parallel lines
Yes, we can determine poles using a bar magnet through attraction/repulsion.
Method:
1. Bring north pole of bar magnet near one end of energized solenoid
2. Observe:
• If attraction → that end is south pole
• If repulsion → that end is north pole
3. Repeat for other end to confirm
Explanation:
Like poles repel, unlike poles attract
• N-N: Repel
• S-S: Repel
• N-S: Attract
• S-N: Attract
Alternative method (compass):
1. Place compass near solenoid end
2. North pole of compass points to south pole of solenoid
3. Opposite end will be north pole
Practical demonstration:
Part 1: How solenoid behaves like a magnet
A solenoid (long coil of wire) carrying current behaves like a bar magnet because:
1. Magnetic field pattern: Identical to bar magnet
2. Definite poles: One end behaves as north, other as south
3. Field lines: Emerge from one end, enter at other end
4. External field: Same as equivalent bar magnet
5. Internal field: Strong, uniform, parallel lines
Right-hand grip rule for solenoid:
Grasp solenoid with right hand, fingers in direction of current
Thumb points toward north pole of solenoid
Part 2: Determining poles using bar magnetGrasp solenoid with right hand, fingers in direction of current
Thumb points toward north pole of solenoid
Yes, we can determine poles using a bar magnet through attraction/repulsion.
Method:
1. Bring north pole of bar magnet near one end of energized solenoid
2. Observe:
• If attraction → that end is south pole
• If repulsion → that end is north pole
3. Repeat for other end to confirm
Explanation:
Like poles repel, unlike poles attract
• N-N: Repel
• S-S: Repel
• N-S: Attract
• S-N: Attract
Alternative method (compass):
1. Place compass near solenoid end
2. North pole of compass points to south pole of solenoid
3. Opposite end will be north pole
Practical demonstration:
Bar magnet N-pole ──[ATTRACTS]── Solenoid end → That end is S-pole
Bar magnet N-pole ──[REPELS]── Solenoid end → That end is N-pole
Electromagnet advantage: Poles can be reversed by reversing current direction!
Bar magnet N-pole ──[REPELS]── Solenoid end → That end is N-pole
Question 9
When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Answer:
The force experienced by a current-carrying conductor in a magnetic field is largest when the conductor is perpendicular to the magnetic field (θ = 90°).
Mathematical explanation:
Force on current-carrying conductor: F = BIL sinθ
Where:
• B = magnetic field strength
• I = current in conductor
• L = length of conductor in field
• θ = angle between conductor and magnetic field
Force is maximum when conductor and field are perpendicular.
Fleming's Left-Hand Rule:
1. Aluminum rod between magnet poles
2. Current perpendicular to magnetic field → maximum deflection
3. Other angles → less deflection
Practical applications:
1. Electric motors: Coils placed perpendicular to field for maximum torque
2. Loudspeakers: Voice coil perpendicular to permanent magnet field
3. Galvanometers: Coil in radial magnetic field for maximum sensitivity
Special case: If conductor is parallel to field (θ = 0°), no force acts on it.
The force experienced by a current-carrying conductor in a magnetic field is largest when the conductor is perpendicular to the magnetic field (θ = 90°).
Mathematical explanation:
Force on current-carrying conductor: F = BIL sinθ
Where:
• B = magnetic field strength
• I = current in conductor
• L = length of conductor in field
• θ = angle between conductor and magnetic field
When θ = 90°: sin90° = 1 → F_max = BIL (maximum)
When θ = 0°: sin0° = 0 → F = 0 (minimum)
When θ = 30°: sin30° = 0.5 → F = 0.5BIL
When θ = 60°: sin60° = 0.866 → F = 0.866BIL
Graphical representation:Force is maximum when conductor and field are perpendicular.
Fleming's Left-Hand Rule:
For direction of force:
• Forefinger → Magnetic field direction
• Middle finger → Current direction
• Thumb → Force direction (motion)
All three mutually perpendicular
Experimental verification (Activity 13.7):• Forefinger → Magnetic field direction
• Middle finger → Current direction
• Thumb → Force direction (motion)
All three mutually perpendicular
1. Aluminum rod between magnet poles
2. Current perpendicular to magnetic field → maximum deflection
3. Other angles → less deflection
Practical applications:
1. Electric motors: Coils placed perpendicular to field for maximum torque
2. Loudspeakers: Voice coil perpendicular to permanent magnet field
3. Galvanometers: Coil in radial magnetic field for maximum sensitivity
Special case: If conductor is parallel to field (θ = 0°), no force acts on it.
Question 10
Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer:
The magnetic field is directed vertically downward (from ceiling to floor).
Step-by-step reasoning:
Step 1: Understand the setup
• You: Sitting, back to one wall
• Electron beam: Back wall → Front wall (toward you)
• Deflection: To your right side
• Charge: Electrons (negative charge)
Step 2: Convert electron motion to conventional current
• Electron direction: Back → Front (toward you)
• Conventional current direction: Opposite to electron flow
• Therefore: Conventional current = Front → Back (away from you)
Step 3: Apply Fleming's Left-Hand Rule
For conventional current (positive charge flow):
• Middle finger: Current direction (away from you, into back wall)
• Thumb: Force direction (to your right)
• Forefinger: Magnetic field direction (what we need to find)
Step 4: Hand positioning
1. Point middle finger away from you (current into back wall)
2. Point thumb to your right (force direction)
3. Forefinger will point downward
For negative charges (electrons), use right hand instead of left:
• Point thumb in electron direction (toward you)
• Point fingers in force direction (to your right)
• Palm faces magnetic field direction (downward)
Verification:
Electron moving toward you + field downward → Force to right (matches observation)
Important note: This is why cathode ray tubes (old TVs, monitors) have deflection coils to control electron beam.
The magnetic field is directed vertically downward (from ceiling to floor).
Step-by-step reasoning:
Step 1: Understand the setup
• You: Sitting, back to one wall
• Electron beam: Back wall → Front wall (toward you)
• Deflection: To your right side
• Charge: Electrons (negative charge)
Step 2: Convert electron motion to conventional current
• Electron direction: Back → Front (toward you)
• Conventional current direction: Opposite to electron flow
• Therefore: Conventional current = Front → Back (away from you)
Step 3: Apply Fleming's Left-Hand Rule
For conventional current (positive charge flow):
• Middle finger: Current direction (away from you, into back wall)
• Thumb: Force direction (to your right)
• Forefinger: Magnetic field direction (what we need to find)
Step 4: Hand positioning
1. Point middle finger away from you (current into back wall)
2. Point thumb to your right (force direction)
3. Forefinger will point downward
Fleming's Left-Hand Rule applied:
Left hand: Middle finger (current) → away from you
Left hand: Thumb (force) → to your right
Left hand: Forefinger (field) → vertically downward ✓
Alternative method (right-hand rule for electrons):Left hand: Middle finger (current) → away from you
Left hand: Thumb (force) → to your right
Left hand: Forefinger (field) → vertically downward ✓
For negative charges (electrons), use right hand instead of left:
• Point thumb in electron direction (toward you)
• Point fingers in force direction (to your right)
• Palm faces magnetic field direction (downward)
Verification:
Electron moving toward you + field downward → Force to right (matches observation)
Important note: This is why cathode ray tubes (old TVs, monitors) have deflection coils to control electron beam.
Question 11
Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer:
Part 1: Labelled diagram of electric motor
Principle: When a current-carrying conductor is placed in a magnetic field, it experiences a force (Fleming's Left-Hand Rule).
Working:
1. Current enters coil through brush X → flows ABCD → exits brush Y
2. Arm AB (current A→B) in magnetic field experiences upward force
3. Arm CD (current C→D) experiences downward force
4. These forces create torque, rotating the coil clockwise
5. After half rotation, split ring commutator reverses current direction
6. Force directions reverse, maintaining continuous rotation
Part 3: Function of split ring commutator
The split ring commutator has two crucial functions:
1. Reverses current direction every half rotation:
• Without reversal, coil would oscillate about mean position
• With reversal, torque always acts in same rotational direction
• Ensures continuous rotation in one direction
2. Maintains electrical contact during rotation:
• Brushes remain stationary
• Split rings rotate with coil
• Continuous electrical connection maintained
How it works:
Practical motors: Use electromagnets, many coil turns, soft iron core for more power.
Part 1: Labelled diagram of electric motor
[Students should draw diagram with these labels:]
1. Armature (rectangular coil ABCD)
2. Field magnets (N and S poles)
3. Split ring commutator (two halves P and Q)
4. Brushes (X and Y)
5. Battery
6. Axle
7. Direction of current
Part 2: Principle of electric motor1. Armature (rectangular coil ABCD)
2. Field magnets (N and S poles)
3. Split ring commutator (two halves P and Q)
4. Brushes (X and Y)
5. Battery
6. Axle
7. Direction of current
Principle: When a current-carrying conductor is placed in a magnetic field, it experiences a force (Fleming's Left-Hand Rule).
Working:
1. Current enters coil through brush X → flows ABCD → exits brush Y
2. Arm AB (current A→B) in magnetic field experiences upward force
3. Arm CD (current C→D) experiences downward force
4. These forces create torque, rotating the coil clockwise
5. After half rotation, split ring commutator reverses current direction
6. Force directions reverse, maintaining continuous rotation
Part 3: Function of split ring commutator
The split ring commutator has two crucial functions:
1. Reverses current direction every half rotation:
• Without reversal, coil would oscillate about mean position
• With reversal, torque always acts in same rotational direction
• Ensures continuous rotation in one direction
2. Maintains electrical contact during rotation:
• Brushes remain stationary
• Split rings rotate with coil
• Continuous electrical connection maintained
How it works:
First half rotation:
Brush X → Split ring P → Coil → Split ring Q → Brush Y
After half rotation:
Brush X now contacts split ring Q
Brush Y now contacts split ring P
Current direction in coil reverses automatically
Comparison with slip rings (AC generator):Brush X → Split ring P → Coil → Split ring Q → Brush Y
After half rotation:
Brush X now contacts split ring Q
Brush Y now contacts split ring P
Current direction in coil reverses automatically
| Feature | Split Ring (Motor) | Slip Rings (Generator) |
|---|---|---|
| Number of segments | Two (split) | Two (continuous) |
| Function | Reverses current | Collects current without reversal |
| Output/Input | DC input | AC output |
Question 12
Name some devices in which electric motors are used.
Answer:
Electric motors are used in numerous devices across various applications:
1. Household Appliances:
• Electric fans (ceiling, table, exhaust)
• Washing machines (spinning drum)
• Mixers/Blenders (rotating blades)
• Food processors
• Vacuum cleaners
• Hair dryers (fan motor)
• Refrigerators (compressor motor)
• Air conditioners (compressor, fan)
• Electric chimneys
2. Kitchen Appliances:
• Electric mixers
• Juicers
• Food processors
• Electric grinders
• Dishwashers
• Microwave ovens (turntable motor)
3. Personal Care:
• Electric shavers
• Hair clippers
• Electric toothbrushes
• Hair dryers
4. Entertainment & Electronics:
• CD/DVD players
• Hard disk drives
• Computer fans
• Record players
• Cassette players
• Video game consoles
5. Tools & Machinery:
• Electric drills
• Circular saws
• Angle grinders
• Lathes
• Conveyor belts
• Industrial fans/blowers
6. Transportation:
• Electric cars (traction motors)
• Electric bikes/scooters
• Trains (electric locomotives)
• Elevators/lifts
• Escalators
7. Office Equipment:
• Printers (paper feed, print head movement)
• Photocopiers
• Scanner motors
8. Toys:
• Remote control cars
• Electric trains
• Robotic toys
Types of motors used:
• DC motors: Battery-operated devices, toys
• AC motors: Household appliances, industrial machines
• Stepper motors: Precision control (printers, robotics)
• Servo motors: Remote control, robotics
Fun fact: An average home has 20-30 electric motors!
Electric motors are used in numerous devices across various applications:
1. Household Appliances:
• Electric fans (ceiling, table, exhaust)
• Washing machines (spinning drum)
• Mixers/Blenders (rotating blades)
• Food processors
• Vacuum cleaners
• Hair dryers (fan motor)
• Refrigerators (compressor motor)
• Air conditioners (compressor, fan)
• Electric chimneys
2. Kitchen Appliances:
• Electric mixers
• Juicers
• Food processors
• Electric grinders
• Dishwashers
• Microwave ovens (turntable motor)
3. Personal Care:
• Electric shavers
• Hair clippers
• Electric toothbrushes
• Hair dryers
4. Entertainment & Electronics:
• CD/DVD players
• Hard disk drives
• Computer fans
• Record players
• Cassette players
• Video game consoles
5. Tools & Machinery:
• Electric drills
• Circular saws
• Angle grinders
• Lathes
• Conveyor belts
• Industrial fans/blowers
6. Transportation:
• Electric cars (traction motors)
• Electric bikes/scooters
• Trains (electric locomotives)
• Elevators/lifts
• Escalators
7. Office Equipment:
• Printers (paper feed, print head movement)
• Photocopiers
• Scanner motors
8. Toys:
• Remote control cars
• Electric trains
• Robotic toys
Types of motors used:
• DC motors: Battery-operated devices, toys
• AC motors: Household appliances, industrial machines
• Stepper motors: Precision control (printers, robotics)
• Servo motors: Remote control, robotics
Fun fact: An average home has 20-30 electric motors!
Question 13
A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?
Answer:
Setup: Coil + Galvanometer circuit, bar magnet
Principle: Electromagnetic induction (Faraday's law)
(i) Bar magnet pushed into the coil:
• Galvanometer shows deflection in one direction
• Reason: Magnetic flux through coil increases
• Induced current flows to oppose this increase (Lenz's law)
• Current direction: Such that coil end near magnet becomes same pole as magnet approaching end (repulsion)
(ii) Bar magnet withdrawn from inside the coil:
• Galvanometer shows deflection in opposite direction
• Reason: Magnetic flux through coil decreases
• Induced current flows to oppose this decrease
• Current direction: Such that coil end near magnet becomes opposite pole to magnet receding end (attraction)
(iii) Bar magnet held stationary inside the coil:
• No deflection in galvanometer
• Reason: No change in magnetic flux
• No induced current (steady magnetic field doesn't induce emf)
• Galvanometer needle returns to zero position
Summary table:
Key points from Faraday's experiments:
1. Relative motion is necessary for induction
2. Faster motion → larger deflection (greater rate of flux change)
3. Stronger magnet → larger deflection
4. More turns in coil → larger deflection
Lenz's Law explanation:
Setup: Coil + Galvanometer circuit, bar magnet
Principle: Electromagnetic induction (Faraday's law)
(i) Bar magnet pushed into the coil:
• Galvanometer shows deflection in one direction
• Reason: Magnetic flux through coil increases
• Induced current flows to oppose this increase (Lenz's law)
• Current direction: Such that coil end near magnet becomes same pole as magnet approaching end (repulsion)
(ii) Bar magnet withdrawn from inside the coil:
• Galvanometer shows deflection in opposite direction
• Reason: Magnetic flux through coil decreases
• Induced current flows to oppose this decrease
• Current direction: Such that coil end near magnet becomes opposite pole to magnet receding end (attraction)
(iii) Bar magnet held stationary inside the coil:
• No deflection in galvanometer
• Reason: No change in magnetic flux
• No induced current (steady magnetic field doesn't induce emf)
• Galvanometer needle returns to zero position
Summary table:
| Magnet Action | Flux Change | Galvanometer | Induced Current |
|---|---|---|---|
| Pushed into coil | Increases | Deflects one way | Yes, momentary |
| Withdrawn from coil | Decreases | Deflects opposite way | Yes, momentary |
| Stationary in coil | No change | No deflection | No |
Key points from Faraday's experiments:
1. Relative motion is necessary for induction
2. Faster motion → larger deflection (greater rate of flux change)
3. Stronger magnet → larger deflection
4. More turns in coil → larger deflection
Lenz's Law explanation:
Induced current always flows in such a direction as to oppose the change causing it.
• Magnet pushed in: Coil opposes entry (repulsion)
• Magnet pulled out: Coil opposes exit (attraction)
Practical application: This is how generators work - coil moves relative to magnet to produce electricity.
• Magnet pushed in: Coil opposes entry (repulsion)
• Magnet pulled out: Coil opposes exit (attraction)
Question 14
Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Answer:
Yes, current will be induced in coil B when current in coil A is changed.
Reason: This is based on the principle of electromagnetic induction and mutual induction.
Step-by-step explanation:
1. Coil A (primary) carries changing current:
• When current in coil A changes (starts, stops, increases, decreases)
• It produces a changing magnetic field around it
2. Coil B (secondary) experiences changing flux:
• Coil B is placed close to coil A
• Changing magnetic field from coil A passes through coil B
• Magnetic flux through coil B changes with time
3. Induced emf in coil B:
• According to Faraday's law, changing flux induces emf
• Induced emf: ε = -dΦ/dt (rate of change of magnetic flux)
• If coil B circuit is closed, induced current flows
4. What happens when:
• Current in A starts: Flux through B increases → current induced in B
• Current in A stops: Flux through B decreases → current induced in opposite direction
• Current in A steady: No flux change → no induced current
• Current in A changes magnitude: Induced current proportional to rate of change
Experimental verification (Activity 13.9):
1. Rate of current change in coil A (faster change → larger induced current)
2. Number of turns in both coils (more turns → stronger induction)
3. Distance between coils (closer → stronger magnetic coupling)
4. Orientation of coils (aligned axes → maximum flux linkage)
5. Core material between coils (iron core greatly increases induction)
Applications of this principle:
• Transformers (change AC voltage)
• Induction coils (car ignition systems)
• Wireless charging
• Metal detectors
• Electric guitar pickups
Key formula: Mutual inductance M = ε_B/(dI_A/dt)
Yes, current will be induced in coil B when current in coil A is changed.
Reason: This is based on the principle of electromagnetic induction and mutual induction.
Step-by-step explanation:
1. Coil A (primary) carries changing current:
• When current in coil A changes (starts, stops, increases, decreases)
• It produces a changing magnetic field around it
2. Coil B (secondary) experiences changing flux:
• Coil B is placed close to coil A
• Changing magnetic field from coil A passes through coil B
• Magnetic flux through coil B changes with time
3. Induced emf in coil B:
• According to Faraday's law, changing flux induces emf
• Induced emf: ε = -dΦ/dt (rate of change of magnetic flux)
• If coil B circuit is closed, induced current flows
4. What happens when:
• Current in A starts: Flux through B increases → current induced in B
• Current in A stops: Flux through B decreases → current induced in opposite direction
• Current in A steady: No flux change → no induced current
• Current in A changes magnitude: Induced current proportional to rate of change
Experimental verification (Activity 13.9):
Coil A (many turns) ── Battery ── Key
Coil B (many turns) ── Galvanometer
When key pressed/released → Galvanometer deflects
Factors affecting induced current:Coil B (many turns) ── Galvanometer
When key pressed/released → Galvanometer deflects
1. Rate of current change in coil A (faster change → larger induced current)
2. Number of turns in both coils (more turns → stronger induction)
3. Distance between coils (closer → stronger magnetic coupling)
4. Orientation of coils (aligned axes → maximum flux linkage)
5. Core material between coils (iron core greatly increases induction)
Applications of this principle:
• Transformers (change AC voltage)
• Induction coils (car ignition systems)
• Wireless charging
• Metal detectors
• Electric guitar pickups
Key formula: Mutual inductance M = ε_B/(dI_A/dt)
Question 15
State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.
Answer:
(i) Magnetic field around straight current-carrying conductor:
(ii) Force on current-carrying conductor in magnetic field:
(iii) Induced current due to coil rotation in magnetic field:
Memory aid:
1. For electrons (negative charge), reverse direction or use opposite hand
2. These rules give direction, not magnitude
3. All assume conventional current (positive charge flow)
(i) Magnetic field around straight current-carrying conductor:
Right-Hand Thumb Rule (Maxwell's Corkscrew Rule):
• Hold the straight conductor in right hand
• Thumb points in direction of conventional current (+, to -)
• Curled fingers show direction of magnetic field lines
• Field lines are concentric circles around wire
Alternative: Corkscrew rule - If corkscrew advances in current direction, rotation gives field direction.• Hold the straight conductor in right hand
• Thumb points in direction of conventional current (+, to -)
• Curled fingers show direction of magnetic field lines
• Field lines are concentric circles around wire
(ii) Force on current-carrying conductor in magnetic field:
Fleming's Left-Hand Rule:
• Stretch left hand's thumb, forefinger, middle finger mutually perpendicular
• Forefinger → Direction of magnetic field (N to S)
• Middle finger → Direction of conventional current
• Thumb → Direction of force (motion of conductor)
All three directions must be perpendicular to each other
Mathematical: F = I(L × B) (cross product)• Stretch left hand's thumb, forefinger, middle finger mutually perpendicular
• Forefinger → Direction of magnetic field (N to S)
• Middle finger → Direction of conventional current
• Thumb → Direction of force (motion of conductor)
All three directions must be perpendicular to each other
(iii) Induced current due to coil rotation in magnetic field:
Fleming's Right-Hand Rule:
• Stretch right hand's thumb, forefinger, middle finger mutually perpendicular
• Forefinger → Direction of magnetic field
• Thumb → Direction of motion of conductor
• Middle finger → Direction of induced current
Used for generators/dynamos
Comparison table:• Stretch right hand's thumb, forefinger, middle finger mutually perpendicular
• Forefinger → Direction of magnetic field
• Thumb → Direction of motion of conductor
• Middle finger → Direction of induced current
Used for generators/dynamos
| Rule | Hand Used | Application | Finger Assignment |
|---|---|---|---|
| Right-Hand Thumb | Right | Field around wire | Thumb: Current, Fingers: Field |
| Fleming's Left-Hand | Left | Motor principle | Forefinger: Field, Middle: Current, Thumb: Force |
| Fleming's Right-Hand | Right | Generator principle | Forefinger: Field, Thumb: Motion, Middle: Current |
Memory aid:
• Left hand = Motor (L and M both have straight lines)
• Right hand = Generator (alternating, dynamo)
• Right-hand grip = Field around wire (grip the wire)
Important notes:• Right hand = Generator (alternating, dynamo)
• Right-hand grip = Field around wire (grip the wire)
1. For electrons (negative charge), reverse direction or use opposite hand
2. These rules give direction, not magnitude
3. All assume conventional current (positive charge flow)
Question 16
Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Answer:
Part 1: Principle of electric generator
Principle: Electromagnetic induction (Faraday's law) - When a conductor moves in a magnetic field or magnetic field changes around a conductor, an electromotive force (emf) is induced.
Specifically: Mechanical energy (rotation) → Electrical energy (current)
Part 2: Labelled diagram
Step 1: Initial position
• Coil ABCD rotates in magnetic field
• Arm AB moves up, CD moves down (say clockwise)
• Using Fleming's right-hand rule: Current induced from A→B→C→D
• Brush B₂ positive, B₁ negative
Step 2: After half rotation
• Arm AB now moves down, CD moves up
• Induced current direction reverses: D→C→B→A
• Brush B₁ now positive, B₂ negative
• Current in external circuit reverses
Step 3: Continuous rotation
• Current direction reverses every half rotation
• Produces Alternating Current (AC)
• Frequency = revolutions per second × 2
Part 4: Function of brushes
Brushes have two main functions:
1. Collect current from rotating parts:
• Slip rings/split rings rotate with coil
• Brushes remain stationary
• Maintain continuous electrical contact
• Transfer current from rotating coil to stationary external circuit
2. Provide low-resistance connection:
• Made of carbon/graphite (good conductor, self-lubricating)
• Press against rings with spring pressure
• Minimize energy loss
• Reduce sparking
Brush construction details:
• Material: Carbon/graphite (sometimes with copper)
• Shape: Rectangular blocks
• Mounting: Spring-loaded to maintain contact
• Function: Allow rotation while maintaining electrical connection
Comparison AC vs DC generator:
Real generators: Have many coils, electromagnets, cooling systems for high power output.
Part 1: Principle of electric generator
Principle: Electromagnetic induction (Faraday's law) - When a conductor moves in a magnetic field or magnetic field changes around a conductor, an electromotive force (emf) is induced.
Specifically: Mechanical energy (rotation) → Electrical energy (current)
Part 2: Labelled diagram
[Students should draw diagram with these labels:]
1. Armature coil (ABCD)
2. Field magnets (N and S poles)
3. Slip rings (R₁ and R₂) [for AC] or Split rings [for DC]
4. Brushes (B₁ and B₂)
5. Galvanometer/External load
6. Axle
7. Direction of rotation
Part 3: Working of AC generator1. Armature coil (ABCD)
2. Field magnets (N and S poles)
3. Slip rings (R₁ and R₂) [for AC] or Split rings [for DC]
4. Brushes (B₁ and B₂)
5. Galvanometer/External load
6. Axle
7. Direction of rotation
Step 1: Initial position
• Coil ABCD rotates in magnetic field
• Arm AB moves up, CD moves down (say clockwise)
• Using Fleming's right-hand rule: Current induced from A→B→C→D
• Brush B₂ positive, B₁ negative
Step 2: After half rotation
• Arm AB now moves down, CD moves up
• Induced current direction reverses: D→C→B→A
• Brush B₁ now positive, B₂ negative
• Current in external circuit reverses
Step 3: Continuous rotation
• Current direction reverses every half rotation
• Produces Alternating Current (AC)
• Frequency = revolutions per second × 2
Part 4: Function of brushes
Brushes have two main functions:
1. Collect current from rotating parts:
• Slip rings/split rings rotate with coil
• Brushes remain stationary
• Maintain continuous electrical contact
• Transfer current from rotating coil to stationary external circuit
2. Provide low-resistance connection:
• Made of carbon/graphite (good conductor, self-lubricating)
• Press against rings with spring pressure
• Minimize energy loss
• Reduce sparking
Brush construction details:
• Material: Carbon/graphite (sometimes with copper)
• Shape: Rectangular blocks
• Mounting: Spring-loaded to maintain contact
• Function: Allow rotation while maintaining electrical connection
Comparison AC vs DC generator:
| Aspect | AC Generator | DC Generator |
|---|---|---|
| Rings | Slip rings (two separate) | Split ring (one, divided) |
| Current | Alternating | Direct (pulsating) |
| Brushes | Two, stationary | Two, stationary |
| Applications | Power stations, homes | Battery charging, some motors |
Question 17
When does an electric short circuit occur?
Answer:
An electric short circuit occurs when there is an abnormally low resistance connection between two points in an electrical circuit that are supposed to be at different voltages.
Common causes of short circuits:
1. Insulation failure:
• Damaged wire insulation
• Aging insulation (becomes brittle)
• Overheating melting insulation
• Rodent damage to cables
2. Direct contact between conductors:
• Live wire touches neutral wire
• Live wire touches earth wire
• Loose connections causing contact
• Water/moisture bridging gaps
3. Equipment/appliance faults:
• Internal wiring faults
• Component failures
• Overloaded circuits
• Manufacturing defects
4. Accidental connections:
• Metallic objects falling across terminals
• Tools creating accidental bridges
• Construction accidents
What happens during short circuit:
1. Resistance drops nearly to zero
2. Current increases dramatically (I = V/R)
3. Excessive heating occurs (Joule heating: H = I²Rt)
4. Possible outcomes:
• Fuse melts/circuit breaker trips
• Wires overheat → fire hazard
• Equipment damage
• Electric shocks
Example calculation:
1. Fuses: Wire melts at rated current
2. Circuit breakers: Automatically switch off
3. MCBs (Miniature Circuit Breakers): Modern homes
4. ELCBs/RCCBs: Earth leakage protection
5. Proper insulation: Prevent accidental contact
Types of short circuits:
• Line-to-neutral: Most common in homes
• Line-to-earth: Live wire touches ground
• Line-to-line: In three-phase systems
• Bolted fault: Solid metal connection
• Arcing fault: Through air (sparking)
Safety first: If you suspect short circuit, turn off power and call electrician.
An electric short circuit occurs when there is an abnormally low resistance connection between two points in an electrical circuit that are supposed to be at different voltages.
Common causes of short circuits:
1. Insulation failure:
• Damaged wire insulation
• Aging insulation (becomes brittle)
• Overheating melting insulation
• Rodent damage to cables
2. Direct contact between conductors:
• Live wire touches neutral wire
• Live wire touches earth wire
• Loose connections causing contact
• Water/moisture bridging gaps
3. Equipment/appliance faults:
• Internal wiring faults
• Component failures
• Overloaded circuits
• Manufacturing defects
4. Accidental connections:
• Metallic objects falling across terminals
• Tools creating accidental bridges
• Construction accidents
What happens during short circuit:
1. Resistance drops nearly to zero
2. Current increases dramatically (I = V/R)
3. Excessive heating occurs (Joule heating: H = I²Rt)
4. Possible outcomes:
• Fuse melts/circuit breaker trips
• Wires overheat → fire hazard
• Equipment damage
• Electric shocks
Example calculation:
Normal: 220V, 44Ω appliance → I = 5A
Short: 220V, 0.1Ω fault → I = 2200A (dangerously high!)
Protection against short circuits:1. Fuses: Wire melts at rated current
2. Circuit breakers: Automatically switch off
3. MCBs (Miniature Circuit Breakers): Modern homes
4. ELCBs/RCCBs: Earth leakage protection
5. Proper insulation: Prevent accidental contact
Types of short circuits:
• Line-to-neutral: Most common in homes
• Line-to-earth: Live wire touches ground
• Line-to-line: In three-phase systems
• Bolted fault: Solid metal connection
• Arcing fault: Through air (sparking)
Safety first: If you suspect short circuit, turn off power and call electrician.
Question 18
What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer:
Part 1: Function of earth wire
The earth wire (ground wire) provides a low-resistance path for electric current to flow to the ground in case of fault, preventing electric shock and fire.
Primary functions:
1. Safety during insulation failure: Diverts leakage current to ground
2. Prevents electric shock: Keeps appliance body at earth potential
3. Protects against lightning: Directs lightning strikes to ground
4. Stabilizes voltage: Provides reference point (zero potential)
Part 2: Why earth metallic appliances
Metallic appliances must be earthed for these crucial safety reasons:
1. Prevent electric shock:
• If live wire touches metal body (insulation failure)
• Without earthing: Body becomes live → shock when touched
• With earthing: Current flows to ground through earth wire
• Circuit breaker/fuse trips, cutting power
2. Low-resistance path:
• Earth wire has very low resistance
• Most current chooses this easy path
• Very little current through human body (if touched)
• Prevents fatal shocks
3. Rapid fault detection:
• Large current flows through earth wire during fault
• Triggers protection devices instantly
• Disconnects power within milliseconds
How it works (fault scenario):
• Red/Brown: Live wire (phase)
• Black/Blue: Neutral wire
• Green/Green-Yellow: Earth wire
Appliances that MUST be earthed:
• Refrigerators, washing machines
• Air conditioners, water heaters
• Electric irons, toasters, ovens
• Computers, microwaves
• Any metal-bodied electrical device
Earthing system components:
1. Earth wire: Green insulated wire
2. Earth electrode: Metal rod/plate buried in ground
3. Earth pit: Chamber with electrode + charcoal/salt
4. Main earth terminal: Connection point in building
Important: Never remove earth pin from plugs or disable earthing - it's a life-saving safety feature!
Part 1: Function of earth wire
The earth wire (ground wire) provides a low-resistance path for electric current to flow to the ground in case of fault, preventing electric shock and fire.
Primary functions:
1. Safety during insulation failure: Diverts leakage current to ground
2. Prevents electric shock: Keeps appliance body at earth potential
3. Protects against lightning: Directs lightning strikes to ground
4. Stabilizes voltage: Provides reference point (zero potential)
Part 2: Why earth metallic appliances
Metallic appliances must be earthed for these crucial safety reasons:
1. Prevent electric shock:
• If live wire touches metal body (insulation failure)
• Without earthing: Body becomes live → shock when touched
• With earthing: Current flows to ground through earth wire
• Circuit breaker/fuse trips, cutting power
2. Low-resistance path:
• Earth wire has very low resistance
• Most current chooses this easy path
• Very little current through human body (if touched)
• Prevents fatal shocks
3. Rapid fault detection:
• Large current flows through earth wire during fault
• Triggers protection devices instantly
• Disconnects power within milliseconds
How it works (fault scenario):
1. Live wire insulation fails, touches metal body
2. Metal body becomes electrically live
3. Current flows through earth wire to ground
4. Large current triggers circuit breaker/fuse
5. Power cuts off in 0.1-0.3 seconds
6. Appliance safe to touch (no power)
Color coding (India):2. Metal body becomes electrically live
3. Current flows through earth wire to ground
4. Large current triggers circuit breaker/fuse
5. Power cuts off in 0.1-0.3 seconds
6. Appliance safe to touch (no power)
• Red/Brown: Live wire (phase)
• Black/Blue: Neutral wire
• Green/Green-Yellow: Earth wire
Appliances that MUST be earthed:
• Refrigerators, washing machines
• Air conditioners, water heaters
• Electric irons, toasters, ovens
• Computers, microwaves
• Any metal-bodied electrical device
Earthing system components:
1. Earth wire: Green insulated wire
2. Earth electrode: Metal rod/plate buried in ground
3. Earth pit: Chamber with electrode + charcoal/salt
4. Main earth terminal: Connection point in building
Important: Never remove earth pin from plugs or disable earthing - it's a life-saving safety feature!