Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
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?
Page 228, Q1: What is the pattern of the magnetic field around a straight current-carrying conductor?
- 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
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.
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
- 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.
Q4. State Fleming's Left-Hand Rule.
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.
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?
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.