Magnetic Effects of Electric Current is Chapter 12 of CBSE Class 10 Science. This chapter explores the connection between electricity and magnetism — how electric current produces a magnetic field, how this principle is used in electromagnets, electric motors, and generators, and how electricity is distributed to our homes safely.
This chapter carries 5–8 marks in board exams. Fleming’s rules, the working of electric motors and generators, and domestic electric circuits are the most frequently tested topics.
Key Concepts
1. Magnetic Field and Field Lines
A magnetic field is the region around a magnet where its influence (force) can be felt. It is represented by magnetic field lines.
Properties of Magnetic Field Lines
- They emerge from the North pole and enter the South pole (outside the magnet)
- Inside the magnet, they go from South to North (forming closed loops)
- They never cross each other
- Closer lines = stronger field; farther apart = weaker field
- They are closed continuous curves
2. Magnetic Field Due to Current-Carrying Conductor
Hans Christian Oersted (1820) discovered that a current-carrying conductor produces a magnetic field around it. This is the link between electricity and magnetism.
a) Magnetic Field Around a Straight Conductor
The magnetic field lines around a straight current-carrying wire are concentric circles centered on the wire.
Right-Hand Thumb Rule: Grip the wire with your right hand so that the thumb points in the direction of current flow. Your curled fingers show the direction of the magnetic field.
- The field is stronger near the wire and weaker farther away
- Increasing the current makes the field stronger
b) Magnetic Field Due to a Circular Loop (Coil)
When current flows through a circular loop, the field lines inside the loop are nearly straight and parallel — creating a pattern similar to a bar magnet.
- One face of the loop acts as North pole, the other as South pole
- More turns in the coil → stronger magnetic field
- More current → stronger magnetic field
c) Magnetic Field Due to a Solenoid
A solenoid is a coil of many turns of insulated wire wrapped in a cylindrical shape. When current flows through it, it produces a uniform magnetic field inside — just like a bar magnet.
- One end becomes North pole, the other becomes South pole
- The field inside the solenoid is strong and uniform (parallel lines)
- The field outside is weak and non-uniform
Electromagnet: A solenoid with a soft iron core inside it. The iron core gets magnetised and greatly increases the magnetic field strength. Electromagnets can be turned on/off by switching the current.
Uses of electromagnets: Cranes for lifting heavy iron/steel, electric bells, MRI machines, speakers, magnetic locks.
3. Force on a Current-Carrying Conductor in a Magnetic Field
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This force is maximum when the conductor is perpendicular to the magnetic field and zero when it is parallel.
Fleming’s Left-Hand Rule (Motor Rule)
Stretch the thumb, forefinger, and middle finger of your left hand at right angles to each other:
- Forefinger: Direction of the magnetic field (B) — North to South
- Middle finger: Direction of the current (I)
- Thumb: Direction of the force/motion on the conductor
Memory aid: FBI — Forefinger = B (field), Middle finger = I (current), Thumb = Force (motion)
4. Electric Motor
An electric motor converts electrical energy into mechanical energy (rotational motion). It works on the principle that a current-carrying conductor in a magnetic field experiences a force (Fleming’s Left-Hand Rule).
Construction
- Armature (coil ABCD): Rectangular coil of insulated copper wire mounted on a shaft that can rotate
- Permanent magnets (N and S): Provide the magnetic field
- Split ring commutator: Two half-rings connected to the ends of the coil; reverses the direction of current every half rotation
- Brushes: Carbon or graphite contacts that connect the rotating commutator to the external battery
Working
- Current flows through the coil → the two arms of the coil experience forces in opposite directions (Fleming’s LHR)
- This creates a torque that rotates the coil
- After half a rotation, the commutator reverses the current direction in the coil
- This ensures the force continues to push the coil in the same direction of rotation
- The coil keeps spinning continuously
Role of commutator: Without it, the coil would rotate half a turn and then stop (forces would balance). The commutator reverses the current every half turn to keep the rotation going.
Uses: Electric fans, washing machines, mixers, drills, electric vehicles.
5. Electromagnetic Induction
Michael Faraday (1831) discovered that a changing magnetic field around a conductor induces an electric current in it. This is called electromagnetic induction.
Ways to induce current:
- Move a magnet towards or away from a coil
- Move a coil towards or away from a magnet
- Change the current in a nearby coil (changing the magnetic field)
Key points:
- The induced current exists only as long as the magnetic field is changing
- If the magnet is stationary (no change), no current is induced
- The direction of induced current reverses when the direction of motion reverses
- Faster motion or more turns in the coil → greater induced current
Fleming’s Right-Hand Rule (Generator Rule)
Stretch the thumb, forefinger, and middle finger of your right hand at right angles:
- Forefinger: Direction of magnetic field (B)
- Thumb: Direction of motion of the conductor
- Middle finger: Direction of the induced current (I)
6. Electric Generator
An electric generator converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction.
Construction (similar to a motor)
- Rectangular coil (armature) that rotates in a magnetic field
- Permanent magnets or electromagnets
- Slip rings (for AC generator) or split ring commutator (for DC generator)
- Carbon brushes
Working
- The coil is rotated mechanically (by turbine, wind, water, etc.) in a magnetic field
- As the coil rotates, the magnetic flux through it changes continuously
- This changing flux induces an EMF (and current) in the coil — electromagnetic induction
- The current is collected by brushes via slip rings
AC vs DC Generator
| Feature | AC Generator | DC Generator |
|---|---|---|
| Current type | Alternating current (changes direction) | Direct current (flows in one direction) |
| Uses | Slip rings (2 complete rings) | Split ring commutator |
| Used in | Power stations, household electricity | Batteries, charging, small devices |
In India: AC is used at 220 V, 50 Hz (the current changes direction 50 times per second).
7. Domestic Electric Circuits
How Electricity Reaches Our Homes
- Power station generates electricity at 11,000–33,000 V (AC)
- Step-up transformer increases voltage for long-distance transmission (reduces energy loss)
- Transmitted at very high voltage via power lines
- Step-down transformers near homes reduce voltage to 220 V
- Two wires enter the house: Live wire (red, at 220 V) and Neutral wire (black, at 0 V)
Components of a Domestic Circuit
| Component | Function |
|---|---|
| Live wire (L) | Red/brown insulation; carries current at 220 V |
| Neutral wire (N) | Black/blue insulation; at approximately 0 V; completes the circuit |
| Earth wire (E) | Green insulation; connected to a metal plate buried in the ground; safety measure — carries leaked current to earth |
| Main fuse | Connected to the live wire; breaks circuit if current exceeds safe limit |
| Electric meter | Measures total energy consumed (in kWh/units) |
| MCB (Miniature Circuit Breaker) | Modern replacement for fuse; automatically trips when current is too high; can be reset |
Important Safety Points
- All appliances are connected in parallel between the live and neutral wires
- Each appliance gets the same voltage (220 V) and can be switched independently
- The earth wire is connected to the metal body of appliances — if a live wire touches the metal body, current flows to the earth instead of through the person → prevents electric shock
- Short circuit: When live and neutral wires come in direct contact (insulation damaged) → very large current flows → overheating → fire risk. The fuse/MCB protects against this.
- Overloading: When too many appliances are connected to a single circuit → total current exceeds the safe limit → overheating. Avoid using too many appliances on one circuit.
Important Definitions
| Term | Definition |
|---|---|
| Magnetic field | Region around a magnet where its magnetic force can be experienced |
| Solenoid | Cylindrical coil of many turns of insulated wire that produces a magnetic field like a bar magnet when current flows |
| Electromagnet | Solenoid with a soft iron core; produces a strong, controllable magnetic field |
| Fleming’s Left-Hand Rule | Gives the direction of force on a current-carrying conductor in a magnetic field (motor rule) |
| Electric motor | Device that converts electrical energy to mechanical (rotational) energy |
| Electromagnetic induction | Generation of electric current by changing the magnetic field around a conductor |
| Fleming’s Right-Hand Rule | Gives the direction of induced current in a conductor moving in a magnetic field (generator rule) |
| Electric generator | Device that converts mechanical energy to electrical energy |
| Commutator | Split ring device that reverses current direction every half rotation in a motor/DC generator |
| Short circuit | Direct contact between live and neutral wires causing very large current flow |
| Overloading | Drawing more current from a circuit than it is designed to handle |
Solved Examples (NCERT-Based)
Example 1
Why does a compass needle get deflected when brought near a current-carrying wire?
Answer: A current-carrying wire produces a magnetic field around it (Oersted’s discovery). A compass needle is a small magnet that aligns itself along the magnetic field. When the compass is near the wire, it experiences the magnetic field of the wire and gets deflected from its normal north-south direction. The greater the current, the greater the deflection.
Example 2
What is the function of the split ring in an electric motor?
Answer: The split ring acts as a commutator. It reverses the direction of current flowing through the coil every half rotation. Without this reversal, the coil would rotate only half a turn and then stop. The commutator ensures the force on the coil arms continues to push in the same rotational direction, keeping the motor spinning continuously.
Example 3
State the difference between an AC generator and a DC generator.
Answer: The key difference is in the output and the type of rings used:
- An AC generator uses slip rings (two complete rings) and produces alternating current that reverses direction periodically.
- A DC generator uses a split ring commutator that reverses the connection every half cycle, producing direct current that flows in only one direction.
Both work on the same principle (electromagnetic induction) and have similar construction otherwise.
Example 4
Why is the earth wire connected to metallic appliances?
Answer: The earth wire provides a low-resistance path for electric current to flow to the ground. If due to a fault, the live wire touches the metal body of an appliance, a large current flows through the earth wire to the ground instead of through the person who touches the appliance. This immediately blows the fuse or trips the MCB, cutting off the supply. Without the earth wire, the person would receive a severe electric shock.
Important Questions for Board Exams
1-Mark Questions
- State Fleming’s Left-Hand Rule.
- What is the function of a commutator in an electric motor?
- What is the frequency of AC supply in India?
- Name the device that converts mechanical energy to electrical energy.
- What is a short circuit?
2-Mark Questions
- What is the difference between Fleming’s Left-Hand Rule and Right-Hand Rule?
- Why does a current-carrying conductor placed in a magnetic field experience a force?
- What is the role of the earth wire in domestic circuits?
- How is an electromagnet different from a permanent magnet?
- What is electromagnetic induction? Who discovered it?
3-Mark Questions
- Draw a labelled diagram of an electric motor. Explain its working principle.
- Draw the magnetic field pattern around a current-carrying solenoid. How is it similar to a bar magnet?
- What is the difference between AC and DC? Name the device used to generate each.
- Explain the working of an AC generator with a labelled diagram.
- What are the causes of short circuit and overloading? How does a fuse protect a circuit?
5-Mark Questions
- Explain the construction and working of an electric motor with a labelled diagram. What is the role of the split ring commutator?
- What is electromagnetic induction? Describe the construction and working of an AC generator. How is it different from a DC generator?
Quick Revision Points
- Oersted: current-carrying conductor produces magnetic field
- Magnetic field around straight wire: concentric circles (Right-Hand Thumb Rule for direction)
- Solenoid: coil producing uniform field inside (like bar magnet)
- Electromagnet: solenoid + soft iron core → strong, switchable magnet
- Force on current-carrying conductor in magnetic field → Fleming’s Left-Hand Rule (F-B-I)
- Electric motor: electrical → mechanical energy; uses commutator to reverse current every half turn
- Electromagnetic induction (Faraday): changing magnetic field → induced current
- Fleming’s Right-Hand Rule → direction of induced current (generator)
- AC generator: slip rings → alternating current; DC generator: split ring commutator → direct current
- India: 220 V, 50 Hz AC
- Domestic circuit: Live (220 V) + Neutral (0 V) + Earth (safety)
- Appliances connected in parallel: same voltage, independent operation
- Safety devices: Fuse (melts), MCB (trips) — protect against short circuit and overloading
- Earth wire: prevents electric shock by providing a low-resistance path to ground
Previous Chapter: Chapter 11 — Electricity
Next Chapter: Chapter 13 — Our Environment
Chapter Navigation
Previous: Electricity Class 10 Notes
Next: Our Environment Class 10 Notes
Related Chapters in Class 10 Science
- Electricity Class 10 Notes
- Light Reflection and Refraction Class 10 Notes
- Human Eye and Colourful World Class 10 Notes
Practice What You Learned
Test yourself with our Class 10 Science Mock Papers to see how well you’ve mastered the concepts.