Class 10 Science Notes Chapter 13 (Magnetic effects of electric current) – Science Book

Detailed Notes with MCQs of Chapter 13: Magnetic Effects of Electric Current. This is a crucial chapter, linking electricity and magnetism, and frequently appears in various government examinations. Pay close attention to the concepts, rules, and applications.

Chapter 13: Magnetic Effects of Electric Current - Detailed Notes

1. Introduction: Oersted's Discovery

• Hans Christian Oersted (1820): Accidentally discovered that a compass needle deflected when an electric current passed through a nearby wire.
• Conclusion: Electric current produces a magnetic field around it. This established the link between electricity and magnetism (Electromagnetism).

2. Magnetic Field and Field Lines

• Magnetic Field: The region around a magnet (or a current-carrying conductor) where its magnetic influence can be detected. It's a vector quantity (has both magnitude and direction).
• Magnetic Field Lines (Lines of Force): Imaginary lines used to represent the magnetic field.
  • Properties:
    • They form continuous closed loops.
    • Outside a magnet, the direction is from the North pole to the South pole.
    • Inside a magnet, the direction is from the South pole to the North pole.
    • They never intersect each other. (If they did, it would mean two directions of the magnetic field at one point, which is impossible).
    • The density (closeness) of the lines indicates the strength of the magnetic field. Stronger field where lines are crowded, weaker where they are far apart.
  • Visualisation: Can be traced using a compass needle or observed using iron filings sprinkled around a magnet.

3. Magnetic Field due to a Current-Carrying Conductor

• a) Straight Conductor:

  • Pattern: Magnetic field lines are concentric circles around the wire.
  • Direction: Given by the Right-Hand Thumb Rule.
    • Rule: Imagine holding the current-carrying wire in your right hand such that your thumb points in the direction of the current. Then, the direction in which your fingers curl gives the direction of the magnetic field lines.
  • Strength: Increases with increasing current; decreases with increasing distance from the wire.

• b) Circular Loop:

  • Pattern: Field lines are circular near the wire but become nearly straight and perpendicular to the plane of the loop at the center.
  • Direction: Can be found using the Right-Hand Thumb Rule for any segment of the loop. At the center, the field lines are along the axis of the loop.
  • Strength:
    • Proportional to the current passing through the loop.
    • Inversely proportional to the radius of the loop.
    • If the loop has 'n' turns, the field strength is 'n' times larger than that produced by a single turn (assuming the same current).

• c) Solenoid:

  • Definition: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
  • Pattern: The magnetic field pattern resembles that of a bar magnet.
    • Inside the solenoid, the field lines are nearly parallel and uniformly distributed, indicating a strong and uniform magnetic field.
    • Outside the solenoid, the field is weak.
  • Polarity: One end acts as a North pole, the other as a South pole (can be determined using the clock face rule or Right-Hand Thumb Rule applied to the coil).
  • Strength: Depends on:
    • The number of turns (n) per unit length.
    • The strength of the current (I).
    • The nature of the core material inside the solenoid.
  • Electromagnet: A strong temporary magnet made by passing current through a coil wrapped around a soft iron core. Soft iron is used because it magnetizes easily when current flows and demagnetizes quickly when current stops.

4. Force on a Current-Carrying Conductor in a Magnetic Field

• Observation: A conductor carrying current experiences a force when placed in an external magnetic field (unless the wire is parallel to the field). This force is maximum when the conductor is perpendicular to the magnetic field.
• Direction: Given by Fleming's Left-Hand Rule.
  • Rule: Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic Field, and the middle finger points in the direction of the Current, then the Thumb points in the direction of the Thrust or Force experienced by the conductor. (Remember: Father Mother Child -> Force Magnetic Field Current)
• Application: This principle is the basis for the working of electric motors.

5. Electric Motor

• Function: A device that converts electrical energy into mechanical energy (rotation).
• Principle: When a rectangular coil carrying current is placed in a magnetic field, it experiences a torque (rotational force) which rotates it continuously.
• Key Components:
  • Armature Coil: Rectangular coil (ABCD) with many turns of insulated copper wire wound on a soft iron core.
  • Strong Magnetic Field: Provided by permanent magnets (North and South poles).
  • Split-Ring Commutator: Two halves of a metallic ring (P and Q). Reverses the direction of current in the coil every half rotation, ensuring continuous rotation in the same direction.
  • Brushes: Carbon brushes (X and Y) that make contact with the rotating commutator rings and supply current to the coil.
  • Battery: Source of electric current.

6. Electromagnetic Induction (EMI)

• Michael Faraday (1831): Discovered that a changing magnetic field linked with a coil can induce an electric current in the coil.
• Definition: The phenomenon of producing an induced electromotive force (e.m.f. or voltage) and hence current in a closed circuit due to a change in the magnetic field associated with it.
• Methods to Induce Current:
  • Moving a magnet towards or away from a coil.
  • Moving a coil towards or away from a magnet.
  • Changing the current in a nearby primary coil (which changes the magnetic field linked with the secondary coil).
• Induced Potential Difference: The potential difference set up across the ends of the coil due to EMI.
• Induced Current: The current that flows in the closed circuit due to the induced potential difference.
• Direction: Given by Fleming's Right-Hand Rule.
  • Rule: Stretch the thumb, forefinger, and middle finger of your right hand mutually perpendicular. If the forefinger points in the direction of the magnetic Field, and the Thumb points in the direction of the Motion of the conductor, then the Middle finger points in the direction of the Induced Current.

7. Electric Generator

• Function: A device that converts mechanical energy into electrical energy.
• Principle: Based on Electromagnetic Induction (EMI). When a conductor (coil) is rotated in a magnetic field, the magnetic field lines linked with it change, inducing a current.
• Key Components: Similar to a motor (Coil, Magnets, Brushes), but uses slip rings for AC generators and a split-ring commutator for DC generators.
  • AC Generator (Alternating Current): Uses two full rings (slip rings) R1 and R2. The direction of current reverses periodically. The current produced in India has a frequency of 50 Hz (changes direction 100 times per second).
  • DC Generator (Direct Current): Uses a split-ring commutator. The commutator ensures that the current in the external circuit always flows in the same direction, although it might be pulsating.

8. Domestic Electric Circuits

• Power Supply: Typically AC, with a potential difference of 220 V and frequency of 50 Hz in India.
• Wiring: Three types of wires are used:
  • Live Wire (Phase): Usually red insulation. Carries current at high potential (220 V).
  • Neutral Wire: Usually black insulation. At or near zero potential. Completes the circuit. The potential difference between live and neutral is 220 V.
  • Earth Wire: Usually green insulation. Connected to a metal plate buried deep in the earth. Provides a safety path for current to flow to the earth in case of leakage to the metallic body of an appliance, preventing electric shock.
• Circuit Components:
  • Electricity Meter: Records energy consumption (in kWh - 'units').
  • Main Switch/Distribution Box: Contains fuses or MCBs (Miniature Circuit Breakers) for different circuits.
  • Parallel Connection: Appliances are connected in parallel across the live and neutral wires. This ensures:
    • Each appliance gets the full voltage (220 V).
    • Switching one appliance on/off does not affect others.
    • Overall resistance decreases, allowing sufficient current draw.
• Safety Measures:
  • Electric Fuse: A safety device connected in series with the live wire. Contains a wire made of an alloy with a low melting point (e.g., tin-lead alloy). If current exceeds a safe limit (due to overloading or short-circuiting), the fuse wire melts, breaking the circuit and preventing damage to appliances and wiring.
  • Earthing: Connecting the metallic body of appliances to the earth wire.
  • MCBs (Miniature Circuit Breakers): Switches that automatically turn off when current exceeds a safe limit. They can be reset manually, unlike fuses which need replacement.
• Hazards:
  • Overloading: Drawing too much current when many high-power appliances are connected to a single circuit. Can cause overheating of wires.
  • Short-Circuiting: When the live wire comes into direct contact with the neutral wire (or earth wire). Resistance becomes very low, leading to a very large current flow, causing sparks and potentially fire.

Multiple Choice Questions (MCQs)

1. Oersted's experiment demonstrated that:
   a) A magnetic field produces an electric current.
   b) An electric current produces a magnetic field.
   c) Moving charges experience force in a magnetic field.
   d) Changing magnetic field induces current.

2. The direction of magnetic field lines inside a bar magnet is:
   a) From North pole to South pole.
   b) From South pole to North pole.
   c) Away from the South pole.
   d) There are no field lines inside.

3. The pattern of magnetic field lines around a straight conductor carrying current is:
   a) Parallel straight lines.
   b) Elliptical loops.
   c) Concentric circles.
   d) Radiating outwards.

4. Which rule is used to determine the direction of the magnetic field produced by a straight current-carrying conductor?
   a) Fleming's Left-Hand Rule
   b) Fleming's Right-Hand Rule
   c) Right-Hand Thumb Rule
   d) Left-Hand Thumb Rule

5. A device that converts electrical energy into mechanical energy is:
   a) Electric Generator
   b) Solenoid
   c) Electric Motor
   d) Electromagnet

6. The phenomenon of electromagnetic induction is:
   a) The process of charging a body.
   b) The process of generating magnetic field due to a current.
   c) The process of producing induced current in a coil due to relative motion between a magnet and the coil.
   d) The process of rotating a coil in a magnetic field.

7. In an electric motor, the component responsible for reversing the direction of current in the coil every half rotation is:
   a) Armature coil
   b) Slip rings
   c) Carbon brushes
   d) Split-ring commutator

8. Fleming's Right-Hand Rule gives the direction of:
   a) Force on a current-carrying conductor in a magnetic field.
   b) Magnetic field due to a current.
   c) Induced current.
   d) Motion of the conductor.

9. For safety, the metallic body of an electric appliance is connected to which wire?
   a) Live wire
   b) Neutral wire
   c) Fuse wire
   d) Earth wire

10. An electric fuse works on the principle of:
    a) Magnetic effect of current
    b) Chemical effect of current
    c) Heating effect of current
    d) Electromagnetic induction

Answer Key for MCQs:

1. b
2. b
3. c
4. c
5. c
6. c
7. d
8. c
9. d
10. c

Study these notes thoroughly. Remember the rules, the principles behind motors and generators, and the safety aspects of domestic circuits. Understanding these fundamentals is key for your exams. Good luck!