Class 10 Notes

Class 10 Science Notes Chapter 13 (Chapter 13) – Examplar Problems (English) Book

Philoid

Philoid

— 8 min read

Examplar Problems (English)
Detailed Notes with MCQs of Chapter 13, 'Magnetic Effects of Electric Current'. This is a crucial chapter, not just for your board exams but also for various government competitive exams where basic science concepts are tested. We'll break down the key ideas from the NCERT Exemplar perspective.

Chapter 13: Magnetic Effects of Electric Current - Detailed Notes

1. Introduction: Magnetism and Electricity

  • Historically, electricity and magnetism were considered separate phenomena.
  • Hans Christian Oersted (1820) discovered that a current-carrying wire deflects a nearby magnetic compass needle. This established the link: Moving electric charges (current) produce magnetic fields.

2. Magnetic Field and Field Lines

  • Magnetic Field: The region surrounding a magnet (or a current-carrying conductor) where its magnetic influence can be detected. It's a vector quantity (has both magnitude and direction).
  • SI Unit of Magnetic Field Strength: Tesla (T). Another common unit is Gauss (1 T = 10⁴ Gauss).
  • Magnetic Field Lines (or Lines of Force): Imaginary lines used to represent the magnetic field visually.
    • Properties:
      • They emerge from the North pole and merge at the South pole outside the magnet.
      • Inside the magnet, their direction is from South to North, forming closed continuous loops.
      • The tangent at any point on a field line gives the direction of the magnetic field at that point.
      • Crucially, two magnetic field lines never intersect. If they did, it would mean there are two directions of the magnetic field at the point of intersection, which is impossible.
      • The relative closeness (density) of the field lines indicates the strength of the magnetic field. Closer lines mean a stronger field.

3. Magnetic Field due to a Current-Carrying Conductor

  • a) Straight Conductor:

    • The magnetic field lines are concentric circles around the wire.
    • The plane of the circles is perpendicular to the wire.
    • Right-Hand Thumb Rule (Maxwell's Corkscrew Rule): If you 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.
    • Magnitude of the field:
      • Directly proportional to the current (I) flowing through the wire. (B ∝ I)
      • Inversely proportional to the distance (r) from the wire. (B ∝ 1/r)

  • b) Circular Loop:

    • At every point on the loop, the field lines are concentric circles, becoming larger as we move away from the wire.
    • Near the center of the loop, the arcs of these circles appear as nearly straight lines, perpendicular to the plane of the loop.
    • The magnetic field produced by the loop is strongest at the center.
    • Direction: Can be found using the Right-Hand Thumb Rule for each segment, or the Clock Face Rule (If current flows clockwise, the face of the loop is a South pole; if anti-clockwise, it's a North pole).
    • Magnitude of the field at the center:
      • Directly proportional to the current (I). (B ∝ I)
      • Inversely proportional to the radius (R) of the loop. (B ∝ 1/R)
      • Directly proportional to the number of turns (N) in the coil. (B ∝ N)

  • c) Solenoid:

    • Definition: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
    • Magnetic Field Pattern: Very similar to that of a bar magnet.
    • The field lines are nearly parallel and uniform inside the solenoid, indicating a strong and uniform magnetic field.
    • Outside the solenoid, the field is weak and non-uniform.
    • One end of the solenoid behaves like a North pole, and the other behaves like a South pole (can be determined using the Clock Face Rule or Right-Hand Thumb Rule applied to the coil).
    • Electromagnet: A strong temporary magnet made by passing current through a solenoid with a soft iron core inside. Soft iron is used because it magnetizes strongly but loses its magnetism quickly when the current is switched off.
    • Strength of the field inside a solenoid depends on:
      • The magnitude of the current (I).
      • The number of turns per unit length (n).
      • The nature of the core material (increases significantly with a soft iron core).

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

  • Ampere's Observation: A current-carrying conductor experiences a force when placed in a magnetic field (provided the conductor is not parallel to the field).
  • This force is maximum when the conductor is perpendicular to the magnetic field.
  • The force is zero when the conductor is parallel to the magnetic field.
  • Fleming's Left-Hand Rule: Used to find the direction of the force.
    • 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 (B),
    • and the Middle finger points in the direction of the Current (I),
    • then the Thumb points in the direction of the Force (or Thrust/Motion) (F) experienced by the conductor.
    • (Remember: Father Mother Child -> Force/Motion, Magnetic Field, Current)
  • Magnitude of the force (F) is proportional to:
    • Magnetic field strength (B)
    • Current (I)
    • Length of the conductor (L) within the field. (F ∝ BIL)

5. Electric Motor

  • Principle: Based on the fact that a current-carrying coil placed in a magnetic field experiences a torque (rotational force), causing it to rotate.
  • Energy Conversion: Converts Electrical Energy into Mechanical Energy.
  • Main Components:
    • Armature Coil (ABCD): Rectangular coil with many turns of insulated copper wire wound on a soft iron core.
    • Strong Magnetic Field: Provided by permanent magnets (field magnets N-S).
    • Split-Ring Commutator: Two halves of a metallic ring (P and Q). Reverses the direction of current in the armature coil every half rotation. This ensures continuous rotation in one direction.
    • Brushes (X and Y): Carbon brushes that press against the commutator halves, providing electrical contact between the battery and the rotating coil.
    • Battery/Power Source: Provides current to the armature coil.
  • Working: Current flows, forces act on opposite arms (AB and CD) in opposite directions (Fleming's Left-Hand Rule), creating torque. Commutator reverses current direction after 180° rotation, reversing forces on arms, ensuring continuous rotation.

6. Electromagnetic Induction (EMI)

  • Michael Faraday (1831): Discovered that a changing magnetic field linked with a coil can induce an electromotive force (voltage) and hence a current in the coil.
  • Definition: The phenomenon of producing an induced current in a coil 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 fixed magnet.
    • Keeping both stationary but changing the magnetic field (e.g., by changing the current in a nearby primary coil - mutual induction).
  • Galvanometer: An instrument used to detect the presence of small currents in a circuit. Its needle deflects when current flows.
  • Induced Current: The current produced due to EMI. Its magnitude depends on the rate of change of the magnetic field and the properties of the coil (number of turns, area).

7. Direction of Induced Current - Fleming's Right-Hand Rule

  • Used to find the direction of the induced current.
  • Stretch the thumb, forefinger, and middle finger of your right hand mutually perpendicular.
  • If the Thumb points in the direction of Motion of the conductor,
  • and the Forefinger points in the direction of the magnetic Field (B),
  • then the Middle finger points in the direction of the Induced Current (I).
    (Remember: Motion, Field, Current)

8. Electric Generator (Dynamo)

  • Principle: Based on Electromagnetic Induction.
  • Energy Conversion: Converts Mechanical Energy into Electrical Energy.
  • Main Components: Similar to a motor (Armature coil, Field Magnets, Brushes) but differs in the ring system and the output connection.
  • Types:
    • AC Generator (Alternating Current): Uses Slip Rings. The direction of current in the external circuit reverses periodically (usually 50 Hz or 60 Hz). The current alternates direction.
    • DC Generator (Direct Current): Uses a Split-Ring Commutator (similar to a motor). The commutator ensures that the current in the external circuit flows in only one direction (though its magnitude may vary - pulsating DC).
  • Alternating Current (AC): Current that reverses its direction periodically. Advantage: Can be transmitted over long distances with less energy loss. Frequency in India is 50 Hz (meaning it changes direction 100 times per second).
  • Direct Current (DC): Current that flows in only one direction. Sources: Batteries, DC generators.

9. Domestic Electric Circuits

  • Wiring System: Power is supplied through mains, typically consisting of three wires:
    • Live Wire (or Phase wire): Usually covered with red insulation. It is at a high potential (e.g., 220 V in India).
    • Neutral Wire: Usually covered with black insulation. It is at or near zero potential. The potential difference between live and neutral is 220 V.
    • Earth Wire: Usually covered with green insulation. Connected to a metal plate buried deep in the earth near the house. It provides a safety path for current in case of leakage.
  • Circuit Connection: Appliances are connected in parallel across the live and neutral wires. Advantages of parallel connection:
    • Each appliance gets the full voltage (220 V).
    • Each appliance can be operated independently with its own switch.
    • If one appliance fails, others continue to work.
  • Safety Devices:
    • Electric Fuse: A safety device connected in series with the live wire. It consists of a wire made of an alloy with a low melting point (e.g., lead-tin alloy).
      • Principle: Works on the heating effect of current (Joule's Law of Heating, H = I²Rt).
      • Function: If the current exceeds a safe limit (due to overloading or short-circuiting), the fuse wire heats up, melts, and breaks the circuit, preventing damage to appliances and wiring. It must have a current rating slightly higher than the normal operating current of the appliance/circuit.
    • Earthing: The metal casing of appliances (like refrigerators, irons, computers) is connected to the earth wire. If there's any leakage of current onto the metal body, the earth wire provides a low-resistance path for the current to flow to the earth, preventing the user from getting a severe electric shock. The large current flow also often blows the fuse.
    • MCB (Miniature Circuit Breaker): An alternative to fuses. It's an automatic switch that trips (turns off) when the current exceeds a safe limit. It can be reset manually, unlike a fuse which needs replacement.
  • Hazards:
    • Overloading: Occurs when too many high-power appliances are connected to a single socket/circuit, drawing excessive current. This can overheat the wires and cause a fire.
    • Short-Circuiting: Occurs when the live wire comes into direct contact with the neutral wire (e.g., due to damaged insulation). This drastically reduces the resistance of the circuit, causing a very large current to flow, leading to overheating and potential fire. Fuses/MCBs protect against both overloading and short-circuiting.

Multiple Choice Questions (MCQs)

  1. 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.

  2. The device used for producing electric current is called a/an:
    (a) generator.
    (b) galvanometer.
    (c) ammeter.
    (d) motor.

  3. 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 magnets.
    (b) DC generator will generate a higher voltage.
    (c) AC generator has slip rings while the DC generator has a commutator.
    (d) AC generator works on EMI, while DC generator works on force on conductor.

  4. At the time of short circuit, the current in the circuit:
    (a) reduces substantially.
    (b) does not change.
    (c) increases heavily.
    (d) vary continuously.

  5. Which rule is used to find 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

  6. A strong bar magnet is placed vertically above a horizontal wooden board. The magnetic lines of force will be:
    (a) only in horizontal plane.
    (b) only in vertical plane.
    (c) in horizontal as well as in vertical planes.
    (d) crowded near the centre of the magnet.

  7. The most important safety method used for protecting home appliances from short-circuiting or overloading is:
    (a) earthing.
    (b) use of stabilizers.
    (c) use of fuse or MCB.
    (d) use of electric meter.

  8. The magnetic field inside a long straight solenoid carrying current:
    (a) is zero.
    (b) decreases as we move towards its end.
    (c) increases as we move towards its end.
    (d) is the same at all points.

  9. An electric motor converts:
    (a) Mechanical energy into electrical energy
    (b) Electrical energy into mechanical energy
    (c) Chemical energy into electrical energy
    (d) Electrical energy into light energy

  10. Two magnetic field lines:
    (a) can intersect at the neutral point.
    (b) can intersect near the poles of the magnet.
    (c) can never intersect each other.
    (d) intersect only in a strong magnetic field.

Answer Key for MCQs:

  1. (c)
  2. (a)
  3. (c)
  4. (c)
  5. (c)
  6. (c) (Field lines exist in 3D space around the magnet)
  7. (c)
  8. (d) (Ideally, the field inside a long solenoid is uniform)
  9. (b)
  10. (c)

Make sure you understand the principles behind each concept, especially the rules (Fleming's Left/Right, Right-Hand Thumb) and the working of motors and generators. Pay attention to the safety aspects of domestic circuits as questions are often framed around these practical applications. Good luck with your preparation!

Read more