Class 7 Notes

Class 7 Science Notes Chapter 14 (Chapter 14) – Examplar Problems Book

Philoid

Philoid

— 5 min read

Examplar Problems
Alright class, let's focus on Chapter 14: Electric Current and its Effects from your Science Exemplar book. This chapter is crucial not just for your school exams but also forms the foundation for many concepts tested in government exams. Pay close attention!

Chapter 14: Electric Current and its Effects - Detailed Notes

1. Introduction to Electric Current & Circuits

  • Electric Current: The flow of electric charge (usually electrons) through a conductor. Think of it like water flowing through a pipe.
  • Electric Circuit: A continuous and closed path along which an electric current can flow.
  • Essential Components of a Simple Circuit:
    • Source of Current: Provides the energy for the charges to flow (e.g., Electric Cell, Battery).
      • Electric Cell: Has a positive (+) and a negative (-) terminal. Current flows from + to - terminal in the external circuit.
      • Battery: A combination of two or more cells connected together. Cells are connected negative-to-positive (-ve of one to +ve of the next) to increase the overall voltage.
    • Load/Appliance: The device that uses the electrical energy (e.g., Bulb, Fan).
    • Switch: A device used to make or break the circuit ('ON' or 'OFF').
    • Connecting Wires: Usually made of copper, provide the path for the current to flow.

2. Symbols for Electric Components

It's essential to represent circuits using standard symbols for clarity:

  • Electric Cell: ⎕ (Longer line is positive terminal, shorter thicker line is negative)
  • Battery: ⎖ (Combination of cells)
  • Switch (Open / 'OFF' position): ⎚ (Gap indicates circuit is broken)
  • Switch (Closed / 'ON' position): ⎛ (Connection indicates circuit is complete)
  • Electric Bulb: ⎗ or 💡 (Circle with a cross or filament shape inside)
  • Connecting Wire: ─── (A straight line)

3. Circuit Diagrams

  • A schematic representation of an electric circuit using standard symbols.
  • Open Circuit: A circuit where the path is broken (e.g., switch is OFF, wire is cut, bulb filament is broken). Current does not flow.
  • Closed Circuit: A circuit where the path is complete (switch is ON). Current flows.

4. Heating Effect of Electric Current

  • Principle: When electric current flows through a wire, the wire offers resistance to the flow. This resistance converts some electrical energy into heat energy, causing the wire to heat up. This is called the heating effect of current or Joule heating.
  • Factors Affecting Heat Produced:
    • Amount of current (More current = More heat)
    • Resistance of the wire (Higher resistance = More heat)
    • Time for which current flows (Longer time = More heat)
  • Applications:
    • Electric Heater, Geyser, Iron: Use coils made of high-resistance materials (like Nichrome) that get very hot when current passes through them.
    • Electric Bulb: The thin filament (usually made of Tungsten, which has a very high melting point) glows white-hot when current passes through it, producing light (and a lot of heat).
    • Electric Fuse: A crucial safety device.

5. Electric Fuse

  • Purpose: To protect electrical appliances and circuits from damage due to excessive current (overloading or short circuits).
  • Construction: A short piece of wire made of a special alloy (like tin-lead or zinc-copper) that has a low melting point and appropriate resistance. It's usually enclosed in a cartridge of porcelain or similar insulating material.
  • Working: When the current in the circuit exceeds a safe limit (the fuse rating), the heat generated melts the fuse wire. This breaks the circuit, stopping the current flow and preventing damage.
  • Important: A blown fuse must be replaced with a new fuse of the correct rating. Using a wire of higher rating or a simple thick wire defeats the purpose and can be dangerous.
  • MCB (Miniature Circuit Breaker): Modern alternative to fuses. These are switches that automatically turn OFF when the current exceeds a safe limit. They can be reset (turned ON again) manually after the fault is corrected. They are safer and more convenient than fuses.

6. Magnetic Effect of Electric Current

  • Discovery: Hans Christian Oersted (in 1820) discovered that a compass needle gets deflected when brought near a wire carrying electric current. This showed that electric current produces a magnetic field around the conductor.
  • Principle: A current-carrying wire or coil behaves like a magnet. The direction of the magnetic field depends on the direction of the current.
  • Electromagnet:
    • Definition: A temporary magnet created by passing electric current through a coil of insulated wire wrapped around a piece of soft iron (called the core).
    • Construction: Insulated copper wire wound around a soft iron core.
    • Working: When current flows through the coil, the soft iron core becomes strongly magnetized. When the current is switched off, the core loses its magnetism almost completely (hence, temporary).
    • Strength Factors: The strength of an electromagnet can be increased by:
      • Increasing the number of turns in the coil.
      • Increasing the current flowing through the coil.
      • Using a soft iron core (it gets magnetized easily).
    • Applications: Electric bells, loudspeakers, cranes for lifting heavy iron objects, separating magnetic materials from junk, in medical equipment (MRI), relays.

7. Electric Bell

  • Working Principle: Based on the magnetic effect of current (using an electromagnet).
  • Components: Electromagnet, soft iron armature with a hammer attached, contact screw, gong, switch, battery.
  • Mechanism:
    1. When the switch is pressed (circuit closed), current flows through the coil of the electromagnet.
    2. The electromagnet attracts the soft iron armature.
    3. The hammer attached to the armature strikes the gong, producing sound.
    4. As the armature moves towards the magnet, it loses contact with the contact screw, breaking the circuit.
    5. The current stops flowing, and the electromagnet loses its magnetism.
    6. The armature is no longer attracted and springs back to its original position, touching the contact screw again.
    7. This completes the circuit, current flows again, and the process repeats rapidly, causing the bell to ring continuously as long as the switch is pressed.

Multiple Choice Questions (MCQs)

Here are 10 MCQs to test your understanding. Choose the best option for each:

  1. Which of the following symbols represents a battery?
    (a) ⎕
    (b) ⎖
    (c) ⎛
    (d) ───

  2. The safety device based on the heating effect of electric current is called:
    (a) Electromagnet
    (b) Electric Bell
    (c) Fuse
    (d) MCB (Miniature Circuit Breaker)
    (Self-correction: Both Fuse and MCB are safety devices based on heating effect indirectly (MCB often uses a bimetallic strip that bends due to heat, or sometimes magnetic effect). Fuse is the more direct answer based on melting due to heat. Let's refine the options or question. Sticking with Fuse as the primary answer based on direct melting principle taught at this level.)
    (Revised thought: Q asks for 'the' safety device. Fuse is the classic example based directly on melting due to heat. MCB often uses heat but sometimes magnetism too. Fuse is the better fit for Class 7 focus.)

  3. An electric bulb produces light due to the:
    (a) Magnetic effect of current
    (b) Heating effect of current
    (c) Chemical effect of current
    (d) Both (a) and (b)

  4. Who discovered the magnetic effect of electric current?
    (a) Michael Faraday
    (b) Alessandro Volta
    (c) Hans Christian Oersted
    (d) André-Marie Ampère

  5. To make a stronger electromagnet, you should:
    (a) Decrease the number of turns in the coil
    (b) Use a core material like wood
    (c) Decrease the current
    (d) Increase the number of turns in the coil

  6. The material used for the filament of an electric bulb is usually:
    (a) Copper
    (b) Nichrome
    (c) Tungsten
    (d) Aluminium

  7. In an electric bell, which part becomes a temporary magnet when current flows?
    (a) Gong
    (b) Hammer
    (c) Coil around the iron core (Electromagnet)
    (d) Armature

  8. What happens when the electric circuit is 'open'?
    (a) Current flows continuously
    (b) Current flows intermittently
    (c) No current flows
    (d) The bulb fuses

  9. A fuse wire should have:
    (a) High melting point and high resistance
    (b) High melting point and low resistance
    (c) Low melting point and specific resistance (appropriate for rating)
    (d) Low melting point and very low resistance

  10. Which device works on the principle of an electromagnet attracting an armature?
    (a) Electric Heater
    (b) Electric Bulb
    (c) Electric Fuse
    (d) Electric Bell

Answer Key for MCQs:

  1. (b)
  2. (c)
  3. (b)
  4. (c)
  5. (d)
  6. (c)
  7. (c)
  8. (c)
  9. (c) (Note: Resistance needs to be appropriate to generate enough heat at overload current, but the key property is the low melting point)
  10. (d)

Revise these notes thoroughly. Understanding the 'why' behind each effect and application is key for competitive exams. Let me know if any part needs further clarification!

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