Class 7 Science Notes Chapter 14 (Electric Current and its Effects) – Science Book

Okay, here are the detailed notes for Chapter 14: Electric Current and its Effects, tailored for government exam preparation based on NCERT Class 7 Science.

Chapter 14: Electric Current and its Effects

1. Introduction to Electric Current and Circuits

• Electric Current: The flow of electric charge through a conductor (like a wire).
• Electric Circuit: A closed, continuous path through which electric current can flow.
• Components of a Simple Circuit:
  • Electric Cell: Source of electric energy. Has two terminals: positive (+) and negative (-). Current flows conventionally from (+) to (-) terminal outside the cell.
  • Battery: A combination of two or more cells connected in series (positive terminal of one cell connected to the negative terminal of the next). Provides higher voltage/energy than a single cell.
  • Switch: A device used to make ('ON' or 'closed' position) or break ('OFF' or 'open' position) an electric circuit.
  • Bulb: An electrical appliance that converts electrical energy into light and heat energy. Acts as an indicator of current flow.
  • Connecting Wires: Conductors (usually metal like copper) that provide the path for current flow between components.

2. Symbols for Electric Components

• Using standard symbols makes drawing circuit diagrams (schematic diagrams) easier and universally understandable.

• Common Symbols:

  Component	Symbol
  Electric Cell	A long line (+) and a shorter, thicker line (-)
  Battery	Combination of cell symbols connected + to -
  Electric Bulb	Circle with a filament shape inside (or a cross)
  Switch (ON / Closed)	Two dots connected by a line
  Switch (OFF / Open)	Two dots with the connecting line lifted
  Connecting Wire	A straight line

3. Electric Circuit Diagrams

• Circuit Diagram: A simplified representation of an electric circuit using standard symbols.
• Open Circuit: A circuit with a break in the path (e.g., switch is OFF, wire is broken, bulb filament is fused). Current cannot flow.
• Closed Circuit: A complete, unbroken path for the current to flow from the source, through the components, and back to the source. Current can flow.

4. Heating Effect of Electric Current

• Phenomenon: When electric current flows through a wire (or any conductor), the wire gets hot. This is known as the heating effect of electric current.
• Cause: Resistance offered by the wire to the flow of current. Electrical energy is converted into heat energy.
• Factors Affecting Heat Produced (Conceptual Level for Class 7):
  • Amount of current flowing.
  • Resistance of the wire (depends on material, length, thickness).
  • Duration for which current flows.
• Applications: Based on producing useful heat.
  • Electric Heater, Electric Iron, Geyser, Hair Dryer: Use heating elements (coils of wire, often made of nichrome alloy) that have high resistance and get very hot.
  • Electric Bulb: The thin wire inside, called the filament, gets heated to a very high temperature and starts glowing.
    • Filament Material: Usually made of Tungsten, which has a very high melting point, allowing it to glow white-hot without melting.
• Waste Heat: Sometimes the heat produced is undesirable (e.g., in computers, motors), representing a loss of energy.

5. Magnetic Effect of Electric Current

• Discovery: Hans Christian Oersted (in 1820) discovered that when electric current passes through a wire, it produces a magnetic field around it. He observed that a magnetic compass needle placed near a current-carrying wire gets deflected.
• Conclusion: Electric current can produce magnetism.
• Electromagnet:
  • Definition: A temporary magnet created by passing electric current through a coil of insulated wire wound around a piece of soft iron (called the core).
  • Properties:
    • It behaves like a magnet only when current flows through the coil.
    • Its magnetic strength can be increased by:
      • Increasing the number of turns in the coil.
      • Increasing the current flowing through the coil.
    • Its polarity (North/South poles) depends on the direction of current flow.
  • Applications:
    • Electric Bell: Uses an electromagnet to strike a gong.
    • Cranes: Used in scrapyards to lift heavy magnetic materials like iron and steel.
    • Medical Equipment: Used in devices like MRI scanners.
    • Loudspeakers, Relays, Motors.

6. Electric Bell

• Working Principle: Based on the magnetic effect of electric current (electromagnetism).
• Main Parts: Electromagnet, Iron Armature (with Hammer attached), Contact Screw, Gong, Switch, Power Source (Battery/Cell).
• Working Mechanism:
  1. When the switch is pressed (ON), the circuit is completed.
  2. Current flows through the coil of the electromagnet.
  3. The electromagnet becomes magnetized and attracts the soft iron armature towards it.
  4. The hammer attached to the armature strikes the gong, producing sound.
  5. As the armature moves towards the electromagnet, it loses contact with the contact screw.
  6. This breaks the circuit, and the current stops flowing through the coil.
  7. The electromagnet loses its magnetism.
  8. The armature is no longer attracted and springs back to its original position, touching the contact screw again.
  9. This completes the circuit, current flows again, and the cycle repeats rapidly, causing continuous ringing as long as the switch is pressed.

7. Electrical Safety Devices

• Need: To protect electrical appliances and wiring from damage due to excessive current, which can cause overheating and potentially lead to fires. Excessive current can be caused by:
  • Overloading: Connecting too many appliances to a single socket/circuit.
  • Short Circuit: When the live wire and neutral wire come into direct contact (e.g., due to damaged insulation).
• Electric Fuse:
  • Purpose: A safety device that breaks the circuit when the current exceeds a safe limit.
  • Principle: Works on the heating effect of electric current.
  • Construction: Consists of a thin wire made of a special material (usually an alloy of tin and lead or tin and copper) that has a low melting point.
  • Working: When excessive current flows, the fuse wire heats up rapidly, melts, and breaks the circuit, stopping the current flow.
  • Placement: Always connected in series with the live wire, usually at the beginning of the circuit (after the meter).
  • Specification: Fuses are rated for specific maximum currents (e.g., 5A, 13A). The correct rating must be used for a circuit.
  • Caution: A blown fuse must be replaced with a new fuse of the correct rating. Never replace it with ordinary wire or a fuse of a higher rating.
• Miniature Circuit Breaker (MCB):
  • Purpose: An automatic switch that trips (turns OFF) when the current in a circuit exceeds a safe limit. Increasingly used instead of fuses in homes and buildings.
  • Principle: Can work on the heating effect or magnetic effect (or both) of the current to trigger the switching mechanism.
  • Working: When excessive current flows, the MCB automatically switches OFF, breaking the circuit.
  • Advantage over Fuse: MCBs are reusable. After the fault causing the overload/short circuit is fixed, the MCB can be manually reset (switched ON again). Fuses need to be replaced.

Key Exam Focus Points:

• Know the symbols of common electric components.
• Differentiate between open and closed circuits.
• Understand the heating effect and its applications (especially bulb filament - Tungsten).
• Know Oersted's discovery regarding the magnetic effect.
• Understand what an electromagnet is, how its strength can be changed, and its applications.
• Be able to explain the working principle of an electric bell.
• Understand the purpose and working principle of a fuse (low melting point material is key).
• Understand the purpose and working of an MCB and its advantage over a fuse (reusability).
• Difference between a cell and a battery.

Remember to revise these points thoroughly and practice drawing circuit diagrams using standard symbols.