Quick Revision Notes

Class 7 Science: Electric Current and its Effects

Madhya Pradesh Board (MP Board)

### 1. Symbols of Electric Components (विद्युत अवयवों के प्रतीक)

In circuit diagrams, electric components are represented by standard symbols. Knowing these symbols helps in easily drawing and reading circuits.

• Electric Cell (विद्युत सेल): Longer line represents the positive terminal (+), and the thicker, shorter line represents the negative terminal (-).
• Electric Bulb (विद्युत बल्ब): Represented by a circle containing a filament loop.
• Switch in 'ON' position (चालू स्थिति में स्विच): A closed continuous path.
• Switch in 'OFF' position (बंद स्थिति में स्विच): A broken or open path.
• Battery (बैटरी): A combination of two or more cells (positive terminal of one cell is connected to the negative terminal of the next).
• Connecting Wire (संयोजक तार): Represented by a straight line.

### 2. Electric Circuit (विद्युत परिपथ)

• Definition: A complete path along which electricity (electric current) can flow from one terminal of the electric cell, through the bulb and switch, back to the other terminal of the cell.
• Complete Circuit (Closed Circuit): The bulb glows because the current flows uninterrupted.
• Incomplete Circuit (Open Circuit): The bulb does not glow because the path is broken (switch is OFF or wire is disconnected).

### 3. Heating Effect of Electric Current (विद्युत धारा का तापीय प्रभाव)

• Definition: When an electric current passes through a wire, the wire gets hot. This is known as the heating effect of electric current.
• Key Factors: The amount of heat produced in a wire depends on its material, length, and thickness.
• Applications (उपयोग):
  • Electric Room Heaters, Electric Irons, Electric Kettles, Hair Dryers, and Immersion Heaters. All these contain a coil of wire called an element (हーター या तापन अवयव).

### 4. Fuse (विद्युत फ्यूज)

• Definition: A safety device used in electrical circuits to prevent damages due to excessive current and short circuits.
• Working Principle: It is made of special wires that melt quickly and break when large electric currents pass through them.
• MCB (Miniature Circuit Breakers): Nowadays, MCBs are often used in place of fuses. These are switches that automatically turn off when the current in a circuit exceeds the safe limit.

### 5. Magnetic Effect of Electric Current (विद्युत धारा का चुम्बकीय प्रभाव)

• Definition: When electric current passes through a wire, it behaves like a magnet. This was discovered by Hans Christian Oersted.
• Compass Needle Effect: If a wire carrying current is placed close to a magnetic compass, the needle of the compass gets deflected (moves from its north-south position).

### 6. Electromagnet (विद्युत चुंबक)

• Definition: An insulated wire wrapped around an iron piece that acts as a magnet when electric current passes through it.
• Properties:
  • It attracts magnetic materials like iron.
  • Its magnetism lasts only as long as the electric current flows through the wire.
• Applications:
  • Electric bells, cranes to lift heavy iron loads, and in medical instruments to remove tiny iron pieces from eyes.

### 7. Electric Bell (विद्युत घंटी)

• Working Principle: Works on the magnetic effect of electric current.
• Components: It consists of an electromagnet, an iron strip with a hammer, a contact screw, and a gong.
• Working Summary: When the circuit is complete, the electromagnet pulls the iron strip. The hammer strikes the gong to make a sound. At the same time, the contact breaks, the current stops, the electromagnet loses its magnetism, and the strip goes back to its original position, repeating the process rapidly.