These DAV Class 7 Science Notes and DAV Class 7 Science Chapter 17 Notes – Electric Current and Its Effects act as excellent revision resources, particularly in preparation for board exams.
Electric Current and Its Effects Class 7 DAV Notes
→ Keating effect of electric current: When electric current flows through a conductor, it can produce heat. This is known as the Heating Effect of Electric Current. This is also known as Joule’s heating effect of electric current.
According to Joule, the amount of heat produced in a conductor depends on following factors:
- Strength of the current through the wire
- Nature of the material of wire
- Length and thickness of wire
- Time for which electric current flows through the wire.
→ Advantages of Heating Effect of Electric Current: Many devices work on heating effect of electric current. These appliances have a ‘heating coil’. The coil is usually made of nichrome wire and is known as element. The coil becomes hot or sometimes red hot due to heating effect of electric current. Due to this, a particular appliance serves some useful purpose for us.
Examples: electric iron, toaster, sandwich maker, geyser, water heater, electric kettle, etc. Electric bulb is a common appliance which works because of heating effect of electric current. The filament of the bulb becomes ‘white hot’ and glows and thus the bulb gives light.
→ Disadvantages of Heating Effect of Electric Current: During transmission of electricity through wires, a major portion of electric current goes waste in the form of heat energy. The heating effect of electric current also breaks the insulation of wiring and various devices.
→ Electric Fuse: Because of the heating effect of electric current, a wire can become hot beyond a limit and can melt down and break. This fact is used for making electric fuse for household circuits and for protection of different electrical and electronic devices.
→ An electric fuse is a safety device. When the electric current goes beyond a limit, the fuse wire melts and breaks the circuit. This helps in protecting the wiring or a device from getting damaged.
→ Following are the reasons for electric current crossing its limit:
- Short circuit: This happens when two wires touch each other.
- Overload: This happens when more than one appliance is connected to a single socket.
- Insulation breakdown: This happens when wires get overheated.
→ MCB (Miniature Circuit Breakers): Electric fuse has been replaced by MCBs in most of the households. MCB is an automatic device which turns off when the current exceeds its safety limit. These switches work on the magnetic effect of electric current.
→ Magnetic Effect of Electric Current: When electric current flows through a conductor, a magnetic field is created around it. This is known as Magnetic Effect of Electric Current. The magnetic effect of electric current was first discovered by Hans Christian Oersted. He observed that when a magnetic compass needle was placed near a current carrying conductor, there was deflection in the compass needle. This happens because of magnetic effect of electric current.
→ Electromagnet: Electromagnet works on the principle of magnetic effect of eiectric current. Electromagnet is made up of a coil which is wrapped around a cylindrical core. When current flows through the coil, the coil behaves like a magnet.
→ Factors affecting the strength of an electromagnet:
- Wrapping the insulated coil around a soft iron core
- Increasing the number of turns in the coil
- Increasing the strength of electric current
→ Practical Uses of Electromagnet:
→ Electromagnets can be made very powerful. Such magnets are used for lifting heavy objects.
- Electromagnets are used in many devices; like microphone, speaker, telephone, etc.
- Electromagnets are used in automatic switches; like in MCBs.
- Electric bell works because of electromagnet.
→ Structure of Electric Bell: An electric bell consists of an electromagnet, a gong, a hammer and electric wires.
→ Working of Electromagnet:
- When current flows through the coil, it becomes electromagnet.
- The electromagnet attracts the soft iron armature. With this, the iron hammer moves and hits the gong to produce sound.
- When the iron hammer and armature move towards the gong, the electric circuit breaks and electromagnet loses its magnetic property.
- The iron armature returns to its original position which helps in completing the circuit again. The process keeps on repeating and the bell produces a continuous ringing sound.
→ Element: The heating coil of an appliance is called element.
→ Short circuit: When a positive wire touches a negative wire, it results in electric spark and sometimes can lead to accidental fire. This is called short circuit.
→ Overload in a circuit: When more than one appliance is attached to a socket, it leads to overload in the circuit.
→ Electric fuse: A safety device which helps in protecting the wiring and electrical appliance.
→ Electromagnet: A coil wrapped around a cylindrical core behaves like magnet when electric current flows through it.
→ Electric bell: A device which produces a ringing sound when electric current flows through it.
→ Miniature Circuit Breaker (MCB): MCBs are now being used in place of electric fuse.
→ Nichrome: An alloy of nickel and chromium. It is used for making resistance coils.