CBSE - VII

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The Magnetic Effect of Electric Current

The Magnetic Effect of Electric Current

When electric current flows through a wire wound around an iron bar, the bar behaves like a magnet.

To view the complete lesson click the video below

Electromagnet

When electric current flows through a wire wound around an iron bar, the bar behaves like a magnet. This magnet is called an electromagnet.
An electromagnet is formed due to the magnetic effect of electric current. This magnetic effect of electric current was discovered by Hans Christian Oersted. Once, while preparing for a lecture, he noticed that there was a deflection in the needle of a magnetic compass kept near a wire that was connected to a battery. This deflection occurred every time the battery was switched on and off. He realised that a magnetic field is created around a current-carrying wire in a circuit.
The components required to create an electromagnet are two pieces of insulated copper wire, a nail, a battery containing two cells, a bulb with a holder, and some paper clips made of iron. The bulb is also used for making an electromagnet since it serves as a resistance to the current in the circuit and it prevents the battery from quick discharge.

Construction of an Electromagnet

  • Take a nail and wind a copper wire around it without any overlap, as shown in the figure.
  • Remove the insulation on the wire at the two ends
  • Connect one end of the wire to the battery and the other to one terminal of the bulb holder.
  • Connect one end of the second wire to the remaining terminal of the bulb holder, and the other end to the battery.
  • Place the paper clips near the nail.
  • When you switch on the current, the paper clips will cling to the nail. This is because the nail becomes an electromagnet.
  • When the battery is disconnected, the nail is no more an electromagnet. The paper clips will not cling to the nail.

Application of Electromagnet

Electromagnets are used in medicines, toys, iron industries, and most commonly in the electric bell. Cranes are used for lifting material, separating iron from scrap in industries, and to lift cars. These work on the principle of the electromagnet. Electromagnets are also used in electromagnetic trains called Maglev’s.

Construction and Working of an Electric Bell

Circuit

  • An electric bell consists of an iron core, on which is wound a wire as a coil.
  • One end of the coil is connected to one terminal of a battery, and the other end to a steel rod that acts like a spring for the hammer touching the screw contact.
  • The other terminal of the battery is connected to the screw contact with a switch in the middle.

Working

  • Electric current flows through the coil when the switch is ON, and the iron core acts as an electromagnet.
  • The iron core attracts the hammer towards it.
  • The hammer hits the bell and produces a sound.
  • The circuit breaks at the screw contact when the hammer moves towards the iron core. At this point, the iron core ceases to be an electromagnet.
  • The hammer is pulled back to its original position due to the spring action of the steel rod, and then touches the contact again to complete the circuit.
  • The circuit is completed and current flows through the coil again, and the hammer strikes the bell again.
  • The process repeats itself and you hear a ringing sound since the hammer keeps hitting the bell, until the switch is released.
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