Notes on Magnetism | Grade 7 > Science > Magnetism | KULLABS.COM

Magnetism

  • Note
  • Things to remember
  • Videos
  • Exercise
  • Quiz

Magnetism

Source: www.thinglink.com Fig: Magnet
Source: www.thinglink.com
Fig: Magnet

The first magnet was found from the naturally occurring mineral called magnetite. The ancient Greeks were the discoverers of magnetite. There is a story about a shepherd named Magners whose shoe nails stuck to a rock containing magnetite while roaming on Mt. Ida. Later, it was found that the mountain was made of black stone with iron that had a special property to attract iron. It was an ore of iron called magnetite. Magnetite occurs all over the world, but there are especially large deposits in Scandinavia. The magnet was first used as a practical magnetic compass by the Vikings in their travels to colonise or in war. A magnet is a substance made of a certain material that has the capacity to attract magnetic substances like iron, nickel, cobalt, etc. The energy possessed by a magnet is called magnetic energy. We use the magnet in our daily life activities. It is used in loudspeaker, telephone, radio, telegraph, dynamos, toys, electric motors, etc. Magnet has always two poles, south pole, and the north pole. Like pole repels and unlike poles attract. The most powerful magnet in the universe is actually a star called a magnetar. Earth itself is like a one big bar magnet. Some animals are affected by magnets.
There are two types of magnets which are given below,

  1. Natural magnet:

    Those magnets which are found in nature is called natural magnet. They are found irregular in shape and it has the weaker force of attraction. Lodestone and magnetite are some examples of the natural magnet.

  2. Artificial magnet:

    Artificial magnets are those magnets that are made by humans by using various methods. They are of various shapes and sizes and has the strong force of attraction. Artificial magnets are generally made from special iron or steel alloys which are usually magnetised electrically. They may be bar-shaped, horseshoe-shaped magnetic needles, magnetic compass, etc. There are two types of artificial magnets which are as follows,
    Source: dir.indiamart.com Fig: Magnetic Compass
    Source: dir.indiamart.com
    Fig: Magnetic Compass


    Temporary magnet: Temporary magnets are the magnets that can retain their magnetic properties for the short period of time. They are made of soft magnetic materials like soft iron, steel, etc. They are magnetised by passing an electric current through an insulated wire, It creates the strong magnetic field due to which they are magnetised. They lose their magnetism when the electric current is stopped or external magnetic field disappears. An electromagnet is one of the examples of a temporary magnet. They are used in an electric bell, electric motor, electric fan, etc.

    Permanent magnet: These magnets retain their magnetic properties for the long period of time. They are made of hard magnetic substances like steel, cobalt, iron, etc. They are made of hard ferromagnetic material. They are used in speaker, transistor, radio, tape recorder, etc.

Activity 1
Bring some pieces of iron and a bar magnet. Then keep the pieces of iron near the bar magnet. What do you observe? What do you conclude from this activity?

Activity 2
Bring a bar magnet and suspend it freely with the help of nylon thread and stand. Then, rotate the magnet and observe the direction of the bar magnet after it comes at rest. Does the magnet show north- south direction after coming at rest? What do you conclude from this experiment?

Methods of Making Magnets

There are various methods of making magnets. Magnets of desired shapes and sizes can be made artificially by using magnetic substances. Some common methods of making magnets are as follows,

Rubbing method
The artificial magnet can be prepared by rubbing strong bar magnet over the surface of magnetic substances for several times. It can be done by two methods,

Source: physicsmax.com Fig: Single touch method and double touch method
Source: physicsmax.com
Fig: Single touch method and double touch method
  1. Single touch method: In single touch method, a single magnet is rubbed over the surface of the magnetic substance in only one direction by using the same pole for several times. Magnetic substances like a blade, iron bar, iron clips, etc. can be magnetised.
  2. Double touch method: In double touch method, two magnets are used for rubbing. Here, unlike poles of two magnets are rubbed at the centre of magnetic substances for more than half an hour.

Electrical method:
Electrical method is used for making temporary magnets like an electromagnet. Here, the magnetic substances are magnetised by passing electricity. It consists of a coil made of insulated copper wire that is wound around the magnetic substance. The two ends of the wire are connected with the battery having a switch. When the switch is made on, the current flows through the coil and the magnetic substance becomes a magnet. When the switch is made off, the magnetic substance loses the magnetic properties. The magnets which are made by passing electricity are called electromagnet.

Source: 4mechtech.blogspot.com Fig: Electrical method
Source: 4mechtech.blogspot.com
Fig: Electrical method

Precaution while making electromagnet

  • Instead of using naked wire, insulated copper wire should be used to prevent from short circuit.
  • The winding should be done only from one side.
  • The strength of the electromagnet can be increased by increasing number of turns of coil, amount of electric current and by using a soft magnetic substance.

Activity 3
Bring a bar magnet, pieces of iron and a steel bar. Then rub the surface of the steel bar by using the same pole of the bar magnet. Repeat this action for several times for more than half an hour. Then keep the pieces of iron near the steel bar. What do you observe? What do you conclude from this activity?

Activity 4
Bring a long insulated wire, pieces of irons, soft magnetic substance (bar), battery and a switch. Then wind the wire over the bar for several times, join both ends of the wire with the battery and switch. Now keep some pieces of iron near the nails and turn the switch on. What happens to the pieces of iron when you turn switch on and off ? What do you conclude from this experiment?

Uses of Magnets

  1. They are used in electric motors and generators.
  2. They are used in electric bells.
  3. They are used in refrigerators to keep the door closed.
  4. They are used in TV screens, computer screens, telephones and in tape recorders.
  5. They are used in the magnetic compass.
  6. Magnets are also used in medicines like in MRI.
  7. They are used to lift the heavy loads.



  • The magnet attracts the magnetic substances like iron, cobalt, nickel, etc.
  • The most powerful magnet in the universe is actually a star called a magnetar.
  • Like pole of the magnet repels and unlike poles attract. 
  • Those magnets which are found in nature is called natural magnet like a lodestone.
  • Those magnets which are made by humans by using various methods are called artificial magnet.
  • An electromagnet is an example of the temporary magnet.
  • Permanent magnets are used in speaker, transistor, etc.
  • Magnets can be prepared by rubbing method and electrical method.
.

Very Short Questions

A magnet is a substance which has the property of attracting iron, cobalt, etc. There are two types of magnet which are as follows;

  • Natural magnet: Lodestone
  • Artificial magnet: Bar magnet

There are two types of magnet which are as follows:

  • Temporary magnet: Those magnets which can retain their magnetic properties for the short period of time is called temporary magnet.
  • Permanent magnet: Those magnets which can retain their magnetic properties for long period of time is called permanent magnet.

Any two uses of magnets are as follows,

  • They are used in electric motors and generators.
  • They are used in TV screens, computer screens, telephones and in tape recorders.

Any two precautions while making electromagnet are as follows,

  • Instead of using naked wire, insulated copper wire should be used to prevent from short circuit
  • The winding should be done only from one side.

Permanent magnets are those magnets which can retain their magnetic properties for a long period of time. Any three devices where permanent magnet is used are as follows;

  • Speaker
  • Radio
  • Tape recorder

An electrical method is used for making temporary magnets like an electromagnet. Here, the magnetic substances are magnetised by passing electricity. It consists of a coil made of insulated copper wire that is wound around the magnetic substance. The two ends of the wire are connected with the battery having a switch. When the switch is made on, the current flows through the coil and the magnetic substance becomes a magnet. When the switch is made off, the magnetic substance loses the magnetic properties. The magnets which are made by passing electricity are called electromagnet.

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Any three devices in which electromagnet is used are as follow,

  • Electric bell
  • Loudspeaker
  • Electric motors

The differences between natural magnet and artificial magnet are as follows,

Natural magnet Artificial magnet
These are the magnets found naturally on earth. These magnets are made by humans by using various artificial methods.
These magnets has weak force of attraction. These magnets has strong force of attraction
Example; lodestone and magnetite Example; bar- magnet and magnetic compass

The differences between permanent magnet and temporary magnet are given below,

Permanent magnet Temporary magnet
They retain magnetic properties for the long period of time. They retain magnetic properties for the short period of time.
Permanent magnets have the magnetic properties even if the external field is zero. Temporary magnets have magnetic properties only in the presence of an external magnetic field
They are used in electric motor and electric fan. They are used in radio and tape recorder.

 

The differences between bar magnet and electromagnet are given below,

Bar magnet Electromagnet
A bar magnet is a permanent magnet. The electromagnet is a temporary magnet.
There is no need of electricity to retain the magnetic properties. Electricity is necessary to retain the magnetic properties.
   

0%
  • A magnets attract_________.

    Plastic
    Wood
    Iron
    Stone
  • A freely suspended bar magnet always rests in ___________ direction.

    North- South
    South- East
    East- West
    West- South
  • Which one of the following is a natural magnet?

    Loadstone
    Magnetic Compass
    U- shaped magnet
    Bar magnet
  • Which one of the following is a temporary magnet?

    Compass
    Electromagnet
    Bar magnet
    Magnetic needle
  • How many types of artificial magnets are there?

    6
    5
    2
    3
  • Who discovered magnets?

    Mangers
    Aristotle
    Isaac Newton
    Niels Bohr
  • How many types of magnets are there?

    3
    6
    4
    2
  • In which one of the following, permanent magnet is not used?

    Tape recorder
    Transistor
    Electric bell
    Speaker
  • In which one of the following, temporary magnet is not used?

    Electric motor
    Generator
    Electric fan
    Transistor
  • Which one of the following is not the method of making magnet?

    Double touch method
    Electrical method
    Distillation method
    Single touch method
  • How many types of methods of making magnet are there in our syllabus?

    2
    3
    6
    7
  • The substance which has the property of attracting iron,cobalt or magnetic substances is known as

    Bar magnet
    Non-magnet
    Attracter
    Magnet
  • Which one of the following is not the artificial magnet?

    U- shaped magnet
    Magnetic needle
    Loadstone
    Bar magnet
  • Electromagnets are not used in

    Loud speakers
    Magnetic compass
    Electric bells
    Electric motors
  • The man- made magnet is called

    Artificial magnet
    Natural magnet
    Loadstone
    Magnetite
  • You scored /15


    Take test again

ASK ANY QUESTION ON Magnetism


You must login to reply

Forum Time Replies Report
bigyan


You must login to reply

why is temorary magnets not used in radios or transistors ?


You must login to reply