Class-8 General Science Notes (By Gaba Books)- Unit-14 -MAGNET AND MAGNETISM - SLO-Based Short and Brief Questions and Answers

Class-8 General Science Notes 
(By Gaba Books)
Unit-14 -MAGNET AND MAGNETISM
SLO-Based Short and Brief Questions and Answers

Q. What is a magnet? Explain briefly.

Ans.

Any substance that attracts small pieces of iron, nickel and cobalt and always attains the north-south direction when freely suspended is called a magnet.

The term magnet is derived from magnetite, an ore of iron (Fe2 04), first discovered by the people of Magnesia.

Q. What are magnetic and nan-magnetic substances?

Ans.

Magnetic Substances:

The substances that are attracted by a magnet and also a magnet itself can be made from it are called magnetic substances. Examples: iron, steel or cobalt. 

Non-Magnetic Substances:

The substances that are not attracted by a magnet and also a magnet itself cannot be made from it are called non-magnetic substances. Examples: Silver, Aluminium, etc.

Q. What is an artificial magnet?

Ans.

The man-made magnet is known as an artificial magnet. 

Q. Explain the terms Temporary magnets and Permanent magnets.

Ans.

Temporary magnets:

The substances that can be easily magnetized or demagnetized and cannot maintain their magnetism for a long time are called temporary magnets. Example: Soft iron is easily magnetised. It is also easily demagnetised. Since soft iron loses its magnetism easily. Magnets made from soft iron are temporary magnets.

Permanent magnets:

The substances that cannot be easily magnetized or demagnetized and can maintain their magnetism for a long time are called permanent magnets. Example: Steel is not easily magnetised. Neither it is easily demagnetised. 

Steel retains its magnetism for a long period, magnets made from it are called permanent magnets.

Q. List four properties of a magnet.

Ans.

A magnet has the following properties:

1. Poles of a Magnet:

A magnet has two poles i.e. North and South poles. The force of attraction of a magnet is maximum at its poles.

2. Directional Properties:

Magnet has a directional property i.e. when a magnet is suspended freely, it comes to rest points towards north-south direction.

3. Like Poles Repel, Unlike Poles Attract

The same poles of two magnets repel each other. However, opposite poles attract each other.

4, Magnetic Poles always exist in Pairs:

Magnetic Poles always exist in Pairs and cannot be separated. When a bar magnet is broken into two pieces, each piece behaves as a whole (single) magnet.

Q. Show or prove that the attraction of a magnet is greater at its pole.

Ans.

Spreading out some iron filings over a sheet of paper. Now by moving a bar magnet in the filings, taking care that all parts of the magnet touch the filings. Picked up the magnet and observed that most of the iron filings clung near the ends of the magnet while there were very few iron filings near the middle. This proves that the poles of the magnet have greater magnetic force than that in the middle.

Q. Prove or show that a freely suspended or floating magnet always comes to rest in the north-south direction.   

Ans.

Took a bar magnet and suspended it freely or floated it on a block of wood in water. The magnet comes to rest after some time. Disturb it a little from its position of rest and see what happens. It is observed that the magnet always comes to rest in the north-south direction.

Q. How will you mark the north and south poles on an unmarked Bar magnet?

Ans.

The end of the freely suspended magnet that points towards the north is called the North Pole (N-Pole) and the other end of the magnet pointing towards the south is called the South Pole (S-Pole)

Q. Show or prove that like poles of a magnet repel each other while unlike poles attract each other.

Ans.

Took two bar magnets. Suspended the one magnet with a piece of thread. After it rests in the north-south direction. Now by bringing the north pole of the magnet in hand near the north pole of the suspended magnet, it is observed that the north pole of the suspended magnet is pushed away i.e. repelled.

Now on bringing the south pole of the magnet in the hand near the north pole of the Suspended magnet. It is attracted towards the south pole. So it is concluded from this activity that in magnets like poles repel and unlike poles attract each other.

Q. What is magnetic induction?

Ans.

The phenomenon by which an ordinary piece of iron temporarily acquires magnetic properties, due to the presence of another magnet near it, is called magnetic induction.

Q. Show magnetic induction by simple experiment. 

Ans. 

Took some iron filings in a beaker. Fixed an iron nail in a wooden stand over iron filings. On bringing near a magnet one pole close to the iron nail. It is noticed that iron nail starts attracting iron filings. This is because the iron nail becomes magnetised by the magnet kept close to it. Now, on removing the magnet, the iron filings fell down. This shows that as soon as the magnet is removed the nail loses its magnetism.

Q. What are magnetic lines of force? show how magnetic lines of force can be mapped?

Ans.

The lines that are formed around the magnet, indicating the region in which the force of the magnet can be detected are called the magnetic lines of force.

Experiment:

Placed a white piece of cardboard on top of a bar magnet. Now carefully sprinkle iron filings over it. By giving gentle tapping, the iron filings arranged to form lines are known as the magnetic lines of force. Magnetic lines of force pass from north to south and they never cross each other.

Q. What is a magnetic field?

Ans.

The area or region within which the effect of magnetic lines of force can be felt is called a magnetic field.

Q. Write the characteristic of magnetic lines of force

Ans.

The magnetic lines of force:

• travels from the north to the south pole of a magnet.
• do not divide into two or more lines.
• do not touch or cross over each other.
• repel each other sideways.
• become more crowded when a magnetic substance is placed in their path.

Q. What is the relationship between current and magnetism?

Or What is an electromagnet?

Ans.

A current is flowing through a wire, or a conductor produces a magnetic field around it. The magnet which is produced by passing an electric current through a conductor is called an electromagnet.

Q. Which type of magnetic field will be formed by a current carrying a long coil.

Ans. 

When a current flows through a long coil, it creates a magnetic field similar to that of a bar magnet.

Q. What is the difference between geographic poles and magnetic poles of the earth?

Ans.

Earth’s geographic poles and magnetic poles are in different places.

Earth’s geographic poles are the points on the Earth's surface that are fixed by the axis of Earth’s rotation where lines of longitude converge.

Magnetic poles, on the other hand, are where the magnetic lines of attraction enter Earth. The Magnetic north and south poles represent lines of magnetic force flowing into Earth in the northern hemisphere, and out of Earth in the southern hemisphere

Q. What is a magnetic meridian?

Ans.

 The circle drawn through the magnetic poles is called the magnetic meridian.

 Q. What is a geographic meridian?

Ans.

The circle drawn through the geographic poles is called the geographic meridian. 

Q. What is a compass? 

Ans.

A compass is an instrument that contains a magnetized pointer and shows the direction. 

Q. What is a pocket compass? explain.

Ans. 

A pocket compass is basically a needle-shaped magnet which is voted on a piece of circular cardboard. The cardboard has the four directions marked on it in a clockwise direction. The magnet always takes a north-south direction, no matter what direction the compass is held in the hand. To read the compass, the north marked on the compass must be in line with the magnet. Only under this condition me compass will give a true indication of direction. 

Q. What is a mariner's compass?

Ans. 

On the mariner's compass, all the directions on the cardboard of the mariner's compass are marked in a clockwise direction. Several magnetized strips, with their north poles facing the north marked on the card, are stuck underneath the card. The card is placed in a bowl containing mercury so that as the ship rolls, the card always remains horizontal. The markings on the compass card show the direction in which the ship is sailing.

Q. Write some use of magnets.

or

Q. Write the name of some instruments that uses magnets.

Ans

Magnets are used in making:

1. Compass: for finding directions.
2. Electrical instruments: for measuring electric currents and voltage, e.g. ammeters and voltmeters.
3. Dynamos: which produce electricity. Power stations generate large amounts of electricity using huge dynamos.

Electromagnets are used in:

• Large cranes: These magnets attract and move heavy loads of scrap iron and iron sheets. The electromagnet only lifts the loads when a current is flowing through it. Once the current is switched off, it ceases to be a magnet and drops the load. In this way, the load can be moved from one spot to another.

• Telephone and electric bells to produce ringing sounds.
• Removing foreign bodies: such as iron nails and other magnetic substances from a patient in the hospital.
• Magnetic separators: to separate magnetic substances (iron, steel, nickel and cobalt) from non-magnetic substances e.g. tin, brass and copper.

Q. Name some devices in which electromagnets are commonly used.

Ans.

Devices in which electromagnet is commonly used are electric motors, diaphragm or cones diaphragms of a loudspeaker, coil of measuring instruments, etc. 

Q. What is an electric motor? Explain how it is working?

Ans.

An electric motor is a device that converts electrical energy into mechanical energy. 

Working:

It consists of a magnet and a coil which can rotate around its axis. When an electrified coil is placed in a magnetic field, forces act on its two sides in opposite directions. The two forces rotate the coil until it becomes parallel to the magnetic field. As soon as the coil becomes parallel to the

magnetic field, the direction of the current is reversed so that the two forces continue to rotate the coil and produce mechanical energy. The turning effect produced by the two forces acting on the coil can be increased by:

• increasing the current in the coil
• increasing the number of turns of the coil
• using a stronger magnet and
• using a bigger coil.

Q. What is a loudspeaker? Explain how it is working?

Ans.

It is a device that changes electrical energy into sound. 

Working:

It consists of a magnet, a coil and a paper cone. A hollow coil is fixed at the centre of the paper cone. The paper cone is so adjusted that the coil can move over a cylindrical magnet. When the coil of the loudspeaker is connected to an amplifier, it moves in and out due to the changing current from the amplifier. The movement of the coil also causes the cone to vibrate with it. As the cone vibrates, it produces sound.

Q. What is an electric generator? Explain how is electricity produced by an electric generator?

Ans.

An electric generator is a device which changes mechanical energy into electrical energy. 

Working: 

A simple generator is an alternating current generator. A coil is rotated about its axis in the magnetic field of a magnet. Each end of the coil is connected with a separate slip ring. The coil and the two slip rings all are mounted on the same shaft. The carbon brushes are used which touch the sip rings to make contact with them. As the coil rotates the magnetic field crossing the coil also changes. This change in the magnetic field generates electricity across the ends of the coil.

Q. What is a microphone? Name it types.

Ans.

A microphone is a device that converts sound energy into electrical energy. There are different types of microphones such as carton microphones, magnetic microphones crystal microphones, capacitor microphones, etc.

Q. Explain the working of a microphones?

Ans.

When someone speaks before a microphone. its aluminium diaphragm vibrates by the sound waves falling over it. This wares the pressure on the carbon granules between the moveable carbon dome and the fixed carbon cap. This varies the current passing through the microphone in the same way as the variation in the sound.

Q. What is a telephone? Explain the Use of microphones in telephones.

Ans.

A telephone is a device which is used to communicate from distance. 

Working:

It consists of a microphone, earphone, alarm system and a dial system.

The mouthpiece contains carbon powder packed between the two plates. One of the plates is connected with a thin metallic diaphragm. When someone speaks before the microphone. the diaphragm vibrates according to the sound waves. This varies the pressure on the carbon particles which results in the variation in the electrical resistance of the carbon powder between the metal plates. Thus, sound waves produce a variation in the electric current.

Q. Explain the working of an earphone.

Ans.

In the earphone, electromagnet is used to convert electrical signals into sound. It consists of an electromagnet and a thin iron disc. When varying electric current passes through the coil of electromagnet, it causes the iron disc to vibrate according to the vibrations in the electric current. As the disc vibrates, it produces sound.

\\Exercise Q/A

Answer these questions:

TEST YOUR KNOWLEDGE

Q. Briefly explain the term magnet.

Ans.

Any substance that attracts small pieces of iron, nickel and cobalt and always attains the north-south direction when freely suspended is called a magnet.

The term magnet is derived from magnetite, an ore of iron (Fe2 04), first discovered by the people of Magnesia.

Q. What are magnetic and non-magnetic substances? Give two examples of each.

Ans.

Magnetic Substances:

The substances that are attracted by a magnet and also a magnet itself can be made from it are called magnetic substances. Examples: iron, steel or cobalt. 

Non-Magnetic Substances:

The substances that are not attracted by a magnet and also a magnet itself cannot be made from it are called non-magnetic substances. Examples: Silver, Aluminium, etc.

Q. State four important properties of a magnet.

Ans.

The four important properties of a magnet are:

1. Poles of a Magnet:

A magnet has two poles i.e. North and South poles. The force of attraction of a magnet is maximum at its poles.

2. Directional Properties:

Magnet has a directional property i.e. when a magnet is suspended freely, it comes to rest points towards north-south direction.

3. Like Poles Repel, Unlike Poles Attract

The same poles of two magnets repel each other. However, opposite poles attract each other.

4, Magnetic Poles always exist in Pairs:

Magnetic Poles always exist in Pairs and cannot be separated. When a bar magnet is broken into two pieces, each piece behaves as a whole (single) magnet.

Q. Show or prove that the attraction of a magnet is greater at its pole.

Ans.

Spreading out some iron filings over a sheet of paper. Now by moving a bar magnet in the filings, taking care that all parts of the magnet touch the filings. Picked up the magnet and observed that most of the iron filings clung near the ends of the magnet while there were very few iron filings near the middle. This proves that the poles of the magnet have greater magnetic force than that in the middle.

Q. How will you find the poles of a magnet? Explain briefly.

Ans.

Took a bar magnet and suspended it freely or floated it on a block of wood in water. The magnet comes to rest after some time. Disturb it a little from its position of rest and see what happens. It is observed that the magnet always comes to rest in the north-south direction. The end of the freely suspended magnet that points towards the north is called the North Pole (N-Pole) and the other end of the magnet pointing towards the south is called the South Pole (S-Pole)

Q. Describe an experiment to show that like poles repel and unlike poles attract each other.

Ans.

Took two bar magnets. Suspended the one magnet with a piece of thread. After it rests in the north-south direction. Now by bringing the north pole of the magnet in hand near the north pole of the suspended magnet, it is observed that the north pole of the suspended magnet is pushed away i.e. repelled.

Now on bringing the south pole of the magnet in the hand near the north pole of the Suspended magnet. It is attracted towards the south pole. So it is concluded from this activity that in magnets like poles repel and unlike poles attract each other.

Q. With the help of an experiment, explain magnetic induction.

Ans.

Took some iron filings in a beaker. Fixed an iron nail in a wooden stand over iron filings. On bringing near a magnet one pole close to the iron nail. It is noticed that iron nail starts attracting iron filings. This is because the iron nail becomes magnetised by the magnet kept close to it. Now, on removing the magnet, the iron filings fell down. This shows that as soon as the magnet is removed the nail loses its magnetism.

Q. What do you understand by a temporary magnet and a permanent magnet?

Ans.

Temporary magnets:

The substances that can be easily magnetized or demagnetized and cannot maintain their magnetism for a long time are called temporary magnets. Example: Soft iron is easily magnetised. It is also easily demagnetised. Since soft iron loses its magnetism easily. Magnets made from soft iron are temporary magnets.

Permanent magnets:

The substances that cannot be easily magnetized or demagnetized and can maintain their magnetism for a long time are called permanent magnets. Example: Steel is not easily magnetised. Neither it is easily demagnetised. 

Steel retains its magnetism for a long period, magnets made from it are called permanent magnets.

Q. Define a magnetic field. Describe how the lines of force of a magnetic field can be drawn on a paper.

Ans.

The area or region within which the effect of magnetic lines of force can be felt is called a magnetic field.

Experiment:

Placed a white piece of cardboard on top of a bar magnet. Now carefully sprinkle iron filings over it. By giving gentle tapping, the iron filings arranged to form lines are known as the magnetic lines of force. Magnetic lines of force pass from north to south and they never cross each other.

Q. Which type of magnetic field will be formed by a current-carrying long coil? Explain it by describing an activity.

Ans.

When a current flows through a long coil, it creates a magnetic field similar to that of a bar magnet.

Take a coil of insulated wire about 4 inches long. Make a slit at the centre of a cardboard and place the wire in it, as shown in the figure. Join the two ends of the coil to a battery of two cells and a switch. This will make an electromagnet. With the help of a magnetic compass needle, the lines of magnetic field can be drawn on the card by a pencil. Mark the N and S poles by such a way that when the end of coil that attracts the north end of the needle will be the S-pole and the end of coil, which attracts the south end of the needle will be N-pole of the electromagnet.

Q. Explain the working principle of an electric motor.

Ans.

An electric motor is a device that converts electrical energy into mechanical energy. 

Working:

It consists of a magnet and a coil which can rotate around its axis. When an electrified coil is placed in a magnetic field, forces act on its two sides in opposite directions. The two forces rotate the coil until it becomes parallel to the magnetic field. As soon as the coil becomes parallel to the

magnetic field, the direction of the current is reversed so that the two forces continue to rotate the coil and produce mechanical energy. The turning effect produced by the two forces acting on the coil can be increased by:

• increasing the current in the coil
• increasing the number of turns of the coil
• using a stronger magnet and
• using a bigger coil.

Q. Describe the working of an electric generator.

Ans.

An electric generator is a device which changes mechanical energy into electrical energy. 

Working: 

A simple generator is an alternating current generator. A coil is rotated about its axis in the magnetic field of a magnet. Each end of the coil is connected with a separate slip ring. The coil and the two slip rings all are mounted on the same shaft. The carbon brushes are used which touch the sip rings to make contact with them. As the coil rotates the magnetic field crossing the coil also changes. This change in the magnetic field generates electricity across the ends of the coil.

Q. What do you know about a microphone.

Ans.

A microphone is a device that converts sound energy into electrical energy. There are different types of microphones such as carton microphones, magnetic microphones crystal microphones, capacitor microphones, etc.

When someone speaks before a microphone. its aluminium diaphragm vibrates by the sound waves falling over it. This wares the pressure on the carbon granules between the moveable carbon dome and the fixed carbon cap. This varies the current passing through the microphone in the same way as the variation in the sound.

Q. How does a loudspeaker produce sound?

Ans.

It is a device that changes electrical energy into sound. 

Working:

It consists of a magnet, a coil and a paper cone. A hollow coil is fixed at the centre of the paper cone. The paper cone is so adjusted that the coil can move over a cylindrical magnet. When the coil of the loudspeaker is connected to an amplifier, it moves in and out due to the changing current from the amplifier. The movement of the coil also causes the cone to vibrate with it. As the cone vibrates, it produces sound.

Q. How does a telephone work?

Ans.

A telephone is a device which is used to communicate from distance. 

Working:

It consists of a microphone, earphone, alarm system and a dial system.

The mouthpiece contains carbon powder packed between the two plates. One of the plates is connected with a thin metallic diaphragm. When someone speaks before the microphone. the diaphragm vibrates according to the sound waves. This varies the pressure on the carbon particles which results in the variation in the electrical resistance of the carbon powder between the metal plates. Thus, sound waves produce a variation in the electric current.

Q. State four important uses of magnets.

Ans.

Magnets are used in making:

1. Compass: for finding directions.
2. Electrical instruments: for measuring electric currents and voltage, e.g. ammeters and voltmeters.
3. Dynamos: which produce electricity. Power stations generate large amounts of electricity using huge dynamos.
4. Large cranes: These magnets attract and move heavy loads of scrap iron and iron sheets. The electromagnet only lifts the loads when a current is flowing through it. Once the current is switched off, it ceases to be a magnet and drops the load. In this way, the load can be moved from one spot to another.

Fill in the following blanks:
Magnetic field of a magnet is stronger at
The magnetic field is
Magnetic field is present around a
at its middle.
carrying conductor.
field.
A force acts on a wire carrying current when placed in a
into sound.
The receiver of a telephone converts the varying into sound with the heith
A loudspeaker converts electric
6.
of an electromagnet
7. Poles of a magnet always exist i
8.
A magnetic compass is a simple device which has alle
9. Lodestone is a natural magnet which is an ore of..................................
10.
are used in a dynamo
C. Decide whether the following statements are TRUE FALSE:
1.
A magnet pulls small objects of copper and brass towards it.
2. The magnetic field of a current-carrying straight wire is in the form of concentric circles.
3. The magnetic field of a current-carrying long coil is similar to that of a bar magnet.
4. The electric motor and electric generator work on the same principle.
5. A speaker converts the electrical vibrations into the sound vibration.
6.
The earth behaves like a large magnet.
7. Nickle is a non magnetic substance.
8. Electromagnets are permanent magnets.
9. A magnet attracts all metals towards itself.
10. A magnet has only one pole.
D. Give one word answer of the following questions:
1. The most popular metal used to make a magnet.
2. Write the name of one substance which is non-magnetic.
3. Which metal is not easily magnetised?
4. In which direction do the lines of force travel?
5. Which thing is itself a large magnet?
Tick mark (✔) the most appropriate answer for the following statements:
Magnetic force is more:
near the poles of the magnet.
c. away from the magnet.
If current is passing through a straight wire:
1. a current starts flowing in other objects.
c. there is no effect on the compass needle.
Magnetic lines of force:
a. cross one another
c. are always parallel.
A microphone:
a. converts electric current into sound.
b. converts electrical energy into mechanical energy.
c. converts sound into current vibrations.
d. is a device used to make an electromagnet.
Discovery of magnetism was made by the people of
a. China
c. Malaysia
b. at the middle of the magnet.
d. far away from the magnet.
b. enter into the north pole.
d. magnetic field is produced.
b. enter into the north pole.
d. are always directed in a straight line.
b. Magnesia
d. England