Magnet and Magnetism


 




Very short Answers:

Q 1. Name the material that is used to make a permanent magnet.
Ans. Steel alloys

Q 2. What are the two ends of a magnet called ?
Ans. Magnetic poles

Q 3. Name the substance that are not attracted by a magnet.
Ans.  Nonmagnetic.  

Q 4. What kind of force experienced when two similar poles are brought together ?
Ans. Repulsive force

Q 5. Which magnetic pole of the earth lies closer to the geographical south pole?
Ans. North pole

Short Answers:

Q 1. Why does a freely suspended bar magnet point in the north-south direction?
Ans. The freely suspended magnet always comes to rest in north – south direction. This is because the earth acts as a big magnet. The north pole of the magnet gets attracted to the south pole of the earth’s field, and south pole of the magnet gets attracted to the north-pole of the earth’s magnetic field.

Q 2. What is magnetic compass ? For what purpose a magnetic compass is used?
Ans. Magnetic compass: A magnetic compass containing a magnetic needle pivoted in a horizontal plane, that indicates the direction of magnetic north at points on the earth's surface. It is used by travellers, sailors and navigators to find the direction.

Q 3. What is meant by magnetic field? On what factors does its strength depend?
Ans. Magnetic field: The region around a magnet where its influence can be felt. The magnetic force of a magnet is maximum near its ends and it decreases as we go towards its mid –point.

Q 4. All conductors of electricity are magnetic substance. Comment on this statement giving reasons.
Ans. No, all the conductors are not made of magnetic material [like iron, cobalt, Nickel]. Some of the best conductors like Silver, Copper, Aluminium are not magnetic. Hence all conductors are not magnetic materials. But current carrying conductor produces magnetic field around them which is used to produce strong electromagnets.

Q 5. How can the self-demagnetisation of magnet be prevented? 
Ans. To prevent self-magnetization, magnets are kept in such a way that their opposite poles are joined. This is done by placing soft iron pieces (called keepers) across both the ends. A wooden piece may be kept in between the two bar magnets.

Long Answers:

Q 1. Define magnetic and nonmagnetic substances. Give one example of each.
Ans. Magnetic Materials: Those materials which are attracted by a magnet are magnetic materials. 
Example: Iron, Nickel, Cobalt and their alloys. 

Non - Magnetic Materials: Those materials which are not attracted by a magnet are called non- magnetic materials. 
Example:  Plastic, Rubber, Water etc. 

Q 2. Write two main properties of a magnet.
Ans. Properties of magnet:

Attractive Property – Magnet attracts ferromagnetic materials like iron, cobalt, and nickel.

Repulsive Property – Like magnetic poles repel each other and unlike magnetic poles attract each other.

Q 3. How can an iron rod be magnetized?
Ans. Magnetizing the Rod:  follow the below steps:

a)  Determine the poles on your magnet. On most magnets, this should be shown with an N on one end and an S on the other.

b)  Decide which pole you want to use to create your magnet. This is purely preference and will not affect the strength of your magnet.

c)   Position the magnet against the iron rod so that only the chosen pole is touching the rod.

d)  Begin at one end of the rod and rub the magnet down the length of the rod in one continuous movement repeat the process 30 to 40 times.

e)  Continue stroking the rod with the magnet until the rod has the desired magnetic strength. You may want to periodically test the magnet, as there is no specific number of strokes to use.

Q 4. Show that a magnet has two poles. What are the properties of the magnetic poles of a magnet?      

Ans. A freely suspended magnet always comes to rest in the north – south direction. This shows that all magnets have two poles.

Properties of magnetic poles:

Attractive Property – Magnet attracts ferromagnetic materials like iron, cobalt, and nickel.

Repulsive Properties – Like magnetic poles repel each other and unlike magnetic poles attract each other.

Directive Property – A freely suspended magnet always points in a north-south direction.

Q 5. Mention three uses of magnets and electromagnets.
Ans. Applications of magnets:
Used in speakers
Used in home as refrigerator magnets hold papers, bottle openers and other small items to the metal refrigerator door. A pocket compass uses a magnetic needle to show which way is north.
Magnets are found in some commonly used medical equipment such as Magnetic Resonance Imaging (MRI) machines.

Applications of Electromagnets: Electromagnets are very widely used in electric and electromechanical devices, including:
Motors and generators.
Transformers.
Electric bells and buzzers

E.  Define the following terms:
1. Magnetic Substance
2. Demagnetisation of magnet
3. Ceramic magnets
4. The poles of a magnet

Ans.
1. Magnetic Substance: Those materials which are attracted by a magnet are magnetic materials. Iron, Nickel, and Cobalt are the magnetic substances as objects made up of these materials are attracted by a magnet.    

2. Demagnetization of a magnet: The process of losing or destroying the magnetic properties of a magnet is called demagnetization of a magnet.

3. Ceramic magnets: Ceramic (ferrite) magnets are made of iron oxide, nickel and beryllium. They are charcoal gray in color and usually appear in the forms of discs, rings, blocks, cylinders.

4. The poles of a magnet: The ends of the magnet where its magnetic force is maximum, are called poles of the magnet or magnetic poles. There are two poles North-Pole and South-Pole. The poles of a magnet occur in a pair and cannot be separated.

D. Tick the Odd-One giving reason

1. Alpins- A magnetic substance and rest are non-magnetic substances.

2. Sawdust -  Others are connected with magnetic effects. 

3. Ironsmith – Term not related to magnet rest are related to history of magnet.

4. Silver – A non-magnetic substance and rest are magnetic substances.

HOTS:

Q 1. You are given a bar magnet. How will you find out the west direction?
Ans: We will tie the magnet with a thin string and suspend it. On suspending it will turn its north pole towards north and south pole towards south. After getting north and south direction we can now easily find out the east or west direction.

Q 2. A pair of bar magnets are kept in a box with keepers at both ends. Would this pair acts as magnet? Give reason in support of your answer.
Ans:  Yes, because the keepers are made up of iron, when the magnet is in contact with it they become temporary magnet. Keepers also have a useful safety function as they stop external metal being attracted to the magnet.
 


 



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