Selina Concise Class 9 Physics Solution Chapter No. 10- ‘Magnetism’ For ICSE Board Students.
Exercise-A Solution
1.) Solution:
Ans:
Loadstone is the first known magnet which is an ore of iron oxide located in large amount in Magnesia.
2.) Solution:
Ans:
- Natural magnets are the pieces of loadstone which are found in nature.
- The limitations of natural magnets are as given below:
- Natural magnets are irregular and odd in shapes.
- Natural magnets are not enough magnetically strong for use.
3.) Solution:
Ans:
- Artificial magnets are made by magnetising the piece of iron.
- Artificial magnets are prepared from iron in different, convenient size and also in shapes such as bar magnets, horse shoe magnets, magnetic needle, magnetic compass etc all are the artificial magnets which are made by magnetising iron pieces.
- Although natural magnets are there but we need artificial magnets because natural magnets are irregular and odd in shapes.
- Also natural magnets are not strongly magnetised for use.
- Because of these reasons we need artificial magnets.
4.) Solution:
Ans:
- When we place a bar magnet near to an iron rod it get magnetised due to magnetic induction produced.
- While when we place the bar magnet near the copper rod it doesn’t get magnetised.
- So we can which rod is made of iron or copper.
5.) Solution:
Ans:
- We already know that, the magnetic bar attracts the iron pieces and when it is suspended freely in air then it rest in only north-south direction.
- Hence, when the magnet bar is suspended freely in air it rest in north-south direction from which we can say that it is the bar magnet only.
- But, when we suspended freely the soft iron bar then it will rest in any direction which indicates that it is not the bar magnet.
6.) Solution:
Ans:
a) Two ends of a magnet are called poles.
b) Unlike poles of a magnet attract each other.
c) Like poles of a magnet repels each other.
d) A freely suspended magnet rest in geographic north-south direction.
7.) Solution:
Ans:
When a small magnet is suspended by a silk thread from a rigid support such that it can swing freely then the magnet will rests in geographic north-south direction as shown in the following figure.
8.) Solution:
Ans:
- When we place the magnetised bar made from magnetic material AB near or in contact with the magnet as shown in figure then the bar AB also acts as the magnet.
- That means it attracts the iron feelings when they are kept near to bar AB.
- Now, if we removed the bar magnet from bar AB then all the iron feelings get removed from the bar AB i.e. the bar AB loses its magnetism.
- Thus, the bar of magnetic material AB acts as a magnet when kept near to the magnet, the magnetism induced in it is called as induced magnetism.
- Thus, we can define the induced magnetism as it is the temporary magnetism induced in the magnetic material when it is kept near or in contact with the bar magnet.
12.) Solution:
Ans:
- We know that, when a small iron bar is kept near the north pole of the bar magnet then magnetisation will be induced in the soft iron bar due to magnetic induction produced.
- That means, the soft iron bar AB also behaves like a magnet.
- After magnetisation the soft iron bar AB acquires the polarity as shown in the following figure.
- Again if we removed the bar magnet from the soft iron bar then there will no magnetisation remain in the soft iron bar and hence it loses all the magnetic properties i.e. magnetism.
13.) Solution:
Ans:
- As we know that, induced magnetism is the magnetisation induced by the magnet within the magnetic material when placed near to it.
- The magnetic material behaves as a magnet till there will be contact of the bar magnet and if we removed the bar magnet from the vicinity of the magnetic bar then the magnetisation induced will be loses. So we can say that, induced magnetism is the temporary.
14.) Solution:
Ans:
- As we know that, when a magnetic material is kept near or in contact with the bar magnet then there will be first magnetisation will be induced in the magnetic material which is only due to magnetic induction.
- And hence the magnetic material acts as the magnet after which there will be attraction between these two magnet takes place.
- Hence, we can say that magnetic induction proceeds attraction.
15.) Solution:
Ans:
A magnetic field lines is the continuous curve in the magnetic field in a such way that if we draw a tangent at any point on this curved line then that tangent gives the direction of magnetic field at that point.
16.) Solution:
Ans:
The following are the properties of magnetic field lines:
- Magnetic field lines are closed and continuous curves.
- Inside the magnet, magnetic field lines are directed from north pole to the south pole of the magnet.
- If we draw a tangent at any point on the magnetic field lines then the that tangent gives the direction of magnetic field at that point.
- The magnetic field lines never intersect each other.
17.) Solution:
Ans:
We know that,
When iron fillings are sprinkled on a sheet of cardboard placed over a bar magnet then due to magnetic induction the iron feelings also acts as the small tiny magnets which are then experiences a force in the direction of magnetic field of the bar magnet at their position or location and hence take a definite pattern by aligning themselves along a curved lines.
18.) Solution:
Ans:
Following is the method of plotting magnetic field lines by using small compass needle:
- Initially we fix the paper sheet on the drawing board with the help of board pins.
- Now, we have placed the small compass needle at position 1 as shown in the figure a).
- Now, looking from the top side of needle we have drawn two pencil dots which are exactly at the two ends of the needle.
- After that, we have placed the needle at position 2 in order to make one end of the needle will coincide with the second pencil dot.
- In this way, we have repeated the process and kept needle at position 3,4…and we have drawn the several pencil dots.
- Now it is observed that, when we join the several dots we will get the straight line. In this way one line of magnetic filed of earth will be drawn. After repeating the above procedure we can draw the number of magnetic lines.
- And these lines are directed from geographic south towards the geographic north as shown in figure which gives the direction of magnetic field lines.
- Finally, we get the lines which are drawn in the figure b).
19.) Solution:
Ans:
The magnetic field lines never intersect one another. Because, if two magnetic field lines get intersected then there will be two directions of magnetic field at that point and which is never possible.
Hence, two magnetic field lines never intersect each other.
20) Solution:
a)
(b)
21.) Solution:
Ans:
The following are the two evidences of the existence of earth’s magnetic field:
- The freely suspended magnetic needle always rests in the geographic north-south direction.
- By plotting the magnetic field lines of the magnet we can get the neutral points on them.
22.) Solution
Ans:
Following figure shows the four magnetic field lines as obtained in a limited space in a horizontal plane in the earth’s magnetic field alone.
23)
a)
b)
Ans:
The magnetic field lines shown in part a) are the non-uniform magnetic field lines.
24)
a)
The following figure shows as given in part a)
b) The following figure indicates the likely positions of the neutral points as indicated by crosses.
c) At neutral points, the magnitude of magnetic field would be zero because at the neutral points the magnetic field of the magnet and the earth’s horizontal magnetic field both are equal but their directions are opposite to each other. Hence, magnitude of the magnetic field at each neutral point will be zero.
25.) Solution:
Ans:
The magnetic field at that point should be zero. Because at that point the earth’s magnetic field is neutralized by the magnetic field of some other magnetic material.
26.) Solution:
Ans:
- Neutral points are those points at which magnetic field is zero. Because at neutral points the magnetic field of magnet and the earth’s horizontal magnetic field both are equal but oppositely directed hence resultant magnetic field would be zero.
- As the resultant magnetic field at neutral point is zero hence magnetic compass needle remains unaffected at neutral points from which we can locate the position of neutral points.
27.) Solution:
Ans:
a) neutral points will be in the east- west direction.
b) neutral points will be in the north – south direction.
28.) Solution:
Ans:
a) If the field lines in a magnetic field are parallel and equidistant, the magnetic field is
b) At a neutral point, the resultant magnetic field is zero.
c) The neutral points of the bar magnet kept with its north pole pointing towards the geographic north are located on either sides of the magnet in East and West direction.
Multiple choice type:
1.) Two like magnetic poles
Ans: a) repel each other
2.) In a uniform magnetic field, the magnetic field lines are
Ans: b) parallel and equidistant straight lines
Exercise-B
1.) What is an electromagnet.
Ans: An electromagnet is made from the piece of soft iron when current is passed through the coil which is wound around it and it is an artificial temporary strong magnet.
2.) Name the material used for preparing electromagnet.
Ans: The material used for preparing an electromagnet is the soft iron pieces.
3.) How is an electromagnet made? Name two factors on which the strength of the magnetic field of an electromagnet depends.
Ans:
- Electromagnets are prepared by winding the insulated copper wire around the soft iron pieces.
- Electromagnets may be made in the shape of solenoid or U-shaped.
- The strength of electromagnets depends on the following factors:
- The number of turns are wounded around the coil.
- And the current flowing through the wire.
4.) You are required to make an electromagnet from a soft iron bar by using a cell, an insulated coil of copper wire and a switch.
a) Draw a circuit diagram to represent the process.
Ans:
5.) Following figure shows the coil wound around the soft iron bar XY.
a) State the polarity at the ends X and Y as the switch is pressed.
b) Suggest one way of increasing the strength of the electromagnet so formed.
Ans: a) When the switch is pressed the polarity at X is the south pole and the polarity at Y is the north pole.
b) The strength of the electromagnet so formed here can be increased by increasing the current flowing through the coil.
6.) A coil of insulated copper wire is wound around a piece of soft iron and current is passed in the coil from a battery. What name is given to the device so obtained? Give one use of the device mentioned by you.
Ans:
- Here, the device formed will be electromagnet.
- In scientific research, to study the magnetic properties of the substance in magnetic field electromagnet are used.
7.) Show with the aid of a diagram how a wire is wound on U – shaped piece of soft iron in order to make it an electromagnet. Complete the circuit diagram and label the poles of the electromagnet.
Ans:
8.) State the two ways through which the strength of an electromagnet can be increased.
Ans: The strength of the electromagnet can be increased by following ways:
- By increasing the number of turns of windings in the solenoid.
- And by increasing the current through the solenoid.
9.) Name one device that uses an electromagnet.
Ans: An electric bell is the device which uses electromagnet.
10.) State two advantages of an electromagnet over a permanent magnet.
Ans: Following are the advantages of electromagnet over permanent magnet:
- An electromagnet can produces the strong magnetic field.
- By changing the number of turns or current through the solenoid the strength of the magnetic field of an electromagnet can be changed.
- By reversing the direction of current through the solenoid we can reverse the polarity of an electromagnet.
11.) State two differences between electromagnet and permanent magnet.
Ans: Electromagnet:
- Electromagnets are made from soft iron.
- The strength of magnetic field of an electromagnet can be changed.
- The magnetic field strength of an electromagnet can be very strong.
- We can also reverse the polarity of an electromagnet.
Permanent magnet:
- Permanent magnet are made from steel.
- Permanent magnet produces permanent magnetic field.
- And hence magnetic field strength of permanent magnet cannot be changed.
- Magnetic field of permanent magnet will not be strong.
- The polarity of permanent magnet cannot be reversed.
12.) Why is the soft iron is used as core of the electromagnet in an electric bell.
Ans:
- When an electric current flows through the solenoid then only soft iron bar shows magnetic properties.
- And when there is no electric current is passed through the solenoid then soft iron bar loses its magnetic properties.
- Due to this reason, soft iron is used as core of the electromagnet in an electric bell.
13.) How is the working of an electric bell affected, if an alternating current is used instead of direct current.
Ans:
- If we used an alternating current source instead of battery then the core of electromagnet may get magnetized but polarity will be changes at its ends.
- As the attraction of armature doesn’t depends on the polarity of the electromagnet hence the bell will ring when switch k is pressed.
14.) Name the material used for making armature of an electric bell. Give a reason for your answer.
Ans: The material used for making armature of an electric bell is soft iron and it induces magnetism suddenly.
Multiple choice type:
1.) Electromagnets are made up of
a) steel
b) copper
c) soft iron
d) aluminium
Ans: c) soft iron
2.) The strength of the electromagnet can be increased by
a) reversing the direction of current.
b) using alternating current of high frequency.
c) increasing the current in the coil
d) decreasing number of turns of a coil
Ans: c) increasing the current in the coil.