Samacheer Kalvi 8th Science Solutions Chapter 7 Pdf
Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 7: Tamilnadu State Board Solution Class 8 Science Chapter 7 – Magnetism.
Samacheer Kalvi 8th Science Solutions Chapter 7: Overview
Samacheer Kalvi 8th Science Solutions Chapter 7 Pdf
(I) Choose the best answer.
(1 ) A magnet attracts iron and steel .
a ) wooden materials
b) any metal
d) iron and steel
(2) One of the following is an example for a permanent magnet.
a ) Electromagnet
c) Soft iron
(3) The south pole of a bar magnet and the north pole of a U- shaped magnet will attract each other.
a ) attract each other
b) repel each other
c) neither attract nor repel each other
d) None of the above
(4) The shape of the Earth’s magnetic field resembles that of an imaginary bar magnet .
a) U – shaped magnet
b) straight conductor carrying current
c) solenoid coil
d) bar magnet
(5) MRI stands for Magnetic Resonance Imaging
a) Magnetic Resonance Imaging
b) Magnetic Running Image
c) Magnetic Radio Imaging
d) Magnetic Radar Imaging
(6) A compass is used for All of these .
a) plotting magnetic lines
b) detection of magnetic field
d) All of these
(II) Fill in the blanks.
1 ) The Magnetic strength is maximum at the poles.
2) A magnet has two magnetic poles.
3) Magnets are used in dynamos for generating electricity.
4) Electromagnets are used to lift heavy iron pieces.
5) A freely suspended bar magnet is always pointing along the geographic north – south direction.
(III) Match the following .
|A tiny pivoted magnet||Natural magnet|
|Closed curves||Ferromagnetic material|
|A tiny pivoted magnet||Compass box|
|Closed curves||Magnetic lines|
(IV) Consider the statements given below and choose the correct option.
1 ) Assertion : Iron fillings are concentrated more at the magnetic poles.
Reason : The magnets are so sharp .
Answer : b) Both assertion and reason are true ,but reason is not the correct explanation of assertion.
There are more concentration of iron fillings at the magnetic poles because there are greater magnetic strength at the ends of the magnet. For this reason, the iron fillings tends to cling more at the poles of the magnet.
2) Assertion : The Earth’s magnetic field is due to iron present in its core.
Reason : At a high temperature a magnet loses its magnetic property or magnetism.
Answer : d) Assertion is false, but reason is true.
Explanation : The electric currents in the liquid outermost core is responsible for the Earth’s magnetic field.
(V) Answer briefly .
1 ) Define magnetic field.
The field of influence of the bar magnet is represented by the space around a bar magnet where there is existence of the arrangement of iron fillings. This is known as the magnetic field. The space around a magnet where its magnetic influence or effect is noticed is called magnetic field. The unit in which it is measured is represented as tesla or gauss . 1 tesla is equivalent to 10000 gauss.
2) What is artificial magnet? Give examples.
Artificial magnets refers to those magnets that are composed by people in the factory or laboratory. These are also called man-made magnets. Natural magnets are not so stronger when compared to the artificial magnets. There are several shapes and dimensions in which artificial magnets can be constructed. Bar magnets, U – shaped magnets, horseshoe magnets , cylindrical magnets, disc magnets, ring magnets and electromagnets are some of the perfect examples of artificial magnets. Artificial magnets are mainly composed of iron, nickel ,cobalt and steel. In order to make artificial magnets, alloy of the metals , Neodynium and Samarium are also used.
3) Distinguish between natural and artificial magnets?
1 ) Natural magnets possess irregular shapes and dimensions and they are mostly found in nature.
2) It is difficult to alter the strength of a natural magnet and it is adequately determined.
3) These magnets lasts for quite a long period of time.
4) These magnets are not used to a great extent.
1 ) There are various shapes and dimensions in which they can be constructed .As artificial magnets are made by people ,for this reason they are called man-made magnets.
2 ) Only a specific strength is required in order to construct these artificial magnets.
3) Their properties are restricted to a certain time period.
4) These magnets are used to a considerable extent in our day to day life activities.
4) Earth acts as a huge bar magnet . Why? Give reasons.
Along the geographical north -south direction, a magnetic needle which is freely suspended comes to rest at a point on the Earth. This clearly depicts that the Earth acts as a huge magnetic dipole, as it’s magnetic poles are situated close to its geographical poles. The approximate pointing of the north pole of a magnetic needle is in the direction of the geographic north .Therefore, it is relevant to affirm that the magnetic south pole of the Earth which is situated at the geographic north attracts the magnetic north pole of the needle. Furthermore, the magnetic north pole of the Earth which is located at the geographic south attracts the magnetic south pole of the needle.
5) How can you identify a non-magnetic materials? Give an example of a non magnetic material.
Non – magnetic materials refers to those materials which does not get attracted by magnets. It is not possible to magnetize non-magnetic materials. They depict extremely minimal reaction in magnetic fields. Glass and rubber are some of the relevant examples of non-magnetic materials. So, in this way we can identify non- magnetic materials.
(VI ) Answer in detail .
1 ) List out the uses of magnets.
In our day to day life we often come into contact with magnets. They are used in a variety of devices. The uses of magnets are as follows:
- In traditional times, the magnets were used by the seamen in the form of ‘direction stone’ for the purpose of finding the directions during a voyage.
- In present times, magnets are used in order to generate electricity in dynamos.
- In our daily life, electromagnets are used to a certain extent.
- Electric bells and electric motors makes use of magnets.
- They are used both in loudspeakers as well as in microphones.
- The fast moving Maglev train makes use of an extensively powerful electromagnet, so that it can continue to float above the tracks.
- Magnetic conveyor belts are widely used in industries, for the purpose of separating magnetic materials from the scraps that are mixed with non-magnetic materials.
- In computer magnets are used in its storage devices as for instance, hard disks.
- In banks, magnets play an important role by facilitating the computers to read the MICR numbers that are printed on a cheque.
- In order to ensure that the screws remains attached to the tip, the screw drivers are constructed in such a manner that it’s tip is made slightly magnetic.
- At hospitals, MRI is used for the purpose of scanning the definite internal organs.
2) How will you convert a ‘nail’ into a temporary magnet?
Firstly, some steel pins should be spread on a wooden board and then an iron nail should be brought close to them. Now, one end of the iron nail should be touched by one of the magnetic poles of the bar magnet.Then, it should be slided along its length in one direction until the arrival of another end. This process should be repeated for at least 20 to 30 times. The magnet should be moved in only one direction. The back and forth swiping of the magnet should be avoided. Then, the iron nail should be brought close to the steel pins. Now, it will be noticed that the steel pins get’s stuck to the iron nails and this happened mainly because the nail has turned into a temporary magnet.
3) Write a note on Earth’s magnetism.
The scientists presumed that the Earth is like a huge magnetic dipole. But, the position of the magnetic poles of the Earth is not clearly outlined in the Earth. It is near the geographic north pole, where the south pole of the earth’s magnet is situated and the north pole of the earth’s magnet is situated close to the geographic south pole. Magnetic axis refers to the lines which joins these magnetic poles. Northern magnetic pole refers to a point where the magnetic axis intersects the geographic north pole . Southern magnetic pole on the other hand, refers to a point where the magnetic axis intersects the geographic south pole. Both the magnetic axis and geographical axis does not correspond with one another. There is an inclination of the magnetic axis of the Earth at an angle of around 10° to 15° with the geographical axis.
The main reason behind the Earth’s magnetism is not known even in the present times. But, some of the essential factors which might be the reason behind the earth’s magnetism are as follows :
- Presence of enormous quantity of magnetic materials in the Earth.
- The radiations coming from the Sun.
- The action executed by the Moon.
So, these are the few important factors that are held responsible for Earth’s magnetism.
(VII) Higher Order Thinking Questions.
1 ) Though Earth is acting as a huge bar magnet it is not attracting other ferromagnetic materials.Why?Give reasons.
It is the external temperature which brings about a major effect on the magnetic character of ferromagnetic materials and for this reason Earth does not attract other ferromagnetic materials. Also, the moment they are heated, these ferromagnetic materials becomes paramagnetic, which means they does not sustain any permanent magnetism at curie point.
2) Why it is not advisable to slide a magnet on an iron bar back and forth during magnetising it?
During magnetising it, it is not preferable to slide a magnet on an iron bar back and forth because of the fact that ,it might operate to nullify each other out ,when it is moved in an opposite direction.
3) Thamizh Dharaga and Sangamithirai were playing with a bar magnet.They put the magnet down and it broke into four pieces. How many poles will be there?
Every piece of the broken magnet will act as a separate individual magnet. So, there will be eight poles as there are four pieces of magnet. Therefore, we can infer that opposite poles of a magnet will attract each other. As we all know that, opposites attract. Each magnet consists of two poles, that is a north pole and south pole. So, when the north pole of one magnet is placed close to the south pole of another magnet ,they will depict the tendency to attract one another.