Chapter -Magnetism

 

 Class 6

Magnetism

Question – Answer

Short-Answer Questions

1.      State two possible origins of the term 'magnetism'.

Answer-  .1.As early as 800 BC, the ancient Greeks probably knew that magnetite, an iron ore mined in the province of Magnesia, attracted iron.  2.Magnet was derived either from Magnesia, or from Magnes, the name of the shepherd who supposedly discovered magnetism when the iron tip of his stick and the nails of his shoes stuck to some rocks containing magnetite.

2. Name three elements that are magnetic materials.

Answer- Three elements that are magnetic materials - Iron, cobalt and nickel

3. How is magnetism of help in navigation?

Answer- Directional property is the principle for magnetic compass formation which always points to earth's North , helping in navigation

4. What is the basic difference between the magnetic properties of iron and steel?

Answer-  Unlike Iron , steel is able to retain its magnetism permanently.

5. State two ways in which a magnet can get demagnetised.

Answer- Two ways in which a magnet can get demagnetised –

    i) by heating

iiHammering.

 Iii). By leaving poles open.

  iv) by placing in magnetic field which changes its direction alternately

6. State one advantage of the shape of a horseshoe magnet.

Answer- The advantage of the shape is that the two poles lie next to each other.Hence their combined attractive power can act on the object 

1.      What is a magnet? What is meant by (a) magnetic materials, and (b) magnetic fields?

Answer-An object which attracts other objects made of magnetic materials is called a magnet.

a)      Magnetic materials - Substances that are strongly attracted by magnets are called magnetic substances.

b)      The region around a magnet, where it’s effects can be felt is called it’s magnetic field. 

2. Why is repulsion a surer test of magnetism than attraction?

Answer- Test for a magnet-

Suppose we have a piece of metal and we want to find out whether it is a magnet. It would appear that we can do this easily by checking whether it attracts a small piece of iron. However, even if there is some attraction between the two, we cannot be sure as to which of them. A surer test would be to take the piece of metal we want to test close to a magnet. Place the two in contact in different positions. If there is repulsion between them at some position, we can be sure that the piece of metal we are testing is a magnet This is summarised as repulsion is a surer test for magnetism than attraction.

3. What is magnetic induction? Describe in detail how a steel needle can be magnetised by induction.

Answer- The process by which a piece of magnetic material starts behaving like a magnet when it is brought close to a magnet is called magnetic induction.

Magnetisation by induction

It is possible to create permanent magnets using the process of magnetic induction.  A steel needle is repeatedly stroked by the north pole of a bar magnet. This means the magnet is moved in only one direction (in this case from right to left) when it is in contact with the needle. It is then lifted and taken to its initial position for the next stroke. After this is done about 30 times, the needle becomes a permanent magnet, with its north pole on the right. (This is as if the south pole is pulled to the left by the north pole of the bar magnet.

4. Explain the directional property of magnets. How do the poles of a magnet get their names? Describe one application of this property.

Answer-

Suspend a bar magnet from a fixed support such that it remains horizontal and can rotate freely.  We find that after some time, it comes to rest along the north-south direction.

The property of a magnet that we have tested i above is called its directional property. This property makes it possible to distinguish between the two poles of a magnet. The pole which points to the north is called the north-seeking pole, or simply the north pole. The other pole is called the south pole.

 

5. Describe a simple experiment by which we can get an idea about the shape of the magnetic field produced by a bar magnet. Make a sketch of the field.

Answer

Sprinkle some iron filings uniformly on a thin sheet of glass or transparent plastic. Place the sheet carefully on top of a bar magnet and tap it a few times. The iron filings will arrange themselves as shown in Figure .This provides something like a photograph of the magnetic field.

6. What is an electromagnet? How is it different from a permanent magnet? Why is iron used as the core of an electromagnet?

Answer – A piece of iron placed inside a current carrying coil called electromagnet , behaves as a magnet only as long as electric current flow through the coil.

 Unlike permanent magnet, electromagnet does not retain its magnetic magnetism when the current is switched off

7. How can a magnet lose its magnetism? How should magnets be stored so that they do not lose their magnetism gradually?

Answer-A magnet  loses its magnetic properties , if it is heated hammered or placed in a field that changes its direction alternately.

8. Distinguish between a permanent magnet and a temporary magnet. Mention two uses of each.

Answer- Permanent magnet detains its magnetic properties even when it is not under the influence of a magnetic field , a temporary magnet on other hand behaves as a magnet only while it is in the magnetic field produced by permanent magnet or electric field.

Uses of permanent magnet – i) magnets are used to used as catches in cupboards doors and windows

 ii) The magnetic strips on credit cards ATM cards etc store information

Uses temporary magnet –

i)                    As electromagnet in door bell

ii)                  As electromagnets used in cranes to pick heavy loads of iron  .

9. What causes the earth to behave as a magnet? Where are the poles of this imaginary magnet?

Answer- Among various complex causes for magnetic behaviour  is the fact that under its surface lie molten rocks rich in iron. The south pole of  this imagine magnet pointing in the direction of geographical north pole and North pole pointing in the direction of geographical south pole.

Objective Questions

Choose the correct option.

1. A single keeper can protect a horseshoe magnet but not a bar magnet.

(a) Iron

(b) Nickel

(c) Cobalt

(d) Stainless steel

2. Which of the following best describes the field due to a magnet?

(a) The region where its effects are felt

(b) The two regions close to its poles

(c) The region between its poles

(d) The region where it aligns itself along the north-south direction

3. Which of the following materials is best suited as the core of an electromagnet?

(a) Steel

(b) Iron

(c) Alnico

(d) Magnetite

4. Which of the following materials is best suited for making magnetic keepers?

(a) Steel

(b) Iron

(c) Alnico

(d) Wood

5. When the north pole of a bar magnet picks up an iron nail, which of the following are induced in the nail?

(a) North poles

(b) South poles

(c) A north pole and a south pole

(d) None of these

Fill in the blanks.

1 . Magnetism was first discovered in  Magnesia

2. Repulsion surer test of magnetism than attraction.

3 Induction precedes attraction in magnetisation.

4. A compass uses the directive property of a magnet.

5. A doorbell uses the attractive property of magnet.

6. The auroras are a result of interactions between solar wind and the earth’s magnetic field

Write true or false.

1. A single keeper can protect a horseshoe magnet but not a bar magnet. True

2. An electromagnet is a permanent magnet. False

3. A permanent magnet can lose its magnetism with time. False

4. Magnetic induction occurs when a piece of magnetic material comes near a magnet. True

5. Ferrites are alloys of iron False

6. A current-carrying coil can be used to make a permanent magnet. False