chapter 6 physics class 6

I. Select the Correct Option

Which of these is not a magnetic material?

(c) Zinc. (Iron and Cobalt are magnetic ).

Which of these is used to make a permanent magnet?

(a) Steel. (Soft iron is typically used for temporary magnets ).

If the north poles of two magnets are placed near one another, there:

(b) is a repulsion between them. (Like poles repel ).

Observe the pictures A and B (toy cars with magnets):

(c) In A, cars 1 and 2 will move away and in B, 3 and 4 will come closer to each other.
Reasoning: In A, like poles ($S$ and $S$) face each other, causing repulsion. In B, unlike poles ($S$ and $N$) face each other, causing attraction.

The correct arrangement to store two magnets is:

(b).
Reasoning: Bar magnets should be stored in pairs with unlike poles on the same side, separated by wood, and with soft iron keepers across the ends.

The strength of these magnets (A, B, and C) based on iron filings:

(b) $A < B < C$.
Reasoning: Magnet C has the most iron filings sticking to its poles, indicating the strongest magnetic force.

The direction of magnetic lines of force outside the magnet is:

(b) from north pole to south pole.

The more dense magnetic lines of force, the stronger is the:

(b) magnetic field.

II. Assertion and Reasoning

Answer: (a) Both A and R are true.
Answer: (a) Both A and R are true.
Answer: (a) Both A and R are true.
Answer: (d) Assertion (A) is true but Reason (R) is false. (Hammering actually destroys alignment ).
Answer: (a) Both A and R are true.

III. Fill in the Blanks

The parts of a magnet where the magnetic force is strongest are called the poles.
A south pole attracts a north pole and repels a south pole.
Soft iron is used to make a temporary magnet.
Like magnetic poles repel each other whereas unlike magnetic poles attract each other.
A compass is useful for finding the direction in an unknown place.
A freely-suspended magnet always rests in the north-south direction.
Magnetic poles always exist in pairs.
The region or space around a magnet in which its influence can be felt is called magnetic field.

IV. True or False

False. Natural magnets (lodestone) were discovered in Greece, not artificial ones.
False. Every magnet, including cylindrical ones, has two poles.
False. Paper is a non-magnetic material.
True. Similar (like) poles repel each other.
True. Magnetic field lines are imaginary lines used to describe the field.
False. They point in the north-south direction.
False. Heating does affect magnetism; it can demagnetise the magnet.

V. Match the Following

Iron — (d) Temporary magnet.
Steel — (c) Permanent magnet.
Magnetic poles — (e) Ends of a magnet.
Electromagnet — (a) Used in a generator.
Effective length — (b) Distance between poles.

VI. Pick the Odd One Out

Lodestone: It is a natural magnet, while the others are artificial magnets.
Steel: It is a magnetic material, while rubber, wood, and glass are non-magnetic.
Nickel: It is a magnetic material, while aluminium, brass, and copper are non-magnetic.

VII. One Word / One Statement

Four materials attracted by magnets: Iron, cobalt, nickel, and steel.
Soft iron piece for storage: Magnetic keeper.
Pole pointing to geographical south: Magnetic south pole (which is near the geographical south).

Correction Note: Usually, the "South seeking" pole of a magnet points North, and the "North seeking" pole points South relative to Earth's magnetic poles.

Classification:

Magnetic: Iron, cobalt, nickel.
Non-magnetic: Zinc, paper, silver, plastic, copper, soil, aluminium, water, phosphorus, mercury.

Types based on nature: Permanent and temporary magnets.
Magnetic keepers: Soft iron pieces used to prevent self-demagnetisation of magnets during storage.
Two appliances using electromagnets: Electric motors and generators.

VIII. Define the Following Terms

1. Magnet: Any substance which has the ability to attract iron, nickel, and cobalt metal towards itself without actually touching them is called a magnet.

2. Magnetic field: The space around a magnet in which its magnetic influence can be felt is called the magnetic field.

3. Electromagnet: A magnet that is produced by passing a powerful direct current through a coil of insulated copper wire wrapped around a soft iron bar is called an electromagnet. It is a temporary magnet.

4. Magnetic induction: The process of producing magnetism in a magnetic material when it is kept near or in contact with a magnet is called magnetic induction. The magnetism produced this way is called induced magnetism.

IX. Answer the following questions in short

1. How can you tell whether a particular material is magnetic or non-magnetic? You can identify this by bringing a magnet near the object ; if the material is attracted to the magnet, it is magnetic, and if it is not attracted, it is non-magnetic.

2. What are magnetic materials? Name some of them. Materials that are attracted by a magnet are called magnetic materials. Examples include iron, cobalt, nickel, and their alloys.

3. State the characteristics of a magnet. A magnet attracts substances like iron , always has two poles (north and south) , and its like poles repel while unlike poles attract. When freely suspended, it always points in a north-south direction.

4. How will you identify the poles of a bar magnet if they are not marked? Suspend the magnet freely from a stand using a thread. After it comes to rest, the end pointing toward the geographical north is the north pole, and the end pointing toward the geographical south is the south pole.

5. Describe the Earth's magnetic field. The Earth behaves like a giant bar magnet buried deep inside its surface, though it is slightly tilted from the Earth's polar axis. Its magnetic poles are near the geographical poles, influencing magnets to align north-south.

6. What type of magnet is an electromagnet? How will you prove that? An electromagnet is a temporary magnet. You can prove this by showing that it only attracts magnetic materials when an electric current is flowing through its coil and loses its magnetism when the current is switched off.

X. Answer the following questions in detail

1. Describe experimentally that a freely-suspended magnet comes to rest in the north-south direction.

Experiment:

Take a bar magnet and suspend it freely using a thread tied around its center.
Ensure the magnet can rotate freely in a horizontal plane without any obstruction.
Observe the position in which the magnet comes to rest.

Observation:

The magnet will align itself in the geographic north-south direction.
The end pointing towards the geographic north is the North Pole of the magnet, and the end pointing towards the geographic south is the South Pole of the magnet.

Explanation:

The Earth behaves like a giant bar magnet with its magnetic field lines running from the geographic South Pole to the geographic North Pole.
The suspended magnet aligns itself with the Earth's magnetic field, which is why it comes to rest in the north-south direction.

2. How will you determine in which direction a freely-suspended magnet, which can rotate freely in a horizontal plane, will come to rest?

A freely-suspended magnet will always come to rest in the geographic north-south direction.
This is due to the influence of the Earth's magnetic field, which causes the magnet to align itself along the field lines.
You can verify this by using a compass. The compass needle, which is a small magnet, will also align in the north-south direction, confirming the alignment of the suspended magnet.

3. Explain the process by which a permanent magnet can magnetize an ordinary piece of iron.

Process:

Take a permanent magnet and place one of its poles (e.g., the North Pole) at one end of the iron piece.
Rub the permanent magnet along the length of the iron piece repeatedly in the same direction.
Ensure that you lift the magnet after each stroke and start again from the same end.
Repeat this process 30-40 times.

Explanation:

The repeated rubbing aligns the magnetic domains in the iron piece in the same direction as the magnetic field of the permanent magnet.
As a result, the iron piece becomes a temporary magnet, with the end where the rubbing started becoming the opposite pole of the permanent magnet used (e.g., if you started with the North Pole, the end of the iron piece will become the South Pole).

4. Suggest an arrangement to store a U-shaped magnet. How is this different from storing a pair of bar magnets?

Arrangement for U-shaped Magnet:

Place a magnetic keeper (a piece of soft iron) across the poles of the U-shaped magnet.
The keeper completes the magnetic circuit, reducing the loss of magnetism by preventing the formation of opposite magnetic poles at the ends.

Arrangement for Bar Magnets:

Store bar magnets in pairs with their unlike poles facing each other.
Place a magnetic keeper across the ends of the pair.
Separate the magnets with a wooden piece to prevent direct contact.

Difference:

A U-shaped magnet uses a single keeper across its poles, while bar magnets are stored in pairs with unlike poles together and a keeper placed across the ends.
The keeper helps maintain the magnetic strength by reducing the magnetic flux leakage.

5. Three identical iron bars are kept on a table. Two out of three bars are magnets. In one of the magnets, the North-South poles are marked. How will you find out which of the other two bars is a magnet? Identify the poles of this magnet.

Procedure:

Take the marked magnet and bring one of its poles (e.g., the North Pole) close to the first end of the first unmarked bar.
Observe if there is attraction or repulsion:

If there is attraction, the end of the unmarked bar is the South Pole (since unlike poles attract).
If there is repulsion, the end of the unmarked bar is the North Pole (since like poles repel).

Repeat the process with the other end of the unmarked bar to confirm its polarity.
Test the second unmarked bar similarly. The bar that shows attraction or repulsion is the magnet, while the one that shows no interaction is just an iron bar.
Once you identify the magnet, use the marked magnet to determine the poles of the unmarked magnet.

6. Why does a freely suspended magnetic needle align itself in the geographic north-south direction? Name a device which is based on this phenomenon.

Explanation: A freely suspended magnetic needle aligns itself in the geographic north-south direction due to the Earth's magnetic field. The Earth behaves like a giant bar magnet, with its magnetic field lines running from the magnetic South Pole to the magnetic North Pole. The magnetic needle aligns itself along these magnetic field lines, pointing towards the magnetic poles.

Device: A compass is a device based on this phenomenon. The needle of a compass is a small magnet that aligns itself with the Earth's magnetic field, helping to determine direction.

XI. Differentiate between the following:

XIII. Crossword Puzzle (Section VI)

· Across:

o 2. Keepers (Used to store permanent magnets).

o 6. Poles (Regions where attractive power is maximum).

o 7. Induction (Property possessed due to a nearby magnet).

o 8. Magnesia (Town where magnets originated).

o 9. Magnet (Substance which attracts iron).

· Down:

o 1. Electromagnet (Functions on electricity).

o 3. Repulsion (A sure test of magnetism).

o 4. Field (Space around a magnet).

o 5. Demagnetisation (Process of destroying magnetic properties).

Sushovan Pal

Search This Blog