Magnetism

A magnet is a special material that produces a magnetic field around itself, which is an invisible force that exerts a push or pull on other magnetic materials without any physical contact. This ability is most prominently seen in its property to attract certain metals like iron, nickel, and cobalt. Every magnet, regardless of its shape, has two distinct regions where the magnetic effect is the strongest, known as poles. These are labeled the North Pole and the South Pole. A fundamental law of magnetism is that like poles repel each other, while unlike poles attract. This force of attraction or repulsion acts along the field lines and is strongest near the poles. If a bar magnet is freely suspended, it will align itself approximately in the North-South direction, which is the principle behind the magnetic compass.

The space around a magnet where its influence can be felt is called the magnetic field. We can visually represent this field by drawing magnetic field lines. These lines are imaginary curves that start from the North Pole and end at the South Pole, outside the magnet. The direction of the field at any point is given by the direction a small compass needle would point. Where the field lines are closer together, the magnetic field is stronger, which is why they crowd near the poles. Importantly, these field lines never intersect each other. This concept helps explain why a magnet can act from a distance; the object is simply interacting with the magnetic field that fills the space around the magnet.

Magnets can be made in several ways. The single-touch and double-touch methods are simple techniques of stroking a magnetic material with a permanent magnet to induce magnetism. A more effective method is electrical magnetization, where a strong magnetic field is created by passing a direct current (DC) through a coil wound around the material. It is crucial to understand the distinction between permanent magnets, which retain their magnetism for a long time, and temporary magnets, which only act as magnets while under the influence of an external magnetic field. To preserve a magnet’s strength, it should be stored properly, often in pairs with opposite poles together and with keepers, which are soft iron pieces that provide a preferred path for the magnetic field, preventing self-demagnetization.

Exercise 10 (A)

Question 1. 

What is a lodestone? 

Ans:

A lodestone is a rather remarkable piece of the natural world. At its heart, it’s a specific kind of mineral called magnetite, which is a common iron ore. What sets a lodestone apart from every other rock is its innate and permanent magnetic field.

Think of it as a natural, permanent magnet. Unlike the magnets we use today that are manufactured, a lodestone acquires its magnetism naturally. Scientists believe this happens through a powerful and rare event, most often a lightning strike. When lightning hits a deposit of magnetite, the immense surge of electrical current can align the magnetic domains within the rock, turning it into a magnet that lasts for centuries.

The most fascinating property of a lodestone, and the reason for its name, is its ability to interact with iron. The Old English word “lodestone” literally means “waystone” or “leading stone.” If you were to dangle a piece of one from a string, it would slowly rotate and align itself with the Earth’s magnetic poles, pointing roughly north-south. This made it the world’s first practical compass, guiding navigators and explorers long before the invention of sophisticated instruments.

Beyond navigation, these stones were surrounded by mystery and superstition. They were thought to have magical properties, capable of warding off spells or even calming storms at sea. Alchemists and early scientists saw in the lodestone a fundamental, invisible force of nature—the mysterious power of attraction at a distance, which laid the groundwork for our modern understanding of magnetism.

Question 2. 

What is a natural magnet? State two limitations of a natural magnet. 

Ans:

What is a Natural Magnet?

A natural magnet is a mineral that occurs in nature and possesses a persistent magnetic field without any human intervention. It is a piece of magnetized rock or ore that has the inherent ability to attract ferromagnetic materials like iron, nickel, and cobalt.

The most common and historically significant natural magnet is a form of the iron oxide mineral called magnetite (Fe₃O₄). When a piece of magnetite is found to have strong magnetic properties, it is often referred to as a lodestone. The fascinating property of lodestone is that if it is freely suspended, it will align itself in a north-south direction, which is why it was used in the earliest compasses for navigation.

Natural magnets acquire their magnetism over geological timescales. The most accepted theory is that they become magnetized when they are struck by lightning or through the process of cooling within the Earth’s magnetic field after being heated above a critical temperature (the Curie point) during volcanic activity.

Two Limitations of a Natural Magnet

While revolutionary in their time, natural magnets have significant drawbacks compared to modern artificial magnets.

1. Weak and Unreliable Magnetic Strength
The magnetic strength of a natural magnet is generally very weak and inconsistent. The potency of a lodestone depends entirely on its specific composition, size, and the random natural events that magnetized it. There is no way to control or standardize its strength. This makes it unsuitable for most modern applications, from electric motors and speakers to medical devices like MRI machines, which require powerful, predictable, and stable magnetic fields. An artificial magnet can be engineered to have a magnetic field thousands of times stronger than any natural magnet.

2. Brittleness and Poor Mechanical Properties
Natural magnets are rocks, and as such, they are hard and brittle. They cannot be easily molded, cut, or shaped into specific forms (like bars, horseshoes, or discs) without risking breakage. This lack of malleability severely limits their practical use. Furthermore, they are susceptible to chipping, cracking, and losing their magnetism if dropped or subjected to physical shock. In contrast, artificial magnets, especially those made from alloys like Alnico or flexible materials like magnetic rubber, can be manufactured in precise, durable shapes tailored for specific functions.

Question 3. 

What is an artificial magnet? State two reasons why we need artificial magnets .

Ans:

An artificial magnet is a man-made object that is magnetized to produce its own persistent magnetic field.

Two reasons why we need artificial magnets are:

1. Specific Shapes and Strength: They can be created in specific, useful shapes (like bars, horseshoes, or discs) and with controlled magnetic strength for particular applications, which is not possible with natural magnets.
2. On-Demand Availability: Natural magnets (lodestone) are rare and weak. Artificial magnets can be produced in large quantities and with much greater magnetic power to meet industrial and technological demands.

Question 4. 

How will you test whether a given rod is made of iron or copper?

[Hint : Iron rod gets magnetised when placed near a bar magnet by magnetic induction, while copper rod does not get magnetised]

Ans:

A Simple Magnet Test: Telling Iron and Copper Apart

Need to quickly figure out if a metal rod is iron or copper? One of the easiest and most reliable methods uses a common tool: a strong magnet. Here’s how to do it and why it works.

What You’ll Need:

• The metal rod in question.
• A strong permanent magnet (a neodymium magnet works best).

The Test:

1. Take your strong magnet and slowly bring one of its poles close to the end or side of the rod.
2. Pay close attention to the sensation. Do you feel a distinct, clear pull? Or does the magnet just sit there, held only by gravity?

Interpreting the Results:

• If the rod is strongly attracted to the magnet, it is Iron.
  • Why this happens: Iron is a ferromagnetic material. This means that when an external magnetic field from your magnet is present, the tiny magnetic regions inside the iron (called domains) all align, turning the rod itself into a temporary magnet. This induced magnetism creates a powerful attraction between the two.
• If the rod shows no reaction to the magnet, it is Copper.
  • Why this happens: Copper is classified as non-magnetic or diamagnetic. Its internal structure is not affected by a magnetic field in a way that produces an attractive force. The magnet will feel nothing more than the smooth surface of the copper rod, with no invisible pull whatsoever.

Question 5. 

You are provided with two similar bars, one is a magnet and the other is a soft iron. How will you distinguish between them without the use of any other magnet or bar?

[Hint : A magnet when suspended freely will rest only in the north-south direction, but the soft iron bar will rest in any direction]

Ans:

Method: The Free Suspension Test

This test relies on the fundamental property of a magnet to align itself with the Earth’s magnetic field.

Procedure:

1. Find a Long, Thin Thread: Take a piece of string or a long, thin thread. A shoe lace or unwaxed dental floss works well.
2. Tie and Suspend: Tie the thread firmly around the center of one of the bars. Suspend the bar freely by holding the other end of the thread. Ensure the bar is hanging in a way that it can rotate easily without any obstructions. It is best to do this in a place with minimal air currents.
3. Observe the Resting Direction: Wait for the bar to come to a complete rest. Note the direction in which it settles.
4. Repeat: Gently disturb the bar to make it swing and rotate. Let it come to rest again. Observe if it settles in the same direction as before.
5. Test the Other Bar: Now, take the thread off and repeat the exact same process with the second bar.

How to Interpret the Results:

• The Magnet: The magnet will consistently come to rest in a North-South direction every single time you perform the test. Even if you twist it and let go, it will swing back to align its north pole with the Earth’s magnetic south and its south pole with the Earth’s magnetic north.
• The Soft Iron Bar: The soft iron bar, not being a permanent magnet, has no inherent polarity. It is simply a piece of magnetic material. Therefore, it will not show a preference for any particular direction. It will rest in whatever random position it was left in and will not consistently point North-South when disturbed and left to settle again.

Why This Works:

• A magnet has its own persistent magnetic field. When suspended freely, it acts like a compass needle, interacting with the Earth’s magnetic field and aligning with it.
• Soft iron is a ferromagnetic material that can be temporarily magnetized when placed in an external magnetic field (like the Earth’s), but it loses this magnetism once removed. The Earth’s field is too weak to induce a strong enough temporary polarity in the soft iron bar to make it consistently align like a compass. It remains essentially non-magnetic for the purpose of this test.

Question 6. 

1. Fill in the blank to complete the sentence : The two ends of a magnet are called __________. 

2. Fill in the blank to complete the sentence : Unlike poles of a magnet ___________ each other. 

3. Fill in the blank to complete the sentence : Like poles of a magnet ______________ each other. 

4. Fill in the blank to complete the sentence : A freely suspended magnet rests in the geographic ___________ direction.

Ans:

1. The two ends of a magnet are called poles.
2. Unlike poles of a magnet attract each other.
3. Like poles of a magnet repel each other.
4. A freely suspended magnet rests in the geographic North-South direction.

Question 7. 

A small magnet is suspended by a silk thread from a rigid support such that the magnet can freely swing. How will it rest? Draw a diagram to show it.

Ans:

A small magnet suspended freely by a silk thread will come to rest in the geographic North-South direction.

This alignment occurs because the suspended magnet is aligning itself with the Earth’s magnetic field. The North-seeking pole (North pole) of the magnet will point towards the Earth’s geographic North, and the South-seeking pole (South pole) will point towards the Earth’s geographic South.

Diagram of a Freely Suspended Magnet

The diagram below illustrates a bar magnet freely suspended from a rigid support, showing its final resting position:

Question 8. 

Explain the meaning of the term induced magnetism.

Ans:

Induced Magnetism

Induced magnetism is the phenomenon where a non-magnetic material (or an unmagnetized magnetic material) temporarily acquires magnetic properties when it is placed near or in contact with a strong magnet.

In simple terms, a material becomes a temporary magnet because of the influence of an external magnetic field.

Key Characteristics:

• Temporary: The induced magnetism is not permanent. The material loses its magnetic properties as soon as the external magnet is removed.
• Mechanism: When the external magnet is brought near, it aligns the tiny, microscopic magnetic regions (domains) within the unmagnetized material. This alignment causes the material itself to act as a magnet.
• Polarity: The end of the temporary magnet closest to the permanent magnet will always acquire the opposite polarity. For example, if a North pole is brought near an iron nail, the end of the nail closest to the magnet will become a South pole, leading to attraction.

Question 9. 

Explain what you understand by magnetic induction. What role does it play in attraction of a piece of iron by a magnet?

Ans:

Magnetic Induction

Magnetic induction is the process by which a non-magnetized magnetic substance, such as a piece of iron, acquires temporary magnetic properties when it is placed near or inside the magnetic field of a permanent magnet.

This induced magnetism is temporary; the substance loses its magnetic properties as soon as the external magnet is removed.

Role in Attraction

Magnetic induction is the direct cause of the attraction between a permanent magnet and a piece of unmagnetized iron:

1. Induction of Polarity: When the permanent magnet is brought near the piece of iron, the external magnetic field of the magnet causes the magnetic domains within the iron piece to align themselves. Crucially, the end of the iron piece closer to the permanent magnet develops a pole of opposite polarity.
   • Example: If the North pole of the permanent magnet is brought near the iron piece, the side of the iron piece closest to the magnet is induced to be a South pole.
2. Attraction: According to the law of magnetism, unlike poles attract each other. Since the iron piece has been induced with an opposite pole facing the permanent magnet, a strong force of attraction develops between them.

Question 10. 

Explain the mechanism of attraction of iron nails by a magnet when brought near them.

Ans:

The attraction of unmagnetized iron nails by a magnet is explained by the principle of magnetic induction.

Here is the step-by-step mechanism:

1. Magnetic Induction (Temporary Magnetization)

• The iron nail is initially unmagnetized, meaning its internal magnetic domains are oriented randomly, canceling out any net magnetic effect.
• When the permanent magnet is brought close to the nail, the strong external magnetic field of the magnet penetrates the iron.
• This external field forces the magnetic domains within the nail to align themselves along the direction of the field.
• As a result, the iron nail temporarily acquires magnetic properties; it becomes an induced magnet.

2. Induction of Opposite Polarity

• A key rule of magnetic induction is that the end of the iron object closest to the permanent magnet acquires the opposite polarity.
  • If the magnet’s North pole (N) is brought near the nail, the end of the nail closest to the magnet becomes a South pole (S).
  • If the magnet’s South pole (S) is brought near the nail, the end of the nail closest to the magnet becomes a North pole (N).

3. Attraction

• The final step involves the Law of Magnetism, which states that unlike poles attract each other.
• Since the permanent magnet induces an opposite pole on the nearest end of the iron nail, a strong force of attraction is generated between the two objects.

Question 11. 

1. Explain the following : When two pins are hung by their heads from the same pole of a magnet, their pointed ends move apart. 

2. Explain the following : Several soft iron pins can cling, one below the other, from the pole of a magnet. 

3. Explain the following :The north end of a freely suspended magnetic needle gets attracted towards a piece of soft iron placed a little distance away from the needle.

Ans:

1. Pins Hanging from the Same Pole Move Apart

This phenomenon is explained by magnetic induction and the rule of magnetic repulsion between like poles.

• Magnetic Induction: When the two pins are hung from the same pole (e.g., the North pole) of a magnet, they each become temporary induced magnets.
• Polarity: The end of each pin touching the magnet acquires the opposite polarity (e.g., South pole). Consequently, the free, pointed end of each pin acquires the same polarity as the magnet’s pole it is touching (e.g., North pole).
• Repulsion: Since the pointed ends of both pins are now like poles (North pole and North pole in this example), they repel each other, causing the pins to swing away and move apart.

2. Soft Iron Pins Cling One Below the Other

This observation is also a clear demonstration of magnetic induction.

• First Pin: The first soft iron pin is attracted to the permanent magnet via magnetic induction. It becomes a temporary magnet with the end closest to the permanent magnet having the opposite polarity.
• Subsequent Pins: The first pin, now an induced magnet, acts as a permanent magnet to the pin below it. It induces magnetism in the second pin, giving the end of the second pin closest to it the opposite polarity, leading to attraction.
• Chain Effect: This process continues, with each pin inducing magnetism in the next unmagnetized pin. This strong, repeated process of magnetic induction allows several soft iron pins to form a temporary magnetic chain.

3. Magnetic Needle Attracted to Soft Iron

This observation involves magnetic induction causing attraction regardless of which pole of the magnet is facing the iron.

• Magnetic Induction in Iron: When the piece of soft iron is placed near the freely suspended magnetic needle, the needle’s magnetic field induces magnetism in the soft iron.
• Induction of Opposite Pole: According to magnetic induction, the end of the soft iron piece closest to the magnet will always acquire the opposite polarity to the magnet’s pole facing it.
  • Since the needle’s North pole is facing the iron, the iron develops a South pole on its nearest end.
• Attraction: The North pole of the magnetic needle is then strongly attracted to the induced South pole on the piece of soft iron (unlike poles attract).

Question 12. 

A small iron bar is kept near the north pole of a bar magnet. How does the iron bar acquire magnetism? Draw a diagram to show the polarity on the iron bar. What will happen if the magnet is removed?

Ans:

Mechanism of Induction and Polarity

1. Induction: When the unmagnetized iron bar is brought near the North pole of the permanent bar magnet, the magnet’s strong external magnetic field forces the magnetic domains within the iron to align themselves. This causes the iron bar to become a temporary magnet.
2. Polarity: The end of the iron bar closest to the magnet’s North pole will acquire the opposite polarity, which is a South pole (S). The far end of the iron bar will acquire the same polarity, a North pole (N).

Diagram Showing Polarity

The diagram below illustrates the induced polarity on the iron bar:

Effect of Removing the Magnet

If the magnet is removed, the induced magnetism in the soft iron bar is lost almost immediately.

• The magnetic domains within the soft iron will quickly return to their original, random orientations.
• The iron bar will cease to be a magnet and will no longer attract other magnetic objects. (This is because soft iron has low retentivity, meaning it is easily magnetized but loses its magnetism just as easily).

Question 13.

 ‘Induced magnetism is temporary’. Comment on this statement.

Ans:

Explanation and Commentary

1. The General Rule (Temporary Nature)

Induced magnetism is typically temporary because it is dependent on an external magnetic field.

• Mechanism: When a magnetic material (like iron) is placed in the field of a permanent magnet, its internal magnetic regions (domains) temporarily align themselves. This alignment makes the material a temporary magnet.
• Loss of Magnetism: As soon as the external permanent magnet is removed, the force holding the domains in alignment is gone. The domains quickly return to their random state due to thermal agitation, and the material loses its induced magnetic properties.
• Material: This characteristic is most true for soft iron, which is described as having low retentivity (it’s easily magnetized but cannot retain the magnetism).

2. The Exception (Residual Magnetism)

The statement is not universally true for all materials.

• Hard Magnetic Materials: Materials like steel have high retentivity (they are difficult to magnetize but retain the magnetism strongly).
• Residual Magnetism: When steel is subjected to magnetic induction, it does not immediately lose all its magnetism upon removal of the external magnet. The portion of magnetism that remains is called residual magnetism.
• This residual magnetism is what allows materials like steel to be used to make permanent magnets.

Question 14.

 ‘Induction precedes attraction’. Explain the statement.

Ans:

The statement “Induction precedes attraction” means that for a magnet to attract an unmagnetized magnetic material (like a piece of iron or a pin), the magnet must first make the material a temporary magnet through the process of magnetic induction.

Here is the step-by-step explanation:

1. Induction Must Happen First

When a permanent magnet is brought near an unmagnetized piece of iron, the iron does not instantly attract the magnet. Instead, the magnet’s strong external magnetic field acts upon the iron.

• Process: The magnetic field forces the internal magnetic domains within the iron to align, making the iron piece a temporary magnet. This process is called magnetic induction.

2. Polarity is Induced

The most critical part of induction is the creation of a pole of opposite polarity on the surface of the iron closest to the magnet.

• Rule: For attraction to occur, you need poles. The induction process automatically ensures this: the North pole of the permanent magnet induces a South pole on the nearest end of the iron, and vice versa.

3. Attraction Follows Induction

Once the opposite pole is induced on the iron, the fundamental rule of magnetism—that unlike poles attract—comes into effect.

• The force of attraction is then exerted between the pole of the permanent magnet and the newly induced, opposite pole on the temporary iron magnet.

Question 15. 

What do you understand by the term magnetic field lines?

Ans:

Magnetic Field Lines (Lines of Force) 

Magnetic field lines, also called lines of magnetic force, are imaginary lines used to visually represent the strength and direction of a magnetic field around a magnet or a current-carrying conductor.

Key Understanding:

• Direction: The lines indicate the direction of the magnetic field at any point. The tangent drawn to a field line at any point gives the direction of the magnetic field strength at that point. Conventionally, they are drawn as closed loops emerging from the North pole and entering the South pole outside the magnet, and traveling from the South pole to the North pole inside the magnet.

• Strength: The closeness (density) of the field lines indicates the strength of the magnetic field. Where the lines are closer together (e.g., near the poles), the field is stronger. 
• Non-intersecting: Two magnetic field lines never intersect (cross each other). If they did, it would mean the magnetic field had two different directions at the point of intersection, which is physically impossible.

Question 16. 

State four properties of magnetic field lines.

Ans:

Here are four key properties of magnetic field lines:

1. Direction: Magnetic field lines are closed loops that emerge from the North pole (N) of a magnet and enter the South pole (S) outside the magnet. Inside the magnet, they travel from the South pole to the North pole.
2. Density/Strength: The closer the field lines are to one another, the stronger the magnetic field. Conversely, where the lines are spread farther apart, the field is weaker. The field is strongest near the poles.
3. Non-Intersection: Magnetic field lines never intersect (cross) each other. If they were to cross, it would mean that the magnetic field has two different directions at that single point, which is physically impossible.
4. Tangent Direction: The tangent drawn to a magnetic field line at any point gives the direction of the net magnetic field at that specific point.

Question 17. 

Explain why iron filings which are sprinkled on a sheet of cardboard placed over a bar magnet take up a definite pattern when cardboard is slightly tapped.

Ans:

The iron filings sprinkled on the cardboard take up a definite pattern when tapped because they are mapping out the magnetic field lines created by the bar magnet.

1. Magnetic Induction

• The small iron filings are made of a ferromagnetic material (iron) and are initially unmagnetized.
• When placed in the magnet’s vicinity, each filing instantly becomes a temporary induced magnet aligned with the magnet’s external field. This process is called magnetic induction.

2. Alignment with Field Lines

• The filings naturally align themselves end-to-end along the direction of the magnetic force because of the strong attraction between the opposite poles induced at their ends.
• They effectively act as tiny compass needles, with their longest dimension lying tangent to the invisible magnetic field line at their location.

3. Role of Tapping

• Initially, the frictional force between the iron filings and the cardboard is too strong to allow them to move freely and align perfectly.
• Tapping the cardboard provides enough energy (vibration) to temporarily reduce this friction. This allows the filings to overcome the resistance and move, rearrange, and settle precisely along the lines of the strongest magnetic force.
• The final, definite pattern that is formed represents the shape of the magnetic field lines, which are strongest and closest together near the poles and spread out farther away.

Question 18.

Explain the method of plotting the magnetic field lines by using a small compass needle.

Ans:

Plotting Magnetic Field Lines using a Compass Needle

The magnetic field lines around a magnet can be plotted using a small compass needle based on the principle that the compass needle aligns itself tangentially to the magnetic field line at any given point.

Materials

• A bar magnet
• A small magnetic compass
• A white sheet of paper
• A pencil

Procedure

1. Preparation:
   • Secure the white sheet of paper to a wooden board (or table).
   • Place the bar magnet at the center of the paper and draw its outline to mark its position.
   • Mark the North (N) and South (S) poles of the magnet.
2. Starting Point:
   • Place the small compass near the North pole (N) of the bar magnet.
   • The compass needle will settle, with its North pole (usually the colored end) pointing away from the magnet’s North pole (due to repulsion).
3. Marking Points:
   • Mark two dots: one at the tip of the compass’s North pole and one at the tip of its South pole.
   • These two points define the direction of the magnetic field at that specific location.
4. Tracing the Line:
   • Move the compass so that its South pole tip is placed exactly over the dot previously marked by its North pole.
   • Mark a new dot at the tip of the compass’s North pole.
   • Repeat this process, moving the compass step-by-step, always placing the South pole of the compass over the previous North pole dot.
5. Drawing the Line:
   • Continue marking points until the compass eventually reaches the South pole (S) of the bar magnet.
   • Carefully draw a smooth curve connecting all the marked points.
   • Draw an arrowhead on the line to indicate the direction, which is conventionally from North (N) to South (S) outside the magnet.
6. Multiple Lines:
   • Repeat the entire process, starting from different points near the North pole, to obtain several distinct magnetic field lines. The resulting pattern shows the complete magnetic field.

Question 19. C

Can two magnetic field lines intersect each other? Give reason to your answer.

Ans:

Reason for Non-Intersection

The reason is directly related to the definition and unique nature of the magnetic field:

1. Unique Direction: A magnetic field line shows the direction of the magnetic field (net force) at every point in space. The direction of the field at any point is given by the tangent drawn to the field line at that point.
2. Physical Impossibility: If two magnetic field lines were to intersect at a single point, it would mean that at the point of intersection, the magnetic field would have two different directions (the directions of the tangents to the two intersecting lines).
3. Conclusion: A magnetic field is a vector quantity and can only have one unique direction at any given point in space. Since two directions at one point are physically impossible, two magnetic field lines cannot intersect.

Question 20. 

1. In the following figure, draw at least two magnetic field lines between the two magnets.

(a)

2. In the following figure, draw at least two magnetic field lines between the two magnets.

(b)

Ans:

1. Magnetic Field Lines: Attraction (Figure a)

Figure (a) shows unlike poles (North and South) facing each other, which results in attraction.

The magnetic field lines in this case emerge from the North pole of the left magnet and enter the South pole of the right magnet, passing directly through the space between them. They form continuous, closed loops.

2. Magnetic Field Lines: Repulsion (Figure b)

Figure (b) shows like poles (North and North) facing each other, which results in repulsion.

The magnetic field lines in this case originate from the North pole of each magnet, but instead of crossing, they exert a lateral pressure on each other, causing them to bend and diverge away from the central region. This bending indicates the force of repulsion.

Question 21. 

State two evidences of the existence of earth’s magnetic field.

Ans:

Here are two primary pieces of evidence for the existence of the Earth’s magnetic field:

1. A Freely Suspended Magnet Rests in the North-South Direction (Compass):
   • When a magnetic compass or a bar magnet is suspended freely (so that it can rotate horizontally), it always settles in a specific direction: its North-seeking pole points approximately towards the Earth’s geographic North, and its South-seeking pole points towards the Earth’s geographic South.
   • This consistent alignment demonstrates that the magnet is responding to an external magnetic force field, which is the Earth’s magnetic field.
2. The Formation of the Neutral Point:
   • When a bar magnet is placed on a horizontal plane (like a table), there are points around it called neutral points where the magnet’s own magnetic field is exactly cancelled out by the Earth’s horizontal magnetic field component.
   • The existence and specific location of these points can be mapped experimentally using a compass and can only be explained if a constant, ambient magnetic field (the Earth’s field) is present and interacting with the bar magnet’s field.

Question 22. 

Sketch four magnetic field lines as obtained in a limited space on a horizontal plane in the earth’s magnetic field alone.

Ans:

To sketch magnetic field lines in a limited space on a horizontal plane in the Earth’s magnetic field alone, we only need to show lines that are parallel, straight, and equally spaced.

This is because the Earth’s magnetic field is considered uniform (constant in strength and direction) over a small, localized region. The lines flow from the geographic South towards the geographic North.

Earth’s Magnetic Field Lines (Horizontal Plane)

Characteristics of the Sketch:

• Direction: The arrows on the lines point from geographic South (S) to geographic North (N), following the convention of the Earth’s magnetic field lines in the horizontal plane.
• Uniformity: The lines are drawn straight and parallel to each other, indicating that the field is uniform (has the same strength and direction) in the limited space.
• Spacing: The lines are equally spaced to further emphasize the uniform nature of the field in that small area.

Question 23. 

1. Draw the pattern of magnetic field lines near a bar magnet placed with its North Pole pointing towards the geographic North. Indicate the position of neutral points by marking X. 

2. State whether the magnetic field lines in part (a) represent a uniform magnetic field or non-uniform magnetic field?

Ans:

1. Magnetic Field Lines and Neutral Points

When a bar magnet is placed with its North Pole (N) pointing towards the Earth’s geographic North, the magnetic field lines between the bar magnet and the Earth’s field interact.

The neutral points (X) are the locations where the horizontal component of the Earth’s magnetic field (BH) is exactly cancelled by the magnetic field of the bar magnet (BM), resulting in a net magnetic field of zero.

In this configuration (N-pole facing geographic North), the fields cancel along the equatorial line (a line perpendicular to the magnet’s axis).

The key features of the diagram are:

• Field lines emerge from the North pole and enter the South pole of the bar magnet.
• The Earth’s magnetic field lines (represented as straight and parallel) run from geographic South to geographic North.
• The field lines created by the magnet and the Earth’s field oppose each other along the equatorial line (the line passing through the centre of the magnet perpendicular to its axis), leading to two neutral points marked by X.

2. Uniform or Non-Uniform Field?

The magnetic field lines in the overall figure (created by the combination of the bar magnet’s field and the Earth’s magnetic field) represent a non-uniform magnetic field.

Reason:

• A uniform magnetic field is represented by straight, parallel, and equally spaced lines.
• The field shown here is a resultant field where the lines are curved and their spacing changes, particularly near the poles of the magnet and where the neutral points form. This variation in direction and strength signifies a non-uniform magnetic field.

Question 24. 

1. Figure. shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic materials nearby.

Insert two magnetic field lines on either side of the magnet using  arrow head to show the direction of each field line.

2. Figure. shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic materials nearby.

Indicate by crosses, the likely positions of the neutral points. 3. Figure. shows a bar magnet placed on the table top with its north pole pointing towards south. The arrow shows the north-south direction. There are no other magnets or magnetic materials nearby.

What is the magnitude of the magnetic field at each neutral point? Give a reason for your answer.

Ans:

The problem describes a bar magnet placed with its North pole (N) facing the geographic South (S) direction. The background field is the Earth’s magnetic field, which runs from geographic South to North.

1. Magnetic Field Lines Drawing

When the North pole of the bar magnet faces the geographic South, the magnetic field lines of the magnet and the Earth’s field reinforce (add up) along the axis of the magnet (the line passing through N and S poles). They oppose each other along the equatorial line (the line perpendicular to the magnet’s axis, passing through its center).

The two magnetic field lines should:

• Start from the N pole and end at the S pole of the bar magnet (outside the magnet).
• Be crowded near the poles (where the field is strong) and spread out farther away.

Question 25. 

What conclusion is drawn regarding the magnetic field at a point if a compass needle at that point rests in any direction? Give reason for your answer.

Ans:

If a compass needle placed at a point rests in any direction, the conclusion drawn is that the net magnetic field at that point is zero (a null field). This location is known as a neutral point.

Reason:

1. Compass Function: A magnetic compass needle is essentially a tiny magnet whose purpose is to align itself with the direction of the net external magnetic field acting on it.
2. Zero Net Force: For a compass needle to be able to rest in any direction, there must be no net magnetic force or torque acting on it to force it into a specific alignment.
3. Conclusion: The only way for the net magnetic force to be zero is if the magnetic field contributions from all surrounding sources (including the Earth’s magnetic field and any nearby magnets or currents) exactly cancel each other out at that specific point. In other words, the magnitude of the net magnetic field at that location is zero.

Question 26. 

What is a neutral point? How is the position of the neutral point located with the use of a compass needle?

Ans:

A neutral point is a location where the total magnetic field strength becomes zero because the fields from two separate sources cancel each other out. Think of it as a spot where competing magnetic pulls are perfectly balanced, resulting in no net force.

A common way to observe this is by using a bar magnet and the Earth’s magnetic field.

Example Setup:
If a bar magnet is placed with its North pole facing the Earth’s geographic North (which is the magnetic South), the Earth’s field pulls a compass needle toward the north, while the bar magnet pulls it toward its own South pole. At the neutral point, these two influences balance exactly.

Finding the Neutral Point with a Compass:

1. Setup: Place the bar magnet on a sheet of paper, with its North pole pointing toward geographic North. Trace the magnet’s outline.
2. Initial Observation: Far from the magnet, the compass needle points toward geographic North, aligned with Earth’s magnetic field.
3. Approach the Magnet: Slowly move the compass toward the magnet from the direction of its North pole. The needle will begin to deflect toward the magnet’s South pole as the magnet’s field dominates.
4. Locate the Neutral Point: At a certain spot, the compass needle will stop pointing in any fixed direction. It becomes sluggish and can settle in any position, indicating that the magnetic fields cancel out here.
5. Mark the Spot: Mark this position on the paper. Another neutral point can usually be found near the magnet’s South pole due to symmetry.

Question 27. 

State the positions of neutral points when a magnet is placed with its axis in the magnetic meridian and with its north pole (i) pointing towards the geographic north and (ii) pointing towards the geographic south.

Ans:

(i) North pole pointing towards geographic north:

• The neutral points (points where the net magnetic field is zero) lie on the equatorial line of the magnet.
• This is because the Earth’s field (pointing north) and the magnet’s field (pointing south at its equator) cancel each other out here.

(ii) North pole pointing towards geographic south:

• The neutral points lie on the axial line of the magnet.
• Here, the Earth’s field (pointing north) and the magnet’s field (also pointing north along its axis outside the south-seeking pole) are in opposite directions and can cancel.

Question 28. 

1. Complete the following sentence : If the field lines in a magnetic field are parallel and equidistant, the magnetic field is __________  . 

2. Complete the following sentence : At a neutral point, the resultant magnetic field is ___________. 

3. Complete the following sentence : The neutral points of a bar magnet kept with its north pole pointing towards geographic north are located ____________.

Ans:

1. If the field lines in a magnetic field are parallel and equidistant, the magnetic field is uniform.
2. At a neutral point, the resultant magnetic field is zero.
3. The neutral points of a bar magnet kept with its north pole pointing towards geographic north are located on the equatorial line (the line perpendicular to the axis of the magnet).

Exercise 10 (A)

Question 1. 

Two like magnetic poles:

1. Repel each other
2. Attract each other
3. First attract each other, then repel
4. Neither attract nor repel.

Question 2. 

In a uniform magnetic field, the field lines are:

1. Curved
2. Parallel equidistant straight lines
3. Parallel but non-equispaced straight lines
4. Nothing can be said.

Exercise 10 (B)

Question 1. 

What is an electromagnet?

Ans:

An electromagnet is a magnet whose magnetic properties are created and sustained by the flow of an electric current.

It is constructed by wrapping a coil of insulated wire around a core, usually made of a readily magnetizable material like soft iron. When current passes through the coil, a magnetic field is generated; when the current stops, the field disappears, making it a temporary magnet.

Key Advantages of Electromagnets

• Switchable: The magnetic field can be instantly activated (turned on) or deactivated (turned off) simply by controlling the electric current supply.
• Adjustable Strength: The strength of the magnet is variable and can be precisely controlled by altering the magnitude of the current or by adjusting the number of wire turns in the coil.
• Reversible Polarity: The North and South poles can be instantly swapped by changing the direction in which the electric current flows through the coil.

Question 2. 

Name the material used for preparing an electromagnet.

Ans:

The core material most commonly used for preparing an electromagnet is soft iron.

Soft iron is preferred because it is a ferromagnetic material with low retentivity (low coercivity). This means it is easily magnetized when current flows through the surrounding coil but loses its magnetism almost completely and instantly when the current is switched off, making the magnet temporary and controllable.

Question 3. 

How is an electromagnet made? Name two factors on which the strength of the magnetic field of the electromagnet depends.

Ans:

How an Electromagnet is Made

An electromagnet is constructed by following these steps:

1. Core Selection: A core of a soft ferromagnetic material, usually soft iron, is chosen. Soft iron is ideal because it is easily magnetized when current flows but quickly loses its magnetism when the current is stopped.
2. Coiling: A length of insulated copper wire is tightly wound around the soft iron core to form a solenoid (a coil).
3. Current Supply: The two ends of the wire coil are connected to a Direct Current (D.C.) source like a battery or power supply.
4. Magnetization: When the current is switched on, the flow of electric charge through the coil generates a magnetic field. This field magnetizes the soft iron core, creating a strong, temporary magnet—the electromagnet.

Factors Affecting the Strength

The strength of the magnetic field (or the magnitude of the force) of an electromagnet depends primarily on the following two factors:

1. Current in the Coil: The strength of the magnetic field is directly proportional to the amount of electric current flowing through the solenoid.
2. Number of Turns in the Coil: The strength of the magnetic field is directly proportional to the number of turns of wire in the coil. Increasing the density of the turns (more turns per unit length) results in a stronger magnetic field.

Question 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. Draw a circuit diagram to represent the process. Label the poles of the electromagnet.

Ans:

Here is a circuit diagram for making an electromagnet:

text

     +———————+

      |                     |

     —                   / \

    |   | Cell             | |  Insulated Copper Coil

     —                   \ /

      |                     |

      |——-Switch——–|

      |                     |

      +—-[Soft Iron]——+

            (Bar)

          N Pole    S Pole

          <<<<<<<<<<<<<<<

Explanation:

• A cell, a switch, and an insulated copper coil wound around a soft iron bar are all connected in series to form a complete circuit.
• When the switch is closed, current flows through the coil, creating a magnetic field and magnetizing the soft iron bar, making it an electromagnet.
• The poles (N and S) are determined by the Right-Hand Thumb Rule. If you curl your fingers in the direction of the conventional current (from positive to negative terminal), your thumb points to the North Pole. In this diagram, the current is flowing clockwise on the side facing us, so that end becomes the South Pole, and the far end becomes the North Pole.

Question 5. 

The following figure shows a coil wound around a soft iron bar XY. State the polarity at the ends X and Y as the switch is pressed. Suggest one way of increasing the strength of electromagnet so formed.

Ans:

Based on the provided figure and the principles of electromagnetism, here is the state of the electromagnet:

Polarity at Ends X and Y

The polarity of the electromagnet is determined using the Right-Hand Grip Rule.

1. Current Direction: When the switch is pressed, the conventional current flows out from the positive terminal of the battery and enters the coil at end Y before exiting at end X.
2. Applying the Rule: If you curl the fingers of your right hand around the coil in the direction of the current (from Y to X, tracing the path on top of the coil), your extended thumb points toward end X.
3. Conclusion:
   • The end X acquires North Pole (N) polarity.
   • The end Y acquires South Pole (S) polarity.

Way to Increase Electromagnet Strength

To increase the strength of the electromagnet formed by the coil and soft iron core, you can utilize the rheostat shown in the circuit diagram:

• Decrease the resistance of the rheostat.

Explanation:

By decreasing the resistance of the rheostat, the total current flowing through the coil from the battery increases (according to Ohm’s Law, I = V/R). Since the strength of an electromagnet is directly proportional to the current flowing through its coil, increasing the current will make the electromagnet stronger.

Question 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:

Electromagnet

The device formed by wrapping a coil of insulated copper wire around a soft iron core and passing an electric current through the coil is called an electromagnet.

Use of the Device

One common use of an electromagnet is in electric bells.

• In an electric bell, the electromagnet is used to repeatedly attract and release an armature (a piece of soft iron) which strikes the gong, producing the ringing sound.

Other uses include:

• Lifting heavy iron scraps in cranes.
• In loudspeakers and headphones.
• In magnetic relays and circuit breakers.

Question 7. 

Show with the aid of a diagram how a wire is wound on a 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:

Question 8. 

State two ways through which the strength of an electromagnet can be increased. 

Ans:

Two common ways through which the strength of an electromagnet can be increased are:

1. Increasing the Current in the Coil:
   • The magnetic field strength is directly proportional to the electric current flowing through the coil. By increasing the voltage of the power source (like using more cells or a stronger battery) or by reducing the resistance in the circuit, you increase the current (I), which results in a stronger magnetic field.
2. Increasing the Number of Turns in the Coil:
   • The magnetic field strength is also directly proportional to the number of turns of wire in the coil (solenoid). By increasing the number of turns per unit length (making the winding denser or using a longer coil), the magnetic field is concentrated and becomes stronger.

Question 9. 

Name one device that uses an electromagnet.

Ans:

One common device that uses an electromagnet is an electric bell.

Question 10. 

State two advantages of an electromagnet over a permanent magnet.

Ans:

Here are two unique advantages of an electromagnet compared to a permanent magnet:

1. Instantaneous Control: An electromagnet offers the benefit of immediate activation and deactivation. Its magnetic field can be established or completely eliminated instantly simply by switching the electric current on or off, which is not possible with a permanent magnet whose field is constant.
2. Dynamic Strength Adjustment: An electromagnet’s force is entirely adjustable; its magnetic strength can be precisely increased or decreased on demand by regulating the current flowing through its coil. A permanent magnet’s strength is fixed once it is manufactured.

Question 11. 

State two differences between an electromagnet and a permanent magnet.

Ans:

1. Controllability of Magnetism:
   • Electromagnet: Its magnetism is temporary and can be instantly turned on or off by controlling the flow of electric current.
   • Permanent Magnet: Its magnetism is always present and cannot be easily turned off.
2. Adjustability of Strength and Polarity:
   • Electromagnet: Its magnetic strength is variable (adjusted by changing the current), and its polarity (North/South poles) can be reversed by reversing the current direction.
   • Permanent Magnet: Its magnetic strength is fixed, and its polarity is unchangeable.

Question 12. 

Why is soft iron used as the core of the electromagnet in an electric bell?

Ans:

Soft iron is used as the core of the electromagnet in an electric bell because it is a material with low retentivity (or low coercivity).

This property is crucial for the function of the bell for the following reasons:

1. Easy Magnetization: Soft iron is a ferromagnetic material, meaning it can be very easily and strongly magnetized when current flows through the coil.
2. Instant Demagnetization: Due to its low retentivity, soft iron loses its magnetism almost instantly when the current is switched off.

In the electric bell, the electromagnet must quickly and repeatedly turn on and off to attract and then release the armature (hammer) that strikes the gong. If a material like steel (which has high retentivity) were used, the core would remain magnetized for a longer period after the circuit is opened, preventing the quick and continuous vibration necessary for the bell to ring.

Question 13. 

How is the working of an electric bell affected, if alternating current is used instead of direct current? 

Ans:

The working of an electric bell is generally not stopped if alternating current (AC) is used instead of direct current (DC), but the mechanism’s behavior and efficiency are affected.

Effect of AC on the Electromagnet and Bell

1. Changing Polarity: When AC is used, the electric current periodically reverses its direction (e.g., 50 or 60 times per second). This causes the polarity (North and South poles) of the electromagnet’s core to also reverse continuously at the same frequency.
2. Attraction Remains: Despite the rapid change in polarity, the attraction of the armature to the electromagnet remains intact. Attraction depends on the presence of a magnetic field, not its specific polarity. In both the positive and negative cycles of the AC, a strong magnetic field is generated.
3. Functionality: Because the armature is attracted by the magnetic field during both halves of the AC cycle, the bell will still ring when the switch is pressed. The core still magnetizes, pulls the armature, and breaks the contact.
4. Efficiency/Design Consideration:
   • The constant, high-frequency reversal of the magnetic field can cause a louder buzzing noise or less clean operation compared to a DC bell.
   • Traditional DC bells rely on a contact breaker mechanism that continuously makes and breaks the circuit to allow the hammer to oscillate. When using AC, the current is constantly reversing, which helps prevent the soft iron core from becoming permanently magnetized (residual magnetism), potentially leading to more reliable operation in some contexts, but it may conflict with the design of the interrupter mechanism built for DC.

Question 14. 

Name the material used for making the armature of an electric bell. Give a reason for your answer.

Ans:

Reason for Using Soft Iron

Soft iron is chosen for the armature because it is a ferromagnetic material with very low retentivity (or low coercivity).

The armature must be quickly and repeatedly attracted to, and released from, the electromagnet to allow the bell to ring continuously.

• Attraction: Soft iron is easily and strongly magnetized when the current flows, allowing the electromagnet to pull the armature quickly.
• Release: Due to its low retentivity, soft iron loses its induced magnetism almost instantly when the circuit contact is broken. This ensures that the spring can immediately pull the armature back to its original position, restoring the contact and allowing the cycle to repeat rapidly.

Exercise 10 (B)

Question 1. 

Electromagnets are made up of :

1. steel
2. copper
3. soft iron
4. Aluminium

Question 2. 

The strength of the electromagnet can be increased by

1. reversing the directions of current
2. using alternating current of high frequency
3. increasing the current in the coil
4. decreasing the number of turns of coil