Frank Brothers Solutions for ICSE Class 9 Physics Chapter 8.3 Electricity And Magnetism

Question 1. Define natural magnet and state its properties.
Answer: Natural magnet is the deposits of the iron ore which has the attracting property of iron The properties of natural magnet are :

• The magnet has the directive property, when it is suspended freely , it always come in north-south direction.
• A magnet attracts magnetic substances like iron, steel etc. so, it has the attractive poperty.
• Like poles repels each other while unlike poles attract each other in magnet.

In simple words: A natural magnet is a special kind of rock found in the ground that can pull iron towards it. It always points North and South if you hang it by a string, and it has "poles" that either push or pull other magnets.

📝 Teacher's Note: Show students a piece of magnetite (loadstone) to demonstrate that magnets occur naturally. Explain that the "directive property" is how the first compasses were made.

🎯 Exam Tip: When listing properties, ensure you include the specific terms "directive property" and "attractive property" to score full marks.

Question 2. What are artificial magnets and why are they used?
Answer: Artificial magnets are the magnets which are formed by bringing the natural magnet near it The artificial magnets have the high retentivity means they remains magnetized for the long period of time and that is why they are used for making strong electromagnets.
In simple words: Artificial magnets are man-made magnets. They are better than natural ones because they stay magnetic for a very long time and can be made much stronger.

📝 Teacher's Note: You can create a temporary artificial magnet in class by rubbing a steel needle with a strong bar magnet in one direction several times.

🎯 Exam Tip: The word "retentivity" is a key technical term; it refers to the ability of a material to stay magnetized after the external magnetic force is removed.

Question 3. What are magnetic poles?
Answer: The magnetic forces appear to be concentrated at the near ends of the magnet. These points are called the magnetic poles.
In simple words: The poles are the two ends of a magnet. They are the "power spots" where the magnet's pull is the strongest.

📝 Teacher's Note: Use iron filings on a sheet of paper over a bar magnet. Students will see the filings cluster heavily at the ends, visually proving where the poles are.

🎯 Exam Tip: Remember that every magnet has exactly two poles, North and South, and they always exist in pairs.

Question 4. How can you distinguish between an iron rod and a copper rod using a magnet?
Answer: Bring the rod near the magnet, iron rod will get attract towards the magnet while copper will not.
In simple words: If you put a magnet near both, the iron one will jump toward the magnet, but the copper one won't move at all. This is because iron is a magnetic material and copper is not.

📝 Teacher's Note: This is a fundamental test for magnetic vs. non-magnetic materials. Copper, aluminum, and gold are common non-magnetic metals that students often find surprising.

🎯 Exam Tip: Use the keyword "attraction" to describe the behavior of the iron rod.

Question 5. How can you identify the North and South poles of a magnet?
Answer: Suspend a magnet freely; it will always point towards the north -south direction. The pole of the magnet which point towards the north is called north pole and the pole which point towards south is called south pole.
In simple words: Hang a magnet by a thread so it can spin. When it stops moving, the end pointing toward the Earth's North Pole is the magnet's North Pole.

📝 Teacher's Note: This works because the Earth itself acts like a giant bar magnet. This is the basic principle behind a navigational compass.

🎯 Exam Tip: The phrase "freely suspended" is important to show that the magnet is not being forced into a direction by anything other than Earth's field.

Question 6. Define Magnetic axis, Poles of the Magnet, and Effective Length of the magnet.
Answer: Magnetic axis: The imaginary line joining the poles of the magnet is called magnetic axis.
Poles of the Magnet: The points of the magnet where the attraction appears to be maximum are called poles of the magnet.
Effective Length of the magnet: The distance between the poles of magnet is the effective length of magnet.
In simple words: The axis is an imaginary center line. The poles are the strong ends. The effective length is the distance between those two ends.

📝 Teacher's Note: Explain that the "effective length" is actually slightly shorter than the "geometric length" (the actual physical size of the metal bar) because the poles are slightly inside the ends.

🎯 Exam Tip: Be careful not to confuse "Magnetic axis" with "Geometric axis"—the magnetic axis specifically passes through the poles.

Question 7. How can you distinguish between an iron bar and a magnet without using another magnet?
Answer: Suspend the iron bar and magnet freely in the space by using thread. And, move them here and there few number of times. The bar which always points in north south direction after coming to rest is a magnet and other bar is the iron bar.
In simple words: Hang them both from strings. The one that acts like a compass and always points North-South is the magnet. The regular iron bar will just point in any random direction.

📝 Teacher's Note: This demonstrates the "directive property" of magnets. It's a reliable way to identify a magnet even if you don't have any other metal nearby.

🎯 Exam Tip: Focus on the "alignment with Earth's magnetic field" as the distinguishing factor.

Question 8. Can we isolate the poles of a magnet?
Answer: No, we cannot isolate the poles of the magnet.
In simple words: You can never have a magnet with only a North pole. If you cut a magnet in half, you don't get one North and one South; you just get two smaller magnets, each with its own North and South.

📝 Teacher's Note: This is the concept of "Magnetic Monopoles" not existing. It's a fundamental law—magnetism always comes in pairs (dipoles).

🎯 Exam Tip: The answer is always "No." Magnetic poles always exist in pairs.

Question 9. Define induced magnetism.
Answer: The magnetism acquired by the magnetic material when it is kept near the magnet is called induced magnetism.
In simple words: If you put a paperclip next to a strong magnet, the paperclip starts acting like a magnet too. That "borrowed" magnetism is called induced magnetism.

📝 Teacher's Note: Show how a magnet can pick up one paperclip, and then that paperclip can pick up a second one. The first paperclip has become an induced magnet.

🎯 Exam Tip: Induced magnetism is usually temporary and disappears when the main magnet is moved away.

Question 10. Define magnetic field lines.
Answer: The lines of the magnetic field are the continuous curve such that the tangent drawn at any point gives the direction of magnetic field at that point.
In simple words: These are invisible "pathway" lines that show the direction and strength of the magnetic force around a magnet.

📝 Teacher's Note: Compare them to the "contour lines" on a map. They don't actually exist on the object, but they represent a physical force we can measure.

🎯 Exam Tip: Mention that field lines go from the North pole to the South pole outside the magnet.

Question 11. What is lodestone?
Answer: The natural magnet which was used for the navigation by the mariners is called lodestone.
In simple words: Lodestone is a natural magnet found in nature. Ancient sailors used it as a simple compass to find their way across the ocean.

📝 Teacher's Note: The word comes from "leading stone." It was the very first tool used for global navigation.

🎯 Exam Tip: Lodestone is chemically known as magnetite (\( Fe_3O_4 \)).

Question 12. Which instrument is used to draw magnetic field lines?
Answer: Magnetic compass is used to draw the magnetic field of lines around the magnet.
In simple words: You can use a small compass to trace the invisible magnetic paths around a magnet by following where the needle points.

📝 Teacher's Note: This is a common lab activity. Students move the compass point-by-point and mark the direction of the needle to "connect the dots" into a field line.

🎯 Exam Tip: The compass needle always aligns itself tangent to the magnetic field line at that point.

Question 13. Can magnetic field lines intersect each other? Why?
Answer: No, Magnetic field of lines cannot intersect each other.
In simple words: Magnetic lines never cross. If they did, a compass needle wouldn't know which way to point at the crossing spot!

📝 Teacher's Note: Explain that at any point in space, the magnetic field can only have one specific direction. If lines crossed, it would imply two different directions at the same spot, which is impossible.

🎯 Exam Tip: The correct scientific reason is: "at the point of intersection, the magnetic field would have two directions, which is not possible."

Question 14. Complete the table showing the interaction of various materials with magnetic poles.
Answer:

In simple words: Non-magnetic things like glass don't react. Magnetic materials like iron always get pulled. Magnets either pull or push depending on which ends are facing each other.

📝 Teacher's Note: This table is a great summary. Emphasize that "No action" means the material is non-magnetic, while "repulsion" only happens between two magnets.

🎯 Exam Tip: Remember: Attraction can happen between a magnet and a magnetic material, but repulsion only happens between two magnets.

Question 15. Describe a simple experiment to prove the laws of magnetic poles.
Answer: The simple experiment to show that like poles repel while unlike poles attract each other is : Suspend a bar magnet and allow it to freely rotate. When it will come to rest, it will point towards the north south direction. Now, bring the N pole of second magnet near the North pole of the suspended magnet , we will see repulsion between the two magnets. But, if we bring south pole of second magnet near the north pole of suspended magnet then we will observe attraction. So, we can say like poles repel while unlike poles attract.
In simple words: If you bring two "North" ends together, they push away. If you bring a "North" and a "South" end together, they pull toward each other.

📝 Teacher's Note: Use the phrase "Like poles repel, unlike poles attract" as a classroom chant. It's the most fundamental rule of magnet behavior.

🎯 Exam Tip: Use the terms "repulsion" and "attraction" clearly in your experimental description.

Question 16. How does a compass needle help in drawing lines of force?
Answer: The directions indicated by the needle of compass will help to draw the magnetic lines of force.
In simple words: Since the compass needle always points along the magnetic path, you can just follow it and draw the line it shows you.

📝 Teacher's Note: Think of the compass needle as a tiny "pointer" that always aligns itself with the invisible magnetic stream around a magnet.

🎯 Exam Tip: The needle of the compass is itself a tiny magnet, which is why it reacts to the larger magnet's field.

Question 17. What are neutral points in a magnetic field?
Answer: Neutral points are the points where the magnetic field of magnet has the magnitude equal to the magnitude of earth’s magnetic field but in opposite direction.
In simple words: A neutral point is a "zero zone" where the magnet's push and the Earth's push are exactly the same strength but in opposite directions, so they cancel out.

📝 Teacher's Note: If you place a compass at a neutral point, it won't point in any specific direction; it will just stay however you leave it because there is no net force.

🎯 Exam Tip: The total magnetic field at a neutral point is exactly zero.

Question 18. What is the use of finding neutral points?
Answer: Neutral points help in finding the magnitude and direction of earth’s magnetic field at that particular point.
In simple words: By finding where a magnet's force is cancelled out, scientists can calculate exactly how strong the Earth's natural magnetism is in that exact spot.

📝 Teacher's Note: Earth's magnetism changes slightly depending on where you are on the planet. Finding neutral points is a clever way to measure these subtle differences.

🎯 Exam Tip: Mention "magnitude and direction" to show a complete understanding of the Earth's field vectors.

Question 19. What is the value of the resultant magnetic field at a neutral point?
Answer: The resultant magnetic field at neutral point is zero.
In simple words: At a neutral point, there is no magnetism at all because the competing fields have perfectly cancelled each other out.

📝 Teacher's Note: This is a direct consequence of the definition of a neutral point. "Zero" is the key concept here.

🎯 Exam Tip: This is a very common one-mark objective question; the answer is always "zero."